Chapter 9:                      Heating and Cooling Systems

This chapter discusses safety and energy-efficiency improve­ments to heating and cooling systems. It is divided into these main sections.

1.      Essential combustion-safety testing

2.      Heating-system replacement

3.      Servicing gas and oil heating systems

4.      Combustion venting

5.      Heating distribution systems

6.      Air-conditioning systems

The SWS requires that weatherization agencies perform a com­bustion-safety evaluation as part of each weatherization work scope. This evaluation is the chapter’s first topic. The chapters other topics are procedures and requirements related to cost-effective ECMs, such as tune-ups and equipment replacement.

Qualified heating technicians should perform the installations, adjustments, and maintenance described in this chapter.

Important Note: Use manufacturer’s specifications and instruc­tions whenever they are available. Many of the specifications in this chapter assume that the manufacturer’s instructions aren’t available. We offer numbers and other specific guidance that experts and reviewers consider correct. In general, the authority having jurisdiction (AHJ) is the most important source of guid­ance. For example: your city or county building department, fire department, or mechanical inspector.

NFPA Codes

The National Fire Protection Association (NFPA) publishes codes and standards used for HVAC installation, maintenance and repair.

•       IFGC: International Fuel Gas Code

•       NFPA 54: The National Fuel Gas Code

•       NFPA 31: Standard for the Installation of Oil-Burning Equipment

•       NFPA 211: Standard for Chimneys, Fireplaces, Vents, and Solid-Fuel-Burning Appliances

•       NFPA 70: National Electrical Code

ANSI/ACCA Manuals

The American National Standards Institute (ANSI) and the Air Conditioning Contractors of America (ACCA) together publish authoritative manuals to size and select HVAC equipment, which are for sale on the ACCA website.

•       Manual J: Residential Load Calculation

•       Manual D: Residential Duct Design

•       Manual S: Residential Equipment Selection

•       Manual N: Commercial Load Calculation

•       Manual CS: Commercial Systems Overview

ICC Codes

he International Code Council (ICC) publishes building codes for residential and commercial buildings, along with codes on energy efficiency and safety.

•       IMC-ICC: International Mechanical Code

•       IRC-ICC: International Residential Code

•       IBC-ICC: International Building Code

•       IECC-ICC: International Energy Conservation Code

If you find a conflict among the listed codes, local codes, manu­als, and manufacturer specifications, comply with the most spe­cific and stringent requirement among them.

9.1   Combustion-Safety Evaluation

At a minimum, evaluate the combustion safety at the weather­ization job’s completion.

See “NFPA Codes” on page 280.

9.1.1   Combustion-Safety Observations

Make the following observations before testing to help you determine the likelihood of carbon monoxide (CO) and spillage problems.

✓     Recognize soot near the draft diverter, barometric damper, or burner of a combustion appliance as a sign that the appliance has produced CO and spilled combustion gases.

✓     Recognize that rust in a chimney or vent connector may also indicate spillage.

✓     Look for irregularities and flaws in the venting system.

✓     Specify that workers seal all accessible return-duct leaks attached to combustion furnaces.

✓     Verify that the home has a working CO alarm. If the home has no working smoke alarm in addition to no CO alarm, install a combination CO-smoke alarm, or separate CO and smoke alarms.

9.1.2   Leak-Testing Gas Piping

Natural gas and propane piping systems may leak at their joints and fittings. Find gas leaks with an electronic combustible-gas detector, also called a gas sniffer. A gas sniffer finds significant gas leaks if used correctly.

✓     Sniff all valves and joints with the gas sniffer.

✓     Accurately locate leaks using a noncorrosive bubbling liq­uid, designed for finding gas leaks.

✓     Repair all gas leaks or label them for a gas service person to seal.

✓     Replace kinked, cracked, or corroded flexible gas connec­tors.

✓     Replace flexible gas lines manufactured before 1973. The line’s manufacture date is stamped on a date ring attached to the flexible gas line or on the body of the hex nut. If a date ring isn’t present and you believe the gas line predates 1973, then replace the flexible gas line.

9.1.3   Carbon Monoxide (CO) Testing

CO testing is essential for evaluating the safety of combustion and venting. Measure CO in the flue gas of every combustion appliance you inspect and service. Measure CO in ambient air in both the home and CAZ as part of inspection and testing of combustion appliances. We strongly recommend using a full-featured electronic combustion analyzer for flue-gas analysis during this essential combustion safety testing. See “Critical Combustion-Testing Parameters” on page 299.

Vent Testing for CO

Testing for CO in the appliance vent is a part of combustion test­ing that happens under worst-case conditions. The DOE and BPI have two separate CO limits depending on the type of appli­ance. If the following CO limits are exceeded in the undiluted combustion byproducts, the appliance fails the CO test under current DOE and BPI standards.

•       Space heaters and water heaters: 100 ppm as measured or 200 ppm air-free

•       Furnaces or boilers: 200 ppm as measured or 400 ppm air-free

Ambient Air Monitoring for CO

The DOE SWS require monitoring of CO during combustion testing to ensure that CO in the combustion appliance zone (CAZ) doesn’t exceed 35 ppm as measured. If ambient CO levels in the combustion zone exceed 35 ppm, stop testing for your own safety. Ventilate the CAZ thoroughly before resuming com­bustion testing. Investigate indoor CO levels, greater than out­door ambient levels, to determine their cause. See "Causes of Carbon Monoxide (CO)" on page 25.

9.1.4   Worst-Case CAZ Depressurization Testing

CAZ depressurization is the leading cause of backdrafting and flame roll-out in furnaces and water heaters that vent into natu­rally drafting chimneys and venting systems.

Worst-case vent testing uses the home’s exhaust fans, air han­dler, and chimneys to create worst-case depressurization in the combustion-appliance zone (CAZ). The CAZ is an area contain­ing one or more combustion appliances. During this worst-case testing, you can measure the CAZ pressure difference with ref­erence (WRT) to outdoors and test for spillage.

Worst-case conditions do occur, and venting systems must exhaust combustion byproducts even under these extreme con­ditions. Worst-case vent testing exposes whether or not the venting system exhausts the combustion gases when the com­bustion-zone pressure is as negative as you can make it. A digital manometer is the best tool for accurate and reliable readings of both combustion-zone depressurization and chimney draft.

Take all necessary steps to reduce CAZ depressurization and minimize combustion spillage, based on your tests.

Worst-Case CAZ Depressurization Test

Follow the steps below to find the worst-case depressurization level in the combustion appliance zone (CAZ).

1.      Close all exterior doors, windows, and fireplace damper(s). Open all interior doors, including closet doors.

2.      Remove furnace filter if it’s dirty. Leave the dirty filter out for the test or replace it with a new filter. Be sure the filter slot is covered for the test.

3.      Record the baseline pressure of the CAZ with reference to outdoors.

4.      Turn on the clothes dryer and exhaust fans. (Clean clothes dryer filter trap)

5.      Open doors to negative zones (rooms with exhaust fans) and close doors to positive zones (bedrooms without returns). Use smoke or a manometer to test room pres­sures if necessary.

6.      Open and close the CAZ door. Record the most nega­tive pressure and note CAZ door position.

7.      Turn on the furnace air handler. Leave it on if the CAZ pressure goes more negative. If it goes more positive, turn off the air handler and proceed to number 8.

8.      Open and close the CAZ door. Record the most nega­tive pressure, and note CAZ door position.

9.      Calculate the net difference between the worst depres­surization found from either #6 or #8 and the baseline pressure from #3. This is the worst-case depressuriza­tion.

10.  Refer to the SWS 2.0299.1 Combustion Appliance Depressurization Limits Table or “Maximum CAZ Depressurization” on page 578.

11.  Specify improvement if the measured worst-case depressurization limit is exceeded. See "Evaluating Combustion Air at Worst-Case" on page 289.

Analyzing CAZ Depressurization

Analyze the negative and positive pressures you measure in the CAZ to find workable solutions, using the troubleshooting table below.

Spillage and CO Testing

Next, verify that the appliance venting systems don’t spill or pro­duce excessive CO at worst-case depressurization. Test each appliance in turn for spillage and CO as described below.

1.      Check for flue-gas flow in the venting system. Feel the vent connector for heat. The vent connector should start warming within 5 seconds if it establishes flue-gas flow. If the vent connector remains cold, stop the test and investigate.

2.      Detect spillage at the draft diverter of each combustion appliance in one of these ways.

a.  Smoke from a smoke generator is repelled by spillage at the draft diverter.

b.  A mirror fogs at the draft diverter

3.      If spillage in one or more appliances continues at worst-case depressurization for 2 minutes or more, take action to correct the problem.

4.      Measure CO in the undiluted flue gases of each space heater or water heater after 2 minutes of operation at worst-case depressurization. If CO in undiluted flue gases is more than 100 ppm as measured or 200 ppm air-free measurement, take action to reduce CO level.

5.      Measure CO in the undiluted flue gases of each furnace or boiler after 2 minutes of operation at worst-case depressurization. If CO in undiluted flue gases is more than 200 ppm as measured or 400 ppm air-free mea­surement, take action to reduce CO level.

6.      Measure draft after 5 minutes.

Spillage and draft: Spillage and draft are two indications of whether the combustion gases are exiting the building as they should. In this guide, we focus on spillage because it’s spillage we’re trying to avoid, and we can detect it easily.

Positive draft indicates spillage, but not as reliably as checking for spillage itself. Evaluate spillage, unless you understand draft and know how to measure it.

9.1.5   Evaluating Combustion Air at Worst-Case

Combustion appliances need combustion air to support com­bustion and to balance the draft in natural-draft chimneys. In most buildings, combustion air comes through the building’s air leaks. If workers seal the building tightly, they may reduce com­bustion air to a level that causes combustion problems or that allows depressurization. The worst-case testing procedure exposes most of these problems. This section tells how to evalu­ate combustion air during the worst-case depressurization test and by using a rule of thumb.

Evaluating the CAZ Volume

In the average building with more than 0.40 natural air changes per hour (ACHn), the combustion appliance zone or CAZ should contain more than 50 cubic feet of volume for each 1000 BTUH of combustion-appliance input. However, a smaller vol­ume may provide adequate combustion air and a larger volume may not depending on the airtightness of the CAZ.

Evaluating Combustion Air by Flue-Gas Analysis

During worst-case testing, use a combustion analyzer to mea­sure both CO and oxygen (O₂). See “Critical Combustion-Testing Parameters” on page 299. 

The O₂ is an indicator of excess combustion air, and high CO may be an indicator of insufficient combustion air.

1.      Sample undiluted flue gases as they leave the appliance’s heat exchanger during worst-case conditions.

2.      If the O₂ reading from the combustion analyzer is more than 5% with an natural-draft burner or more than 3% with a power burner or well adjusted and maintained barometric draft control, combustion air is probably adequate assuming CO is minimal.

3.      If the O₂ reading from the combustion analyzer is less than the above O₂ values, this indicates that combus­tion air is inadequate or that the appliance is over-fired. We would expect significant CO to accompany such low O₂ readings.

4.      If O₂ is too low, measure fuel input to verify that the fir­ing rate is at or below the manufacturer’s BTUH specifi­cations for input. An excessive firing rate could also cause low O₂ and high CO.

5.      If O₂ is too low at the correct firing rate, open a door or a window connected to the CAZ. If opening the CAZ door, a nearby window, an exterior door, or any combi­nation of these increases the O₂ reading and decreases CO, then consider the options specified in “Combus­tion-Air-Related Solutions” on page 294.

9.1.6   Mitigating CAZ Depressurization and Spillage

If you find problems with CAZ depressurization or spillage, consider the improvements discussed next to solve the prob­lems.

If the appliance spills or shows inadequate draft, open a window, exterior door, or interior door to observe whether the additional combustion airflow through that opening stops the spillage.

1.      If this additional air improves draft, the problem is usu­ally depressurization.

2.      If this additional air doesn’t stop the spillage, inspect the chimney. The chimney may be obstructed, undersized, oversized, or leaky.

Improvements to Mitigate CAZ Depressurization

This list of improvements may solve spillage problems detected during the previous tests on a forced air heating system.

✓     Seal all return-duct leaks in the CAZ.

✓     Isolate combustion appliances from exhaust fans, clothes dryers, and return registers by air sealing between the CAZ and zones containing these depressurizing devices as described on page 294.

✓     Provide make-up air for dryers and exhaust fans. See page 348. 

✓     Reduce the CFM of exhaust appliances.

Chimney Improvements to Mitigate Spillage Problems

Suggest the following chimney improvements to mitigate spill­age problems detected during the previous testing.

•       Remove chimney obstructions.

•       Repair disconnections or leaks at joints and where the vent connector joins a masonry chimney.

•       Measure the size of the vent connector and chimney and compare to vent-sizing information listed in Chapter 13 of the National Fuel Gas Code (NFPA 54). A vent connector or chimney liner that is either too large or too small can reduce draft.

•       If wind interferes with draft, install a wind-dampening chimney cap.

•       If the masonry chimney is deteriorated, install a new chim­ney liner.

•       Increase the pitch of horizontal sections of vent.

•       Increase the vertical rise of the vent connector, directly off the appliance vent fitting.

Fixing Persistent Depressurization and Spillage Problems

Sometimes buildings and their combustion appliances don’t respond to the possible solutions listed previously. For persistent depressurization, spillage, and make-up air consider the follow­ing solutions.

•       Replace open combustion appliances with sealed-combus­tion appliances.

•       For an orphaned water heater either reline the chimney with a correctly sized chimney liner or replace gas or oil-fired water heaters with electric water heaters.

•       If opening the CAZ door reduces worst-case CAZ depres­surization, consider providing a vent between the CAZ and surrounding zones.

9.1.7   Combustion-Air-Related Solutions

If combustion air is inadequate after trying the solutions in the previous sections, consider replacing open-combustion appli­ances with sealed-combustion appliances. The options dis­cussed here have a risk of failure because of the unknowns with installing supplemental combustion air and isolating CAZs from the remainder of the building. Sealed-combustion is the ultimate answer to the problems of combustion air, depressur­ization, and spillage.

Installing Supplemental Combustion Air

Combustion-air vents should be no less than 3 inches in their smallest dimension. A vent with a louver or grille has a net free area (NFA) that is smaller than actual vent area because NFA accounts for the blocking effect of louvers and grilles. Metal grilles and louvers provide about 75% of their area as net-free area while wood louvers provide only about 25%.

The table shown here gives sizing guidelines for combustion air openings. If testing indicates the need for supplemental com­bustion air, install openings to one of these spaces.

•       Another indoor space.

•       A ventilated intermediate zone, such as a ventilated attic or ventilated crawl space.

•       From outdoors into an isolated CAZ.

•       From outdoors to the appliance by replacing natural-draft combustion appliances with sealed-combustion (direct-vent) appliances.

After installing supplemental combustion air, repeat the worst-case testing to verify that the combustion air problem is solved and that the combustion is safe.

Zone Isolation for Natural-Draft Vented Appliances

An isolated CAZ improves the safety of natural-draft vented appliances in some cases if replacing natural-draft appliances with sealed-combustion isn’t an option. The CAZ is isolated if it receives combustion air only from outdoors or a ventilated intermediate zone. Inspect the CAZ for connections with the home’s main zone and make sure it is isolated.

1.      Seal all connections between the isolated CAZ and the home. Examples include joist spaces, forced-air grills, transfer grills, leaky doors, and holes for ducts or pipes.

2.      Measure a base pressure from the CAZ to outdoors.

3.      Set-up house in worst case, and verify that the set-up doesn’t affect the CAZ pressure.

4.      Measure CO and O₂ at worst-case and evaluate com­bustion air as described in “Evaluating Combustion Air by Flue-Gas Analysis” on page 290.

5.      If the CAZ-to-outdoors pressure changed during worst-case, air-seal the zone, and retest as described in steps 2 and 3.

6.      If you can’t air-seal the CAZ adequately to isolate the zone, solve worst-case depressurization and spillage problems as described in “Evaluating Combustion Air at Worst-Case” on page 289.

7.      Provide outdoor combustion air.

8.      If the zone isolation fails, replace natural-draft appli­ances with sealed-combustion appliances.

9.2   Electronic Combustion Analysis

The goal of a combustion analysis is to quickly analyze combus­tion safety and efficiency. When the combustion appliance reaches steady-state efficiency (SSE), you can measure its most critical combustion parameters. This information saves time and informs both service and installation adjustments.

Modern combustion analyzers measure O₂, CO, and flue-gas temperature. Some models also measure draft. Combustion analyzers also calculate combustion efficiency or steady-state efficiency (SSE), which are synonymous.

9.2.1   Critical Combustion-Testing Parameters

These furnace-testing parameters tell you how efficient and safe the furnace currently is and how much you might be able to improve efficiency. Use these measurements to analyze the com­bustion process.

Carbon monoxide (CO) (ppm):

Poisonous gas indicates incom­plete combustion. Modern combustion analyzers let you choose between an as-measured value or a calculated value that states the concentration of CO in theoretical air-free flue gases. Adjusting combustion to produce less than 100 ppm as mea­sured or 200 ppm air-free is almost always possible with fuel-pressure adjustments, air adjustments, or burner maintenance.

Oxygen (percent):

 Indicates the percent of excess air and whether fuel-air mixture is within a safe and efficient range. Efficiency increases as oxygen decreases because excess air, indi­cated by the O₂ carries heat up the chimney. Percent O₂ may also indicate the cause of CO as either too little or too much combustion air. Technicians used to measure CO₂, but O₂ is eas­ier to measure, and you only need to measure one of these two gases.

Flue-gas temperature:

Flue-gas temperature is directly related to furnace efficiency. Too high flue-gas temperature wastes energy and too-low flue-gas temperature causes corrosive condensation in the venting system.

Smoke number

For oil only, this measurement compares the stain made by flue gases with a numbered stain-darkness rating called smoke number. Smoke number should be 1 or lighter on a 1-to-10 smoke scale.

Draft

The pressure in the chimney or vent connector (chimney draft or breech draft). Also the pressure in the combustion chamber (over-fire draft), used primarily with oil power burn­ers.

9.3   Heating System Replacement

This section discusses replacing combustion furnaces and boil­ers. We’ll also discuss gas heating-replacement and oil-heating-replacement specifications.

See “NFPA Codes” on page 280 and “ANSI/ACCA Manuals” on page 280.

9.3.1   Combustion Furnace Replacement

This section discusses air handlers of combustion furnaces and also heat pumps. Successful air-handler replacement requires selecting the right heat (and cooling) input, blower model, and blower speed. The installation must include repairs to ducts and other remaining components, and testing to verify that the new air handler operates correctly.

See “NFPA Codes” on page 280 and “ANSI/ACCA Manuals” on page 280.

Preparation

✓     Recover refrigeration in the existing heating-cooling unit according to EPA regulations.

✓     Disconnect and remove the furnace or heat pump, attached air-conditioning equipment, and other materials that won’t be reused.

✓     Transport these materials off the customer’s property to a recycling facility.

✓     Verify that all accessible ducts were sealed as part of the furnace’s installation, including the air handler, the ple­nums, and the branch ducts.

Equipment Selection

✓     Evaluate the building to determine the correct size of the furnace, using ACCA Manual J or equivalent method. Select the smallest BTUH output furnace that your pre­ferred manufacturer offers and that exceeds your heat loss calculation.

✓     Select the air handler using ACCA Manual S or equivalent method along with manufacturers’ air-handler specifica­tions. Consider blower airflow requirements for air condi­tioning if there is existing central air conditioning.

✓     Select the supply and return registers using ACCA Manual T or equivalent method.

Air-Handler Installation

✓     Install MERV 6 or higher filter inside or outside of the new furnace.

✓     The filter must be easy to replace.

✓     The filter retainer must hold the filter firmly in place.

✓     The filter must provide complete coverage of blower intake or return grille. The filter compartment must not permit air to bypass the filter.

✓     If flue-gas temperature or supply air temperature are unusually high, check static pressure and fuel input. See “Ducted Air Distribution” on page 348.

✓     Attach the manufacturer’s literature including, operating manual and service manual, to the furnace.

Supporting Air Handlers

Support the new air handlers using these specifications.

•       Support horizontal air handlers from below with a non-combustible, water-proof, and non-wicking material. Or support the horizontal air handler with angle iron and threaded rod from above.

•       Support upflow air handlers with corner support legs, bricks, or pads from below when necessary to hold it above a damp basement floor.

•       Support downflow air handlers with a strong, airtight sup­ply plenum. Insulate this supply plenum on the exterior of the plenum.

9.3.2   Gas-Fired Heating Installation

The goals of gas-appliance replacement are to save energy and improve heating safety. The heating replacement project should produce a gas-fired heating system in virtually new condition, even though existing components like the gas lines, chimney, pipes, or wiring may remain.

See “NFPA Codes” on page 280 and “ANSI/ACCA Manuals” on page 280.

Include maintenance or repair of existing components as part of the installation. Analyze design defects in the original system, and correct the defects during the heating system’s replacement.

✓     If possible, install a condensing sealed-combustion (direct vent) furnace or boiler with a 90+ AFUE.

✓     Install non-condensing furnaces and boilers with a mini­mum AFUE of 80%, if the 90% replacement unit isn’t cost-effective or practical.

✓     Select the most energy efficient blower available. Prefer electrically commutated motors (ECM) when possible.

✓     Install new gas-fired unit with adequate clearances to allow maintenance.

✓     Follow manufacturer’s venting instructions along with the National Fuel Gas Code (NFPA 54) to install a proper venting system. See “Inspecting Venting Systems” on page 333.

✓     Check clearances of the heating unit and its vent connec­tor to nearby combustibles, according to NFPA 54. See page 333. 

✓     Measure the new unit’s gas input, and adjust the gas input if necessary.

Testing New Gas-Fired Heating Systems

✓     Do a combustion test, and adjust fuel-air mixture to mini­mize O₂. However don’t allow CO beyond 200 ppm as measured or 400 ppm air-free with this adjustment. See pages 283 and 317. 

✓     Verify that the gas water heater vents properly after instal­lation of a sealed-combustion or horizontally vented fur­nace or boiler. Install a chimney liner if necessary to provide right-sized venting for the water heater.

9.3.3   Combustion Boiler Replacement

Technicians replace boilers as an energy-conservation measure or for health and safety reasons.

Boiler piping and controls present many options for zoning, boiler staging, and energy-saving features. Dividing homes into zones, with separate thermostats, can significantly improve energy efficiency compared to operating a single zone.

Follow these specifications when recommending a replacement boiler.

See “NFPA Codes” on page 280 and “ANSI/ACCA Manuals” on page 280.

Boiler Design

A boiler’s seasonal efficiency is more sensitive to correct sizing than is a furnace’s efficiency.

✓     Consider weatherization work that reduced the heating load serviced by the previous boiler when sizing the new boiler.

✓     Determine the correct size of the boiler. Use ACCA Man­ual J or Manual N.

✓     Use the current version of ANSI/ACCA Manual S or Man­ual CS or equivalent procedures to select the boiler.

✓     Along with calculations from these manuals, consider the total installed radiation surface area connected to the boiler and also the radiator sizes in individual rooms.

✓     Select heating equipment of the lowest capacity required to meet the design heating load and provide sufficient vol­ume for components of existing distribution system that will remain in place.

✓     Size new radiators according to room heat loss and design water temperature.

✓     Select a boiler that is ENERGY STAR® certified or equiva­lent.

✓     Install unit in a dry location and within conditioned space when possible.

✓     Provide ease of access for routine maintenance/service on all system components requiring maintenance or service.

✓     Specify radiator temperature controls (RTCs) for areas with a history of overheating.

✓     A functioning pressure-relief valve, expansion tank, air-excluding device, back-flow preventer, and an automatic fill valve must be part of the new hydronic system.

Pump and Piping

✓     Verify that all supply piping is insulated with foam or fiberglass pipe insulation.

✓     Suggest that the pump be installed near the downstream side of the expansion tank to prevent the suction side of the pump from depressurizing the piping, which can pull air into the piping system.

✓     Replace the expansion tank, unless it’s the proper size for the new system. Adjust the expansion tank for the correct pressure during boiler installation. See page 381. 

✓     Extend new piping and radiators to conditioned areas, like additions and finished basements, which are currently heated by space heaters.

Controls

✓     Maintaining a low-limit boiler-water temperature is waste­ful. Boilers should be controlled for a cold start, unless the boiler is used for domestic water heating.

✓     For large boilers, install reset controllers that adjust supply water temperature according to outdoor temperature and prevent the boiler from firing when the outdoor tempera­ture is above a setpoint where heat isn’t needed.

✓     Verify that return-water temperature is above 130° F for gas and above 150° F for oil, to prevent acidic condensa­tion within the boiler, unless the boiler is designed for condensation. Install piping bypasses, mixing valves, pri­mary-secondary piping, or other strategies, as necessary, to prevent condensation within a non-condensing boiler.

Combustion Testing

✓     Inspect the chimney and upgrade it if necessary.

✓     Verify that flue-gas oxygen and temperature are within the ranges specified in these two tables.

a.  “Combustion Standards for Gas Furnaces and Boilers” on page 300

b.  “Minimum Oil Burner Combustion Standards” on page 323

Steam Boilers

Steam-boiler performance is heavily dependent on the perfor­mance of the existing steam distribution system. The boiler installer should know how the distribution system performed when it was connected to the old boiler.

The new boiler’s water line should be at the same height as the old boiler’s water line, or the installers should know how to compensate for the difference in water-line levels. See "Steam Heating and Distribution" on page 385.

9.3.4   Oil-Fired Heating Installation

Oil-heating replacement should provide an oil-fired heating sys­tem in virtually new condition, even though components like the oil tank, chimney, piping, and wiring may remain in place.

Any maintenance or repair for these remaining components should be part of the replacement job. Analyze design defects of the original system, and correct them during the heating-system replacement.

✓     New oil-fired furnaces and boilers should have a mini­mum AFUE of 83%.

✓     Install new oil-fired furnaces and boilers with adequate clearances to facilitate maintenance.

✓     Inspect the existing chimney and the vent connector. Re-place the vent connector with Type L double-wall vent pipe if necessary.

✓     Install a stainless steel chimney liner if necessary. See "Spe­cial Venting Considerations for Gas" on page 343.

✓     Verify that the clearances between the vent connector and nearby combustibles are adequate. See “Clearances to Combustibles for Vent Connectors” on page 335.

✓     Install a new fuel filter, and purge the fuel lines as part of the new installation.

See “NFPA Codes” on page 280 and “ANSI/ACCA Manuals” on page 280.

Controls

✓     Verify that a working emergency shut-off is installed in the living space.

✓     Look for a control that interrupts power to the burner in the event of a fire.

✓     Measure the transformer voltage to verify that it complies with the manufacturer’s specifications.

✓     Measure the control circuit amperage, and adjust the ther­mostat’s heat anticipator to match the amperage. Or, follow the thermostat manufacturer’s instructions for adjusting cycle length.

Testing New Oil-Fired Heating Systems

✓     Verify that the oil pressure matches the manufacturer’s specifications, but isn’t less than 100 psi.

✓     If the flue-gas temperature is too high, adjust oil pressure per manufacturers instructions or replace nozzle as neces­sary to produce the correct input and flue-gas tempera­ture.

✓     Verify that the spray angle and spray pattern fit the size and shape of the combustion chamber.

✓     Adjust oxygen, flue-gas temperature, and smoke number to match manufacturer’s specifications or specifications given here. Smoke number should be zero on all modern oil-fired equipment.

9.3.5   Evaluating Oil Tanks

Inspect the oil tank, and remove dirt and moisture at bottom of the tank. Verify that the oil tank and oil lines comply with NFPA 31.

Oil tanks are now almost always installed above ground. But many old oil tanks are still buried. Inspect above-ground tanks to find leaks.

Below-ground tanks and above-ground tanks can both be evalu­ated by tests for water in the fuel system.

1.      Start by inspecting the oil filter for corrosion. Corrosion in the oil filter indicates a high probability of water and corrosion in the tank.

2.      Next use water-finding paste, applied to the end of a probe, to detect water at the bottom of the oil tank. For indoor tanks, you’ll need a flexible probe because of the ceiling-height limitations.

See “NFPA Codes” on page 280 and “ANSI/ACCA Manuals” on page 280.

Inspecting Above-Ground Oil Tanks

Indoor oil leaks are usually accompanied by petroleum smells. Inspect the oil tank as well as all the oil piping between the oil tank and the oil-fired furnace.

✓     Look for different colors on the tank from condensation, corrosion, or fuel leaks.

✓     Look at the bottom of the oil tank and see if oil is dripping from a leak.

✓     Look for patches from previous leaks.

✓     If the oil tank is new, don’t mistake previous oil-tank leaks for leaks in the new tank.

✓     Use the water test described previously.

If you smell oil but you can’t see the leak, consider the following tests.

✓     Use the water test described previously.

✓     For hidden leaks, consider ultrasound leak detection by a oil-tank specialist.

Advice for Below-Ground Oil Tanks

Leaky below-ground oil tanks are a financial problem and a major environmental problem. Local, state, or federal authori­ties may require homeowners to remove the tank, abandon it in place, or have it leak-tested by one of the following methods.

✓     Use the water testing described previously.

✓     A tank specialist collects multiple soil samples from around the tank and analyzes them for petroleum contam­ination by an approved method.

9.4   Combustion Space Heater Replacement

Space heaters are inherently more efficient than central heaters, because they have no ducts or distribution pipes.

See “NFPA Codes” on page 280 and “ANSI/ACCA Manuals” on page 280.

Weatherization agencies replace space heaters as an energy-con­servation measure or for health and safety reasons. Choose a sealed-combustion space heater. Inspect existing space heaters for health and safety problems.

✓     If power outages are common, select a space heater that will work without electricity.

✓     Follow manufacturer’s venting instructions carefully. Don’t vent sealed-combustion or induced-draft space heaters into naturally drafting chimneys.

✓     Verify that flue-gas oxygen and temperature are within the ranges specified in Table 9-4 on page 300. 

✓     If the space heater sits on a carpeted floor, install a fire-rated floor protector.

✓     Install the space heater away from traffic, draperies, and furniture.

✓     Provide the space heater with a correctly grounded duplex receptacle for its electrical service.

9.4.1   Space Heater Operation

Communicate the following operating instructions to the occu­pants.

✓     Don’t store any objects near the space heater that would restrict airflow around it.

✓     Don’t use the space heater to dry clothes or for any pur­pose other than heating the home.

✓     Don’t allow anyone to lean or sit on the space heater.

✓     Don’t spray aerosols near the space heater. Many aerosols are flammable or they can corrode the space heater’s heat exchanger.

9.4.2   Unvented Space Heaters

Unvented space heaters include ventless gas fireplaces and gas logs installed in fireplaces previously designed for wood-burn­ing or coal-burning. These unvented space heaters create indoor air pollution because they deliver all their combustion byprod­ucts to the indoors. Unvented space heaters aren’t safe. Replace them with vented space heaters or electric space heaters.

DOE forbids unvented space heaters as primary heating units in weatherized homes. However, unvented space heaters may be used as secondary heaters, under these four requirements.

1.      The heater must have an input rating less than 40,000 BTUH.

2.      If located in a bedroom, the heater must have an input rating of less than 10,000 BTUH.

3.      The heater must be equipped with an oxygen-depletion sensor.

4.      The room containing the heater must have adequate combustion air.

5.      Home must have adequate ventilation: See “Whole-Dwelling Ventilation Requirement” on page 419.

9.5   Gas Burner Safety & Efficiency Service

Gas burners should be inspected and maintained during a ser­vice call. These following specifications apply to gas furnaces, boilers, water heaters, and space heaters.

See “NFPA Codes” on page 280.

9.5.1   Combustion Efficiency Test for Furnaces

Perform the following procedures at steady-state to verify a fur­nace’s correct operation.

•       Perform a combustion test using a electronic flue-gas ana­lyzer. Recommended flue-gas temperature depends on the type of furnace and is listed in the table titled, “Combustion Standards for Gas Furnaces and Boilers” on page 300.

•       Measure temperature rise (supply minus return tempera­tures). Temperature rise should be within the manufac­turer’s specifications for a furnace or boiler: between 30° and 70°.

•       If O₂ is high, or the estimated output from the table is low, increase gas pressure until you measure 6% O₂ if possible, as long as you don’t create CO in the process.

•       Increase gas pressure if needed to increase temperature rise and flue-gas temperature.

If you know the airflow through the furnace from measure­ments described in “Ducted Air Distribution” on page 348, you can use the table, “Carbon Monoxide Limits” on page 579, to check whether output is approximately what the manufacturer intended. Dividing this output by measured input as described above gives you another check on the steady-state efficiency.

9.5.2   Inspecting Gas Combustion Equipment

Perform the following inspection procedures on all gas-fired furnaces, boilers, water heaters, and space heaters, as necessary.

✓     Look for soot, melted wire insulation, and rust in the burner and manifold inside and outside the burner com­partment. These signs indicate flame roll-out, combustion gas spillage, CO, and incomplete combustion.

✓     Inspect the burners for dust, debris, misalignment, flame-impingement, and other flame-interference problems. Clean, vacuum, and adjust as needed.

✓     Inspect the heat exchanger for cracks, holes, or leaks.

✓     Verify that furnaces and boilers have dedicated circuits with safety shutoffs near the appliance. Verify that all 120-volt wiring connections are enclosed in covered electrical boxes.

✓     Verify that pilot is burning (if equipped) and that main burner ignition is satisfactory.

✓     Check venting system for proper diameter and pitch. See page 333. 

✓     Check venting system for obstructions, blockages, or leaks.

✓     Observe flame characteristics. Flames should be blue and well shaped. If flames are white or yellow, the burner may suffer from faulty combustion.

9.5.3   Testing and Adjustment

The goal of these measures is to reduce carbon monoxide (CO), stabilize flame, and verify the operation of safety controls.

✓     Do an electronic combustion analysis and note the oxygen, CO, and flue-gas temperature.

✓     Test for spillage or measure draft. Take action to improve the draft if it is inadequate because of improper venting, obstructed chimney, leaky chimney, or depressurization. See page 289. 

✓     If you measure CO and the measured oxygen level is low, open a window while observing CO level on the meter to see if CO is reduced by increasing the available combus­tion air through the open window. See page 348. 

✓     Adjust gas input if combustion testing indicates over-firing or under-firing.

✓     For programmable thermostats, read the manufacturer’s instructions about how to control cycle length. These instructions may be printed inside the thermostat.

Burner Cleaning

Clean and adjust the burner if any of these conditions exists.

•       CO is greater than 100 ppm as measured or 200 ppm air-free measurement for space heaters and water heaters and 200 ppm as measured or 400 air-free for furnaces or boil­ers.

•       Visual indicators of soot or flame roll-out exist.

•       Burners are visibly dirty.

•       Measured draft is inadequate. See page 333. 

•       The appliance has not been serviced for two years or more.

Maintenance and Cleaning

Gas-burner and gas-venting maintenance should include the following measures.

✓     Remove causes of CO and soot, such as over-firing, closed primary air intake, flame impingement, and lack of com­bustion air.

✓     Remove dirt, rust, and other debris that may be interfering with the burners. Clean the heat exchanger if there are signs of soot around the burner compartment.

✓     Seal leaks in vent connectors and chimneys.

9.6   Oil Burner Safety and Efficiency Service

Oil burners require annual maintenance to maintain acceptable safety and combustion efficiency. Use combustion analysis to evaluate the oil burner and to guide maintenance and adjust­ment. These procedures apply to oil-fired furnaces, boilers, and water heaters. Use other test equipment as discussed to measure other essential operating parameters and to make adjustments as necessary.

See “NFPA Codes” on page 280 and “ANSI/ACCA Manuals” on page 280.

9.6.1   Oil Burner Testing and Adjustment

Unless the oil-fired heating unit is very dirty or disabled, techni­cians should do combustion testing and adjust the burner for safe and efficient operation.

Combustion Testing and Adjustment

Combustion testing is essential to understanding the current oil burner performance and potential for improvement.

✓     Sample the undiluted flue gases with a smoke tester, after reading the smoke tester instructions. Compare the smoke smudge left by the gases on the filter paper with the manu­facturer’s smoke-spot scale to find the smoke number.

✓     If the smoke number is higher than 3, take steps to reduce smoke before sampling the gases with a combustion ana­lyzer to prevent the smoke from fouling the analyzer.

✓     Sample undiluted flue gases between the barometric draft control and the appliance. Analyze the flue gas for O₂, flue-gas temperature, CO, and steady-state efficiency (SSE).

✓     Measure the overfire draft over the fire inside the firebox through a plug in the heating unit.

✓     A flue gas temperature more than 450° F is a sign that a clean heating unit is oversized. Exceptions: steam boilers and boilers with tankless coils. If the nozzle is oversized, replace the burner nozzle after selecting the correct nozzle size, spray angle, and spray pattern.

✓     Adjust the barometric damper for a negative overfire draft of–0.020 IWC or –5 pascals at a test plug in the heating unit.

✓     Adjust the air shutter to achieve the oxygen and smoke values, specified in Table 9-6 on page 323. 

✓     Adjust oxygen, flue-gas temperature, CO, and smoke number to match manufacturer’s specifications or specifi­cations given here. Smoke number should be near zero on all modern oil-fired equipment.

Other Efficiency Testing and Adjustment

✓     Adjust the gap between electrodes and their angle for proper alignment.

✓     Measure the control-circuit amperage. Adjust the thermo­stat’s heat anticipator to match the amperage, or read the thermostat manufacturer’s instructions for adjusting cycle length.

✓     Measure the oil-pump pressure, and adjust it to manufac­turer’s specifications if necessary.

✓     Measure the transformer voltage, and adjust it to manufac­turer’s specifications if necessary.

✓     Adjust the airflow or the water flow to reduce high flue-gas temperature if possible, but don’t reduce flue-gas tempera­ture below 350°F.

9.6.2   Oil Burner Inspection and Maintenance

Use visual inspection and combustion testing to evaluate oil burner operation. An oil burner that passes visual inspection and complies with the specifications on page 323 may need no maintenance. Persistent unsatisfactory test results may indicate the need to replace the burner or the entire oil-fired heating unit.

Safety Inspection, Testing, and Adjustment

✓     Inspect burner and appliance for signs of soot, overheat­ing, fire hazards, corrosion, or wiring problems.

✓     Inspect heat exchanger and combustion chamber for cracks, corrosion, or soot buildup.

✓     If the unit smells excessively of oil, test for oil leaks and repair the leaks.

✓     Time the flame sensor control or stack control to verify that the burner shuts off, within either 45 seconds or a time specified by the manufacturer, when the cad cell is blocked from seeing the flame.

✓     Measure the high limit shut-off temperature and adjust or replace the high limit control if the shut-off temperature is more than 200° F for furnaces, or 220° F for hot-water boilers.

Oil Burner Maintenance

After evaluating the oil burner’s operation, specify some or all of these maintenance tasks as necessary, to optimize safety and efficiency.

✓     Clean the burner’s blower wheel.

✓     Clean dust, dirt, and grease from the burner assembly.

✓     Replace oil filter(s) and nozzle.

✓     Clean or replace air filter.

✓     Remove soot from combustion chamber.

✓     Remove soot from heat exchange surfaces.

✓     Adjust gap between electrodes to manufacturer’s specs.

✓     Check if the nozzle and the fire ring of the flame-retention burner is appropriate for the size of the combustion cham­ber.

✓     Repair the ceramic combustion chamber, or replace it if necessary.

✓     Verify correct flame sensor operation.

After these maintenance procedures, the technician carries out the diagnostic tests described previously to evaluate improvement made by the maintenance procedures and to determine whether more adjustment or maintenance is required.

9.7   Inspecting Furnace Heat Exchangers

Leaks in heat exchangers are a common problem, causing the flue gases to mix with house air. Ask customers about respira­tory problems, flue-like symptoms, and smells in the house when the heat is on. Also, check around supply registers for signs of soot, especially with oil heating. All furnace heat exchangers should be inspected as part of weatherization. Con­sider using one or more of these six options for evaluating heat exchangers.

1.      Look for rust at exhaust ports and vent connectors.

2.      Look for flame-impingement on the heat exchanger during firing and flame-damaged areas near the burner flame.

3.      Observe flame movement, change in chimney draft, or change in CO measurement when blower is activated and deactivated.

4.      Measure the flue-gas oxygen concentration before the blower starts and then again just after the blower starts. There should be no more than a 1% change in the oxy­gen concentration.

5.      Examine the heat exchanger by shining a bright light on one side and looking for light on the other side using a mirror to look into tight locations.

6.      Employ chemical detection techniques, according to the manufacturer’s instructions.

See “NFPA Codes” on page 280.

9.8   Wood Stoves

Wood heating is a popular and effective auxiliary heating source for homes. However, wood stoves and fireplaces can cause indoor air pollution and fire hazards. Inspect wood stoves to evaluate potential hazards.

9.8.1   Wood Stove Clearances

Stoves that are listed by a testing agency like Underwriters Labo­ratory have installation instructions stating their clearance from combustibles. Unlisted stoves must adhere to clearances speci­fied in NFPA 211.

9.8.2   Stove Clearances

Look for metal tags on the wood stove that list minimum clear­ances. Listed wood stoves may be installed to as little as 6 inches away from combustibles, if they incorporate heat shields and combustion design that directs heat away from the stove’s back and sides.

Unlisted stoves must be at least 36 inches away from combusti­bles. Ventilated or insulated wall protectors may decrease unlisted clearance from one-third to two thirds, according to NFPA 211. Always follow the stove manufacturer’s or heat-shield manufacturer’s installation instructions.

Floor Construction and Clearances

The floor of a listed wood stove must comply with the specifica­tions on the listing (metal tag). Modern listed stoves usually sit on a 1-inch thick non-combustible floor protector that extends 18 inches beyond the stove in front.

The floor requirements for underneath a unlisted wood stove depends on the clearance between the stove and the floor, which depends on the length of its legs. Unlisted wood stoves must have floor protection underneath them unless they rest on a floor of non-combustible construction. An example of a non­combustible floor is one composed of only masonry material sitting on sand or gravel.

An approved floor protector is either one or two courses of hol­low masonry material (4 inches thick) with a non-combustible quarter-inch surface of steel or other non-combustible material on top of the masonry. This floor for a non-listed wood stove must extend no less than 18 inches beyond the stove in all direc­tions.

Vent-Connector and Chimney Clearance

Interior masonry chimneys require a 2-inch clearance from combustibles and exterior masonry chimneys require a 1-inch clearance from combustibles. All-fuel metal chimneys (insu­lated double-wall or triple wall) usually require a 2- inch clear­ance from combustibles.

Double-wall stove-pipe vent connectors require a 9-inch clear­ance from combustibles or a clearance listed on the product. Single wall vent connectors must be at least 18 inches from com­bustibles. Wall protectors may reduce this clearance up to two-thirds.

See also “Wood Stove Clearances” on page 329 and “Stove Clear­ances” on page 329.

9.8.3   Wood Stove Inspection

All components of wood stove venting systems should be approved for use with wood stoves. Chimney sections penetrat­ing floor, ceiling, or roof should have approved thimbles, sup­port packages, and ventilated shields to protect nearby combustible materials from high temperatures. Perform or specify the following inspection tasks.

✓     Inspect stove, vent connector, and chimney for correct clearances from combustible materials as listed on stoves and vent assemblies or as specified in NFPA 211.

✓     Each wood stove must have its own dedicated flue pipe. Two wood stoves may not share a single flue.

✓     If the home is tight (<0.35 ACH), the wood stove should be equipped with a dedicated outdoor combustion-air duct.

✓     Inspect vent connector and chimney for leaks. Leaks should be sealed with a high temperature sealant designed for sealing wood stove vents.

✓     Galvanized-steel pipe must not be used to vent wood stoves.

✓     Inspect chimney and vent connector for creosote build-up, and suggest chimney cleaning if creosote build-up exists.

✓     Inspect the house for soot on seldom-cleaned horizontal surfaces. If soot is present, inspect the wood stove door gasket. Seal stove air leaks or chimney air leaks with stove cement. Improve draft by extending the chimney to reduce indoor smoke emissions.

✓     Inspect stack damper and/or combustion air intake damper.

✓     Check catalytic converter for repair or replacement if the wood stove has one.

✓     Assure that heat exchange surfaces and flue passages within the wood stove are free of accumulations of soot or debris.

✓     Wood stoves installed in manufactured homes must be approved for use in manufactured homes.

9.9   Inspecting Venting Systems

Combustion gases are vented through vertical chimneys or other types of approved horizontal or vertical vent piping. Iden­tifying the type of existing venting material, verifying the cor­rect size of vent piping, and making sure the venting conforms to the applicable codes are important tasks in inspecting and repairing venting systems. Too large a vent often leads to con­densation and corrosion. Too small a vent can result in spillage. The wrong vent materials can corrode or deteriorate from heat.

See “NFPA Codes” on page 280.

9.9.1   Vent Connectors

A vent connector connects the appliance’s venting outlet collar with the chimney. Approved vent connectors for gas-fired units are made from the following materials.

•       Type-B vent, consisting of a galvanized steel outer pipe and aluminum inner pipe for gas-fired units.

•       Type-L vent connector with a stainless-steel inner pipe and a galvanized-steel outer pipe for oil-fired units.

•       Double-wall stove-pipe vent connector with a stainless-steel inner pipe and a black-steel outer pipe for solid-fuel units.

•       Galvanized steel pipe for gas or oil-fired units only: See table.

Double-wall vent connectors are the best option, especially for appliances with some non-vertical vent piping. A double-wall vent connector maintains flue gas temperature and prevents condensation. Gas appliances with draft hoods, installed in attics or crawl spaces must use a Type-B vent connector. Use Type-L double-wall vent pipe for oil vent connectors in attics and crawl spaces.

Vent-Connector Requirements

Verify that vent connectors comply with these specifications.

•       Vent connectors must be as large as the vent collar on the appliances they vent.

•       Single wall vent-pipe sections must be fastened together with 3 screws or rivets.

•       Vent connectors must be sealed tightly where they enter masonry chimneys.

•       Vent connectors must be free of rust, corrosion, and holes.

•       Maintain minimum clearances between vent connectors and combustibles.

•       The chimney combining two draft-hood vent connectors must have a cross-sectional area equal to the area of the larger vent connector plus half the area of the smaller vent connector. This common vent must be no larger than 7 times the area of the smallest vent connector. For specific vent sizes, see the NFPA codes listed on page 333. 

•       The horizontal length of vent connectors shouldn’t be more than 75% of the chimney’s vertical height or have more than 18 inches horizontal run per inch of vent diameter.

•       Vent connectors must have upward slope to their connec­tion with the chimney. NFPA 54 requires a slope of at least ¹/₄-inch of rise per foot of horizontal run so that combus­tion gases rise through the vent. The slope also prevents condensation from collecting in the vent and corroding it.

•       When two vent connectors connect to a single chimney, the vent connector servicing the smaller appliance must enter the chimney above the vent for the larger appliance.

9.10   Chimneys

There are two common types of vertical chimneys for venting combustion fuels that satisfy NFPA and ICC codes. First there are masonry chimneys lined with fire-clay tile, and second there are manufactured metal chimneys, including all-fuel metal chimneys, Type-B vent chimneys for gas appliances, and Type L chimneys for oil appliances.

See “NFPA Codes” on page 280.

9.10.1   Masonry Chimneys

Verify the following general specifications for building, inspect­ing, and repairing masonry chimneys.

•       A masonry foundation should support every masonry chimney.

•       Existing masonry chimneys should be lined with a fire clay flue liner. There should be a ¹/₂-inch to 1-inch air gap between the clay liner and the chimney’s masonry to insu­late the liner. The liner shouldn’t bond structurally to the outer masonry because the liner needs to expand and con­tract independently of the chimney’s masonry structure. The clay liner can be sealed to the chimney cap with a flex­ible high-temperature sealant.

•       Masonry chimneys should have a cleanout 12 inches or more below the lowest inlet. Clean mortar and brick dust out of the bottom of the chimney through the clean-out door, so that this debris won’t eventually interfere with venting.

•       Seal the chimney’s penetrations through floors and ceilings with sheet metal and high-temperature sealant as a fire-stop and air barrier.

•       Re-build deteriorated or unlined masonry chimneys as specified above or reline them as part of a heating-system replacement or a venting-safety upgrade. Or, install a new metal chimney instead of repairing the existing masonry chimney.

Metal Liners for Masonry Chimneys

Install or replace liners in unlined masonry chimneys or chim­neys with deteriorated liners as part of heating system replace­ment. Orphaned water heaters may also need a chimney liner because the existing chimney may be too large. Use a correctly sized Type-B vent, a flexible or rigid stainless-steel liner, or a flexible aluminum liner.

Flexible liners require careful installation to avoid a low spot at the bottom, where the liner turns a right angle to pass through the wall of the chimney. Comply with the manufacturer’s instructions, which usually require stretching the liner and fas­tening it securely at both ends, to prevent the liner from sagging and creating a low spot.

Flexible liners are easily damaged by falling masonry debris inside a deteriorating chimney. Use B-vent, L-vent, or single-wall stainless steel pipe instead of a flexible liner when the chim­ney is significantly deteriorated.

To minimize condensation, insulate the flexible liner — espe­cially when installed in exterior chimneys. Consider fiberglass-insulation jackets or perlite, if the manufacturer’s instructions allow. Wood-stove chimney liners must be stainless steel and insulated.

Sizing flexible chimney liners correctly is very important. Over­sizing is common and can lead to condensation and corrosion. The manufacturers of the liners include vent-sizing tables in their specifications. Liners should display a label from a testing lab like Underwriters Laboratories (UL).

Masonry chimneys as structural hazards: A building owner may want to consider reinforcing a deteriorated chimney by re-pointing masonry joints or parging the surface with reinforced plaster. Other options include demolishing the chimney or fill­ing it with concrete to prevent it from damaging the building by collapsing during an earthquake.

Solutions for Failed Chimneys

Sometimes a chimney is too deteriorated to be re-lined or repaired. In this case, abandon the old chimney, and install one of the following.

•       A double-wall horizontal sidewall vent, equipped with a barometric draft control and a power venter mounted on the exterior wall. Maintain a 4-foot clearance between the ground and the vent’s termination if you live where it snows.

•       A new heating unit, equipped with a power burner or draft inducer, that is designed for horizontal or vertical venting.

•       A new manufactured metal venting system.

9.10.2   Manufactured Chimneys

Manufactured metal chimneys have engineered parts that fit together in a prescribed way. Parts include: metal pipe, weight-supporting hardware, insulation shields, roof jacks, and chim­ney caps. One manufacturer’s chimney may not be compatible with another’s connective fittings.

All-fuel chimneys (also called Class A chimneys) are used pri­marily for the solid fuels: wood and coal. All-fuel metal chim­neys come in two types: insulated double-wall metal pipe and triple-wall metal pipe. Comply with the manufacturer’s specifi­cations when you install these chimneys.

Type-B vent double-wall pipe is permitted as a chimney for gas appliances. Type BW pipe is manufactured for gas space heaters in an oval shape to fit inside wall cavities.

Type L double-wall pipe is used for oil chimneys.

9.10.3   Chimney Terminations

Masonry chimneys and all-fuel metal chimneys should termi­nate at least three feet above the roof penetration and two feet above any obstacle within ten feet of the chimney outlet.

B-vent chimneys can terminate as close as one foot above flat roofs and above pitched roofs up to a ⁶/₁₂ roof pitch. As the pitch rises, the minimum required termination height, as mea­sured from the high part of the roof slope, rises as shown in this table.

9.10.4   Air Leakage through Masonry Chimneys

The existing fireplace damper or “airtight” doors seldom pro­vide a good air seal. Help the customer decide whether the fire­place will be used in the future or whether it can be taken out of service. Consider these solutions for chimneys with ineffective or missing dampers.

•       Install an inflatable chimney seal along with a notice of its installation to alert anyone wanting to start a fire to remove the seal first.

•       Install an operable chimney-top damper and leave instruc­tions on how to open and close it. Also notify users of which position is open and which is closed.

•       Air seal the chimney top from the roof with a watertight, airtight seal. Also seal the chimney from the living space with foam board and drywall. If you install a permanent chimney seal such as this, post a notice at the fireplace say­ing that it is permanently disabled.

9.11   Special Venting Considerations for Gas

The American Gas Association (AGA) publishes a classification system for venting systems attached to natural-gas and propane appliances. This classification system assigns Roman numerals to four categories of venting based on whether there is positive or negative pressure in the vent and whether condensation is likely to occur in the vent.

.

A majority of gas appliances found in homes and multifamily buildings are Category I, which have negative pressure in their vertical chimneys. We expect no condensation in the vent con­nector or chimney.

Condensing furnaces are usually Category IV, have positive pressure in their vent, and condensation occurring in both the appliance and vent. Category III vents are rare, however a few fan-assisted furnaces and boilers vent their flue gases through airtight non-condensing vents. Category II vents are very rare and beyond the scope of this discussion.

9.11.1   Venting Fan-Assisted Furnaces and Boilers

Newer gas-fired fan-assisted central furnaces and boilers elimi­nate dilution air and may have slightly cooler flue gases com­pared to their predecessors. The chimney may experience more condensation than in the past. Inspect the existing chimney to verify that it’s in good condition when considering replacing an old natural-draft unit. Reline the chimney when you see any of these conditions.

•       When the existing masonry chimney is unlined.

•       When the old clay or metal chimney liner is deteriorated.

•       When the new furnace has a smaller input (BTUH) than the old one, the liner should be sized to the new furnace and the existing water heater.

Liner Materials for 80+ Furnaces

For gas-fired 80+ AFUE furnaces, a chimney liner should con­sist of one of these four materials.

1.      A type-B vent

2.      A rigid or flexible stainless steel liner (preferably insu­lated)

3.      A poured masonry liner

4.      An insulated flexible aluminum liner

Chimney relining is expensive. Therefore consider a power-vented sealed-combustion unit when an existing chimney is inadequate for a new fan-assisted appliance.

9.11.2   Venting Sealed-Combustion Furnaces and Boilers

Some space heaters, furnaces, and boilers use factory-built metal chimneys with single stainless steel liners that vent hori­zontally under positive pressure.

Condensing furnaces usually employ horizontal or vertical plas­tic-pipe chimneys.

9.11.3   Sidewall Power Venting

Stainless-steel vents powered by fans in gas and oil appliances exit through walls and don’t require vertical chimneys.

9.12   Ducted Air Distribution

The forced-air system consists of an air handler (furnace, heat pump, air conditioner) with its heat exchanger along with attached ducts. The annual system efficiency of forced-air heat­ing and air-conditioning systems depends on the following issues.

•       Duct leakage

•       System airflow

•       Blower operation

•       Balance between supply and return air

•       Duct insulation levels

See “NFPA Codes” on page 280 and “ANSI/ACCA Manuals” on page 280.

9.12.1   Sequence of Operations

The evaluation and improvement of ducts has a logical sequence of steps.

✓     Solve the airflow problems because a contractor might have to replace ducts or install additional ducts.

✓     Determine whether the ducts are located inside the ther­mal boundary or outside it.

✓     Evaluate the ducts’ air leakage and decide whether duct-sealing is important and if so, find and seal the duct leaks.

✓     If supply ducts are outside the thermal boundary or if con­densation is an air-conditioning problem, insulate the ducts.

9.12.2   Solving Airflow Problems

You don’t need test instruments to discover dirty blowers or dis­connected branch ducts. Find these problems before measuring duct airflow, troubleshooting the ducts, or sealing the ducts. These steps precede airflow measurements.

1.      Ask the customer about comfort problems and tem­perature differences in different rooms of the home.

2.      Based on the customers comments, look for discon­nected ducts, restricted ducts, and other obvious prob­lems.

3.      Inspect the filter(s), blower, and indoor coil for dirt. Clean them if necessary. If the indoor coil isn’t easily visible, a dirty blower means that the coil is probably also dirty.

4.      Look for dirty or damaged supply and return grilles that restrict airflow. Clean and repair them.

5.      Look for closed registers or closed balancing dampers that could be restricting airflow to uncomfortable rooms.

6.      Notice moisture problems like mold and mildew. Mois­ture sources, like a wet crawl space, can overpower air conditioners by introducing more moisture into the air than the air conditioner can remove.

Measuring Total External Static Pressure (TESP)

The blower creates the duct pressure, which is measured in inches of water column (IWC) or pascals. The return static pres­sure is negative and the supply static pressure is positive. Total external static pressure (TESP) is the sum of the absolute values of the supply and return static pressures. Absolute value means that you ignore the positive or negative signs when adding sup­ply static pressure and return static pressure to get TESP. This addition represents the distance on a number line as shown in the illustration here.

TESP gives a rough indicator of whether airflow is adequate. The greater the TESP, the less the airflow. The supply and return static pressures by themselves can indicate whether the supply or the return or both sides are restricted. For example, if the supply static pressure is 0.10 IWC (25 pascals) and the return static pressure is –0.5 IWC (-125 pascals), you can assume that most of the airflow problems are due to a restricted or under­sized return. The TESP give a rough estimate of airflow if the manufacturer’s graph or table for static pressure versus airflow is available.

1.      Attach two static pressure probes to tubes leading to the two ports of the manometer. Attach the high-side port to the probe inserted downstream of the air handler in the supply duct. The other tube goes upstream of the air handler in the return duct. The manometer adds the supply and return static pressures to measure TESP.

2.      Consult manufacturer’s literature for a table of TESP versus airflow for the blower or the air handler. Find airflow for the TESP measured in Step 1.

3.      Measure pressure on each side of the air handler to obtain both supply and return static pressures sepa­rately. This test helps to locate the main problems as related to either the supply or the return.

Static Pressure Guidelines

Air handlers deliver their airflow at TESPs ranging from 0.30 IWC (75 Pascals) to 1.0 IWC (250 Pascals) as found in the field. Manufacturers maximum recommended static pressure is usu­ally a maximum 0.50 IWC (125 pascals) for standard air han­dlers. TESPs greater than 0.50 IWC indicate inadequate airflow in standard residential forced-air systems.

The popularity of pleated filters, electrostatic filters, and high-static high-efficiency evaporator coils, prompted manufacturers to introduce premium air handlers that can deliver adequate air­flow at a TESPs of greater than 0.50 IWC (125 pascals). Pre­mium residential air handlers can provide adequate airflow with TESPs of up to 0.90 IWC (225 pascals) because of their more powerful blowers and variable-speed blowers. TESPs greater than 0.90 IWC indicate the possibility of inadequate airflow in these premium residential forced-air systems.

9.12.3   Unbalanced Supply-Return Airflow Test 

Closing interior doors often separates supply registers from return registers in homes with central returns. A bedroom door with no return register and a closed door restricts the bedroom air from returning to the air handler. This restriction pressurizes bedrooms and depressurizes the central areas near return regis­ters. These pressures can drive air leakage through the building shell, create moisture problems, and bring pollutants in from the crawl space, garage, or CAZ.

The following test uses only the air handler and a digital manometer to evaluate whether the supply air can flow back to the return registers relatively unobstructed. Activate the air han­dler and close interior doors.

1.      Measure the pressure difference between the home’s central living area and the outdoors with a digital manometer.

2.      Measure the bedrooms’ pressure difference with refer­ence to outdoors.

If the sum of these two measurements is more than 3.0 pascals with the air handler operating, consider pressure relief.

•       Like TESP, disregard the positive or negative signs, and add the absolute values.

•       Or instead, you can measure the pressure difference between the central zone and the bedroom as shown in the next illustration.

To estimate the amount of pressure relief needed, slowly open the bedroom door until the pressure difference drops below 1 pascal.

Estimate the surface area of that door opening. This is the area of the permanent opening required to provide pressure relief. Pressure relief may include undercutting the door, installing transfer grilles, or installing jumper ducts.

9.12.4   Evaluating Furnace Performance

The effectiveness of a furnace depends on its temperature rise, fan-control temperatures, and flue-gas temperature. For effi­ciency, you want a low temperature rise. However, you must maintain a minimum flue-gas temperature to prevent corrosion in the venting of 70+ and 80+ AFUE furnaces. Apply the follow­ing furnace-operation standards to maximize the heating sys­tem’s seasonal efficiency and safety.

✓     Perform a combustion analysis as described in “Gas Burner Safety & Efficiency Service” on page 317.

✓     Check temperature rise after 5 minutes of operation. Refer to manufacturer’s nameplate for acceptable temperature rise (supply temperature minus return temperature). The temperature rise should be the minimum and maximum temperature rise on the nameplate (usually 40°F and 70°F). Prefer the lower end of this range for energy effi­ciency.

✓     With temperature-activated controls, verify that the fan-on temperature is 120–140° F. The lower the better.

✓     With time-activated fan controls, verify that the fan is switched on with the shortest time delay available if it is adjustable. The appliance should be switched off with the time delay that achieves a fan off temperature of 20° to 30° above the measured return-air temperature.

✓     Verify that the high limit controller shuts the burner off before the furnace temperature reaches 200°F.

✓     Verify that there is a strong noticeable airflow from all supply registers.

✓     Adjust fan control to conform to these standards, or replace the fan control if adjustment fails. Some fan con­trols aren’t adjustable.

✓     Adjust the high limit control to conform to the above stan­dards, or replace the high limit control.

✓     All forced-air heating systems must deliver supply air and collect return air only from inside the intentionally heated portion of the house. Taking return air from an un-heated area of the house such as an unconditioned basement or a crawl space isn’t acceptable.

9.12.5   Recommended Airflow for Air Handlers

The air handler’s recommended airflow depends on its heating or cooling capacity. For combustion furnaces, provide 11-to-15 cfm of airflow for each 1000 BTUH of output. Verify at least 2 square inches of cross-sectional area for each 1000 BTUH of furnace input in both the supply plenum and the return plenum in order to achieve this airflow.

Central air conditioners and heat pumps should deliver 400 cfm ±20% of airflow per ton of cooling capacity (one ton equals 12,000 BTUH). This airflow standard typically requires a duct system with at least 6 square inches of cross-sectional area of both supply plenum and return plenum, connected to the air handler, for each 1000 BTUH of air-conditioning or heat-pump capacity.

9.12.6   Improving Forced-Air System Airflow

Inadequate airflow is a common cause of comfort complaints. When the air handler is on there should be a strong flow of air out of each supply register. Low register airflow may mean that a branch duct is blocked or separated, or that return air from the room to the air handler isn’t sufficient. When low airflow is a problem, consider specifying the following improvements as appropriate from your inspection.

✓     Clean or change filter. Select a less restrictive filter if you need to reduce static pressure substantially.

✓     Clean air handler’s blower.

✓     Clean the air-conditioning coil or heat pump coil. If the blower is dirty, the coil is probably also dirty.

✓     On a condensing furnace, clean the secondary heat exchanger coil.

✓     Increase blower speed.

✓     Verify that balancing dampers to rooms that need more airflow are wide open.

✓     Lubricate the blower motor, and check tension on drive belt.

Duct Improvements to Increase Airflow

Consider specifying the following duct changes to increase sys­tem airflow and reduce the imbalance between supply airflow and return airflow.

✓     Modify the filter installation to allow easier filter chang­ing, if filter changing is currently difficult.

✓     Install a slanted filter bracket assembly or an enlarged filter fitting to accommodate a larger filter with more surface area and less static-pressure drop compared to the existing filter.

✓     Remove obstructions to registers and ducts such as rugs, furniture, and objects placed inside ducts (children’s toys and water pans for humidification, for example).

✓     Remove kinks from flex duct, and replace collapsed flex duct and fiberglass duct board.

✓     Remove excessive lengths of slacking flex duct, and stretch the duct to enhance airflow.

✓     Perform a Manual D sizing evaluation to evaluate whether to replace branch ducts that are too small.

✓     Install additional supply ducts and return ducts as needed to provide heated air throughout the building, especially in additions to the building.

✓     Undercut bedroom doors, especially in homes without return registers in bedrooms.

✓     Repair or replace bent, damaged, or restricted registers. Install low-resistance registers and grilles.

9.13   Evaluating Duct Air Leakage

Duct leakage is a major energy-waster in homes where the ducts are located outside the home’s thermal boundary in a crawl space, attic, attached garage, or leaky unconditioned basement. When these intermediate zones remain outside the thermal boundary, duct sealing is usually cost-effective.

Duct leakage within the thermal boundary isn’t usually a signifi­cant energy problem.

See “NFPA Codes” on page 280.

9.13.1   Troubleshooting Duct Leakage

There are several simple procedures for finding the locations of the duct leaks and evaluating their severity.

Finding Duct Leaks Using Touch and Sight

One of the simplest ways of finding duct leaks is feeling with your hand for air leaking out of supply ducts, while the ducts are pressurized by the air handler’s blower. Duct leaks can also be located using light. Use one of these 4 tests to locate air leaks.

1.      Use the air handler blower to pressurize supply ducts. Closing the dampers on supply registers temporarily or partially blocking the register with pieces of carpet, magazines, or any object that won’t be blown off by the register’s airflow increases the duct pressure and make duct leaks easier to find. Dampening your hand makes your hand more sensitive to airflow, helping you to find duct air leaks.

2.      Place a trouble light, with a 100-watt bulb, inside the duct through a register. Look for light emanating from the exterior of duct joints and seams.

3.      Determine which duct joints were difficult to fasten and seal during installation. These joints are likely duct-leakage locations.

4.      Use a trouble light, flashlight, and mirror or a digital camera to help you to visually examine duct interiors.

Feeling air leaks establishes their exact location. Ducts must be pressurized in order to feel leaks. You can feel air leaking out of pressurized ducts, but you can’t feel air leaking into depressur­ized return ducts. Pressurizing the home with a blower door forces air through duct leaks, located in intermediate zones, where you can feel the leakage coming out of both supply and return ducts.

Pressure Pan Testing

Pressure pan tests can identify leaky or disconnected ducts located in intermediate zones. With the house depressurized by the blower door to either –25 pascals or –50 pascals, make pres­sure pan readings at each supply and return register.

If the ducts are in a basement and the basement is conditioned, pressure pan testing isn’t necessary, although air sealing the return ducts for safety is still important.

If instead, the basement is unconditioned, close any openings between the basement and conditioned space. Measure and record the zone pressure of the basement with reference to the conditioned space before pressure pan testing.

1.      Install blower door and set-up house for winter condi­tions. Open all interior doors.

2.      If the basement is conditioned living space, open the door between the basement and upstairs living spaces.

3.      If the basement isn’t conditioned living space, close the door between basement and upstairs. Then measure and record the zone pressure of the basement with ref­erence to the conditioned space.

4.      Turn furnace off at the thermostat or main switch. Remove furnace filter, and temporarily tape filter slot if one exists. Be sure that all grilles, registers, and damp­ers are fully open.

5.      Temporarily seal any outside fresh-air intakes to the duct system.

6.      Seal supply registers in unoccupied zones that aren’t intended to be heated — an unconditioned basement or crawl space, for example.

7.      Open attics, crawl spaces, and garages as much as possi­ble to the outdoors so they don’t create a secondary air barrier.

8.      Connect hose between pressure pan and the input tap on the digital manometer. Leave the reference tap open.

9.      With the blower door’s manometer reading –25 or –50 pascals, place the pressure pan completely over each grille or register one by one to form a tight seal. Leave all other grilles and registers open when making a test. Record each reading, which should give a positive pres­sure.

10.  If a grille is too large or a supply register is difficult to cover with the pressure pan (under a kitchen cabinet, for example), seal the grille or register with masking tape. Insert a pressure probe through the masking tape and record the reading. Remove the tape after the test.

11.  Use either the pressure pan or tape to test each register and grille in a systematic way.

Pressure Pan Duct Standards

If the ducts are perfectly sealed with no leakage to the outdoors, you won’t measure any pressure difference (0.0 pascals) during a pressure-pan test. The higher the measured pressure reading, the more connected the duct is to the outdoors.

•       If the median pressure pan reading is 4 pascals or more and/or if one reading is more than 8 pascals, duct-sealing is usually cost-effective.

•       Following duct-sealing work, no more than three registers should have pressure-pan readings greater than 2 pascals. No single reading should be greater than 4 pascals.

•       The reduction you achieve depends on your ability to find the leaks and whether you can access the leaky ducts. The best weatherization agencies use 1 pascal or less as a gen­eral goal for all registers.

Examine the registers connected to ducts that are located in areas outside the conditioned living space. Unconditioned spaces containing ducts include attics, crawl spaces, garages, and unoccupied basements. Also evaluate registers attached to stud cavities or panned joists used as return ducts. Leaky ducts, located outside the conditioned living space, may lead to pres­sure-pan measurements more than 30 pascals if these ducts have large holes.

9.13.2   Measuring Duct Air Leakage with a Duct Blower

Pressurizing the ducts with a duct blower measures total duct leakage. The duct blower is the most accurate common measur­ing device for duct air leakage. It consists of a fan, a digital manometer or set of analog manometers, and a set of reducer plates for measuring different leakage levels. Using a blower door with a duct blower measures leakage to outdoors.

Measuring Total Duct Leakage

The total duct leakage test measures leakage to both indoors and outdoors. The house and intermediate zones should be open to the outdoors by way of windows, doors, or vents. Opening the intermediate zones to outdoors insures that the duct blower is measuring only the ducts’ airtightness — not the airtightness of ducts combined with other air barriers like roofs, foundation walls, or garages.

Supply and return ducts can be tested separately, either before the air handler is installed in a new home or when an air handler is removed during replacement.

Follow these steps when performing a duct airtightness test.

1.      Install the duct blower in the air handler or to a large return register, either using its connector duct or simply attaching the duct blower itself to the air handler or return register with cardboard and tape.

2.      Remove the air filter(s) from the duct system.

3.      Seal all supply and return registers with masking tape or other non-destructive sealant.

4.      Open the house, basement or crawl space, containing ducts, to outdoors.

5.      Drill a ¹/₄ or ⁵/₁₆-inch hole into a supply duct a short distance away from the air handler and insert a manometer hose. Connect a manometer to this hose to measure duct with reference to (WRT) outdoors. (Indoors, outdoors, and intermediate zones should ide­ally be opened to each other in this test).

6.      Connect an airflow manometer to measure fan WRT the area near the fan.

Check manometer(s) for proper settings. Digital manometers require your choosing the correct mode, range, and fan-type settings.

1.      Turn on the duct blower and pressurize the ducts to 25 pascals.

2.      Record duct-blower airflow.

3.      While the ducts are pressurized, start at the air handler and move outward feeling for leaks in the air handler, main ducts, and branches.

4.      After testing and associated air-sealing are complete, restore filter(s), remove seals from registers, and check air handler.

Measuring Duct Leakage to Outdoors

Measuring duct leakage to outdoors gives you a duct-air-leakage value that is directly related to energy waste and the potential for energy savings.

1.      Set up the home in its typical heating and cooling mode with windows and outside doors closed. Open all indoor conditioned areas to one another.

2.      Install a blower door, configured to pressurize the home.

3.      Connect the duct blower to the air handler or to a main return duct.

4.      Pressurize the ducts to +25 pascals by increasing the duct blower’s speed until this value is reached.

5.      Pressurize the house until the pressure difference between the house and duct is 0 pascals (house WRT ducts). See "Blower-Door Test Procedures" on page 554.

6.      Read the airflow through the duct blower. This value is duct leakage to outdoors.

9.13.3   Measuring House Pressure Caused by Duct Leakage

The following test measures pressure differences between the house and outdoors, caused by duct leakage. Try to correct pres­sure differences greater than +2.0 pascals or more negative than –2.0 pascals because of the shell air leakage that the pressure dif­ferences create.

1.      Close all windows and exterior doors. Turn-off all exhaust fans.

2.      Open all interior doors, including door to basement.

3.      Measure the baseline house-to-outdoors pressure dif­ference and zero it out using the baseline procedures described in “Blower-Door Test Procedures” on page 554.

4.      Turn on air handler.

5.      Measure the house-to-outdoors pressure difference. This test indicates dominant duct leakage as shown here.

A positive pressure indicates that the return ducts (which pull air from leaky intermediate zones) are leakier than the supply ducts. A negative pressure indicates that the supply ducts (which push air into intermediate zones through their leaks) are leakier than return ducts. A pressure at or near zero indicates equal supply and return leakage or else little duct leakage.

9.14   Sealing Duct Leaks

Ducts located outside the thermal boundary or in an intermedi­ate zone like a ventilated attic or crawl space should be sealed. The following is a list of duct leak locations in order of their rel­ative importance. Leaks nearer to the air handler are exposed to higher pressure and are more important than leaks further away.

See “NFPA Codes” on page 280.

9.14.1   Duct Repair and Sealing Methods

Duct Repair and Fastening

Before you air seal ducts, make necessary repairs using these general guidelines.

✓     Attach flex duct to metal duct or duct board with a rigid metal coupling, using two tensioned tie bands per joint.

✓     Fasten round ducts to round ducts or fittings with a mini­mum of three equally spaced galvanized or stainless steel fasteners.

✓     Fasten duct board to duct board, using overlapping joints, UL 181 fiber mesh tape or aluminum tape, mastic, stitch staples, or other approved products.

✓     Fasten duct boots to wood using a minimum of 1 stainless steel or galvanized fastener per side.

✓     Fasten duct boots to drywall with mesh tape or a duct-boot hanger, if the boot is accessible.

✓     Support flexible and duct board ducts and plenums with 1-¹/₂-inch wide or greater material, installed every 4 feet or less. Don’t pinch the duct or reduce its interior dimen­sions.

✓     Support metal ducts with ¹/₂-inch-wide or greater eigh­teen-gauge metal straps, 12-gauge galvanized wire, or metal rods every 10 feet or less.

General Duct-Sealing Methods

Duct sealers install duct mastic and fiberglass mesh to seal duct leaks. When they need reinforcement or temporary closure, the duct sealers use tape or sheet metal. Observe these standards.

✓     Remove any substance that would prevent sealant adhe­sion (tape, oil, dirt) using appropriate methods and sol­vents.

✓     Seal seams, cracks, joints, and holes, less than ¼ inch using mastic and fiberglass mesh.

✓     Bridge seams, cracks, joints, holes, and penetrations, between ¼ and ¾ inch, with sheet metal or tape. Then cover the metal or tape completely with mastic reinforced by mesh at seams.

✓     Repair leaks larger than ¾ inch using a rigid duct patch. Mechanically fasten patch before applying mastic. Install mesh and mastic over seams, overlapping repair joint by at least one inch on all sides

✓     Overlap the mastic and mesh at least one inch beyond the seams, repairs, and reinforced areas of the ducts.

9.14.2   Sealing Return Ducts

Return leaks are important for combustion safety and for effi­ciency. Use the following techniques to seal return ducts.

✓     First, seal all return leaks within the combustion zone to prevent this leakage from depressurizing the combustion zone and causing backdrafting.

✓     Seal all return ducts in crawl spaces for indoor air quality.

✓     Seal filter slots with a tight-fitting, durable, user-friendly filter-slot cover to allow easy removal for filter-changing.

✓     Seal the joint between the furnace and return plenum with a removable sealant such as foil tape.

Panned or Cavity Return Ducts

✓     Seal panned return ducts using mastic to seal all cracks and gaps within the return duct and register.

✓     Seal leaky joints between building materials composing cavity return ducts, like panned floor cavities and furnace return platforms. Remove the panning to seal cavities con­taining joints in building materials.

✓     Carefully examine and seal leaks at transitions between panned floor joists and metal trunks that change the direc­tion of the return ducts. You may need a mirror to find some of the biggest return duct leaks in these areas.

9.14.3   Sealing Supply Ducts

Inspect these places in the duct system and seal them as needed.

✓     Plenum joint at air handler: Technicians might have had problems sealing these joints because of a lack of space. Seal these plenum connections thoroughly even if you must cut an access hole in the plenum. Use silicone caulk­ing or foil tape instead of mastic and fabric mesh here for future access — furnace replacement, for example.

✓     Joints at branch takeoffs: Seal these important joints with a thick layer of mastic. Fabric mesh tape should cover gaps and reinforce the seal at gaps.

✓     Joints in sectioned elbows: Known as gores, these are usu­ally leaky and require sealing with duct mastic.

✓     Tabbed sleeves: Attach the sleeve to the main duct with 3-to-5 screws and apply mastic plentifully. Or better, remove the tabbed sleeve and replace it with a manufactured take­off.

✓     Flexduct-to-metal joints: Apply a 2-inch band of mastic to the end of the metal connector. Attach the flexduct’s inner liner with a UL-181-approved tie band, tightening it with a tie-band tensioner. Attach the insulation and outer liner with another tie band.

✓     Damaged flex duct: Replace flex duct when it is punctured, deteriorated, or otherwise damaged.

✓     Deteriorating ductboard facing: Replace ductboard, prefer­ably with metal ducting, when the facing deteriorates because this deterioration leads to a lot of air leakage.

✓     Consider closing supply and return registers in unoccu­pied basements or crawl spaces.

✓     Seal penetrations made by wires or pipes traveling through ducts.

✓     Seal the joint between the boot and the ceiling, wall, or floor between conditioned and unconditioned areas.

Duct Support

✓     Support rigid ducts and duct joints with duct hangers at least every 5 feet or as necessary to prevent sagging of more than one-half inch.

✓     Support flexible ducts and duct board every 4 feet using a minimum of 1 ½” wide support material.

9.14.4   Materials for Duct Sealing

Duct mastic is the best duct-sealing material because of its supe­rior durability and adhesion. Apply mastic at least ¹/₁₆-inch thick, and use reinforcing mesh for all joints wider than ¹/₈-inch or joints that may move. Install screws to prevent joint move­ment or separation.

Aluminum foil or cloth duct tape aren’t good materials for duct sealing because their adhesive often fails. Consider covering tape with mastic to prevent tape’s adhesive from drying out and failing.

9.15   Duct Insulation

Insulate supply ducts that are installed in unconditioned areas outside the thermal boundary such as crawl spaces, attics, and attached garages with vinyl- or foil-faced duct insulation. Don’t insulate ducts that run through conditioned areas unless they cause overheating in winter or condensation in summer. Use these best practices for installing insulation.

✓     Always perform necessary duct sealing before insulating ducts.

✓     Duct-insulation R-value must be ≥R-8 indoors and ≥R-12 outdoors.

✓     Select insulation with a flame spread and smoke developed index listed at 25/50.

✓     Insulation should cover all exposed supply ducts, with no significant areas of bare duct left uninsulated.

✓     Insulation’s compressed thickness must be more than 75% of its uncompressed thickness. Don’t compress duct insu­lation excessively at corner bends.

✓     Fasten insulation using mechanical means such as stick pins, twine, staples, or plastic tie bands.

✓     Cover the insulation’s joints with UL 181 tape to seal all gaps.

✓     Install the duct insulation 3 inches away from heat-pro­ducing devices such as recessed light fixtures.

✓     Post a dated receipt, signed by the installer, that includes: Installed insulation type, coverage area, installed thick­ness, and installed R-value.

Caution: Burying ducts in attic insulation is common in some regions and it reduces energy losses from ducts. However, con­densation on ducts in humid climates is common during the air-conditioning season, so don’t allow cellulose to touch metal ducts to avoid corrosion from cellulose’s Borate fire retardant.

Important Note: Tape can be effective for covering joints in the insulation to prevent air convection, but the tape fails when expected to resist the force of the insulation’s compression or weight. Outward-clinch staples or plastic tie bands can help hold the insulation facing and tape together.

9.15.1   Spray Foam (SPF) Duct Insulation

High-density spray foam insulation is also a good duct-insula­tion option, assuming it is listed as ASTM E-84 or UL 723. Spray foam is effective in areas where the foam can seal seams and insulate in one application. However, the spray foam application is limited by the available space around the duct to a greater amount than wrapping ducts with fiberglass blankets.

✓     Select foam insulation with a flame spread and smoke developed index listed at 25/450.

✓     Prepare surfaces to satisfy manufacturer’s specifications for cleanliness, moisture content, and temperature.

✓     Cover all holes, cracks, and gaps where SPF may enter the duct with a backing material, such as foil tape.

✓     Separate foam insulation from living spaces with a thermal barrier or ignition barrier as required by local codes.

✓     Post a dated receipt, signed by the installer, that includes: Installed insulation type, coverage area, installed thick­ness, and installed R-value.

9.16   Hot-Water Space-Heating Distribution

The most significant energy wasters in hot-water systems are poor steady-state efficiency, off-cycle flue losses stealing heat from the stored water, and boilers operating at a too-high water temperature. For information about boiler installation, see page 307. 

See “NFPA Codes” on page 280.

9.16.1   Boiler Efficiency and Maintenance

Monitor boiler performance and efficiency in the following ways.

•       Corrosion, scaling, and dirt on the water side of the heat exchanger.

•       Corrosion, dust, and dirt on the fire side of the heat exchanger.

•       Excess air during combustion from air leaks and incorrect fuel-air mixture.

•       Off-cycle air circulation through the firebox and heat exchanger, removing heat from stored water.

Boiler Efficiency Improvements

Consider the following maintenance and efficiency improve­ments for both hot-water and steam boilers based on boiler inspection.

✓     Check for leaks on the boiler, around its fittings, or on any of the distribution piping connected to the boiler.

✓     Clean fire side of heat exchanger of noticeable dirt.

✓     Drain water from the boiler drain until the water flows clean. Verify that the fill valve replaces the water you drained.

9.16.2   Distribution System Improvements

Hydronic distribution systems consist of the supply and return piping, the circulator, expansion tank, air separator, air vents, and heat emitters. A properly designed and installed hydronic distribution system can operate for decades without service. However, many systems have installation flaws or need service.

Note: You can recognize a hot-water boiler by its expansion tank, located somewhere above the boiler. The expansion tank provides an air cushion to allow the system’s water to expand and contract as it is heated and cooled. Without a functioning expansion tank excessive pressure in the boiler discharges water through the pressure-relief valve.

Safety Checks and Improvements

Work with contractors and technicians to specify and verify the following safety and efficiency tests and inspections.

✓     Verify the existence of a 30-psi-rated pressure relief valve. The pressure relief valve should have a drain pipe that ter­minates 6 inches from the floor. Replace a malfunctioning valve, or install a pressure relief valve if none exists. Look for signs of leakage or discharges. Find out why the pres­sure relief valve is discharging.

✓     Verify that the expansion tank isn’t waterlogged or isn’t too small for the system. A waterlogged expansion tank can make the pressure relief valve discharge. Measure the expansion tank’s air pressure. A common pressure for one and two-story buildings is 12 psi. The pressure in taller buildings should be approximately one (1) psi per 2.3 feet of the distribution system’s height.

✓     If you observe rust in venting, verify that the return water temperature is warmer than 130°F for gas and warmer than 140°F for oil. These minimum water temperatures prevent acidic condensation.

✓     Verify that high-limit control extinguishes the burner at a water temperature of 200°F or less.

✓     Lubricate circulator pump(s) if necessary.

Simple Efficiency Improvements

Do the following energy-efficiency improvements.

✓     Repair water leaks in the system.

✓     Remove corrosion, dust, and dirt on the fire side of the heat exchanger.

✓     Check for excess air during combustion from air leaks and incorrect fuel-air mixture.

✓     Bleed air from radiators and piping through air vents on piping or radiators. Most systems fill automatically through a shutoff and pressure-reducing valve connected to the building’s water supply.

✓     If there is a shutoff and no pressure-reducing valve, install one. Set the pressure-reducing valve to the hydronic-sys­tem pressure. Then check the system pressure at the expansion tank, and adjust the pressure as necessary.

✓     Vacuum and clean fins of fin-tube convectors if you notice dust and dirt there.

✓     Insulate all supply and return piping, passing through unheated areas, with pipe insulation, at least R-3, rated for temperatures up to 200° F and compliant with fire safety codes. Secure seams with a durable sealant or zip ties.

✓     Install radiator reflector (insulated or uninsulated) behind radiators. Maintain a continuous air space between the reflector and the radiator. Meet applicable fire code requirements.

Improvements to Boiler Controls

Consider these improvements to control systems for hot-water boilers.

✓     Install outdoor reset controllers to regulate water tempera­ture, depending on outdoor temperature.

✓     If possible, operate the boiler without a low-limit control for maintaining a minimum boiler-water temperature. If the boiler heats domestic water in addition to space heat­ing, the low-limit control may be necessary.

✓     After control improvements like two-stage thermostats or reset controllers, verify that return water temperature is high enough to prevent condensation and corrosion in the chimney as noted previously.

✓     Install electric vent dampers on natural-draft gas- and oil-fired high-mass boilers.

9.17   Steam Heating and Distribution

Steam heating is less efficient than hot-water heating because steam requires higher temperatures than hot water. For single-family homes, consider replacing a steam heating system with a hot-water or forced-air system. Multifamily buildings, especially multi-story buildings, may have little choice but to continue with steam because of the high cost of switching systems.

Operate steam systems at the lowest steam pressure that heats the building adequately. Two psi on the boiler-pressure gauge is a practical limit for many systems although some systems can operate at pressures down to a few ounces per square inch of pressure. Traps and air vents are crucial to operating at a low steam pressure.

Post on the equipment or in a conspicuous location, a list of all systems and components inspected, results, and services per­formed. Include service personnel name, contact information, and date of service.

Note: You can recognize a steam boiler by its sight glass, which indicates the boiler’s water level. Notice that the water doesn’t completely fill the boiler, but instead allows a space for the steam to form above the boiler’s water level.

See “NFPA Codes” on page 280.

9.17.1   Steam System Maintenance

Do these safety and maintenance tasks on steam systems.

✓     Verify that steam boilers have functioning high-pressure limits and low-water cut-off controls.

✓     Verify that flush valves on low-water cutoffs flush water but don’t leak water.

✓     Verify the function of the low-water cutoff by flushing the low-water cutoff with the burner operating. Combustion should stop when the water level in the boiler drops below the level of the float. If combustion continues, repair or replace the low-water cutoff.

✓     Consider flushing the boiler if you find dirt in the water from flushing the low-water cutoff.

✓     Clean heat exchangers and burners if you see significant dirt deposits.

✓     Suggest that the building manager begin a schedule of boiler blow-down, chemical analysis, and chemical treat­ment.

✓     Inspect: automatic fill valves, zone valves, condensate tanks, and air vents.

✓     Specify that technicians drain mud legs on return piping.

9.17.2   Steam System Energy Conservation

Specify the following efficiency checks and improvements for steam systems.

General Steam System Improvements

✓     Repair leaks on the steam supply piping or on condensate return piping.

✓     Consider a flame-retention burner and electric vent damper as retrofits for steam boilers.

✓     Insulate steam pipes with pipe insulation designed for steam, R-3 or greater, and compliant with fire safety codes. Secure seams with a durable sealant or zip ties. Post a dated receipt signed by the installer that includes: insula­tion type, coverage area, installed thickness, and installed R-value.

✓     Install radiator reflector (insulated or uninsulated) behind radiators. Maintain a continuous air space between the reflector and the radiator. Meet applicable fire code requirements.

One-Pipe Steam

✓     Verify that high-pressure limit control is set at or below 1 (one) psi or as low as acceptable in providing heat to the far ends of the building.

✓     Verify that steam reaches all radiators during every steam cycle. Steam need not fill radiators on every cycle. In mild weather, steam partly fills radiators before the boiler cycles off.

✓     Radiator air vents should be open to release air while the system is filling with steam, then closed when steam reaches the vents. Replace malfunctioning radiator air vents as necessary. However, don’t over-vent radiators because this can cause water hammer.

✓     Verify air vents function and that all steam radiators receive steam during every cycle. Unplug air vents or replace malfunctioning vents as necessary. Add vents to steam lines and radiators as needed to get steam to all the registers.

Two-Pipe Steam

✓     When you can gain access to all the system’s steam traps, repair leaking steam traps or replace them. All failed traps should be replaced at the same time to prevent new traps failing because of water hammer from steam leakage through neighboring failed traps. The only 100% reliable way to test a steam trap is to connect it to a test apparatus and see if it allows steam to pass. However if you have an accurate thermometer, the temperature on the radiator side of a functioning trap should be more than 215°F and the temperature on the return side of that trap should be less than 205°F when steam is in the radiator.

✓     When you can’t gain access to all the system’s steam traps at the same time, consider abandoning failed steam traps and installing radiator-inlet orifices in two-pipe steam radiators. The orifices limit steam flow to an amount that can condense within the radiator. Orifices can also reduce steam delivery to oversized radiators by 20% or a little more.

✓     Consider controlling radiators with thermostatic radiator valves (TRVs) except for radiators in the coolest rooms. TRVs can be used with systems equipped with either traps or orifices. For effective temperature control, the thermo­static element of the TRVs must be located in the path of air moving toward the radiator or convector. TRVs are available with sensors located remotely from the valve, which solves the problem of a valve located where the radiator heats a valve-mounted sensor, fooling it into clos­ing.

✓     Inspect return lines and condensate receiver for steam coming back to the boiler. Check radiator and main line traps.

✓     Check steam traps with a digital thermometer or listening device to detect any steam escaping from radiators through the condensate return. Replace leaking steam traps or their thermostatic elements.

✓     Consider installing remote sensing thermostats that vary cycle length according to outdoor temperature and include night-setback capability.

9.18   Replacing Thermostats

Installing Thermostats

✓     Verify the number of thermostat wires available to meet the needs of the replacement thermostat.

✓     Install thermostat where it accurately reflects the tempera­ture and humidity of the zone which it controls.

✓     Don’t install a thermostat in a place exposed to extreme temperatures, radiant heat sources, or drafts.

✓     Seal penetrations for control wire with a durable sealant.

✓     Provide occupants/owners with user's manual, warranty information, installation instructions, and installer contact information.

Programmable Thermostats

A programmable thermostat may be a big energy saver if the building’s occupants understand how to program it. However, a programmable thermostat won’t save any energy if occupants already control day and night temperatures effectively.

✓     Before you replace the existing thermostat, discuss the operation of programmable thermostats with occupants.

✓     If they can use a programmable thermostat effectively, then install one.

✓     Educate occupants on setting the programmable thermo­stat and leave a copy of manufacturer’s directions with them.

Note: Programmable thermostats may not work effectively with hydronic heating systems because of slow temperature change in those systems.

Heat Pump Thermostats

✓     Select a double-setback programmable thermostat that allows for full functionality of the installed HVAC system, including supplementary heat, emergency heat, fan only, and ventilation control.

✓     Connect supplementary heat to second-stage heating ter­minal in accordance with manufacturer specifications.

✓     Install outdoor temperature sensor, compatible with the thermostat, according to manufacturer specifications.

Many models of programmable thermostats have settings that you select from inside the thermostat. These settings include the heat-anticipator setting, which adjusts the cycle length of the heating or cooling system.

9.19   Electric Heat

Electric heaters are usually 100% efficient at converting the elec­tricity to heat in the room where they are located.

See “NFPA Codes” on page 280.

9.19.1   Electric Baseboard Heat

Electric baseboard heaters are zonal heaters controlled by ther­mostats within the zone they heat. Electric baseboard heat can help to minimize energy costs, if residents take advantage of the ability to heat by zones.

Baseboard heaters contain electric resistance heating elements encased in metal pipes. These are surrounded by aluminum fins to aid heat transfer. As air within the heater is heated, it rises into the room. This draws cooler air into the bottom of the heater.

•       Make sure that the baseboard heater sits at least an inch above the floor to facilitate good air convection.

•       Clean fins and remove dust and debris from around and under the baseboard heaters as often as necessary.

•       Avoid putting furniture directly against the heaters. To heat properly, there must be space for air convection.

The line-voltage thermostats used with baseboard heaters some­times don’t provide good comfort. This is because these thermo­stats allow the temperature in the room to vary by 2°F or more. Newer, more accurate thermostats are available. Programmable thermostats for electric baseboard heat use timers or a resident-activated button that raises the temperature for a time and then automatically returns to the setback temperature. Some base­board heaters use low-voltage thermostats connected to relays that control baseboard heaters in rooms.

9.19.2   Electric Furnaces

Electric furnaces heat air moved by its fan over several electric-resistance heating elements. Electric furnaces have two to six elements — 3.5 to 7 kW each — that work like the elements in a toaster. The 24-volt thermostat circuit energizes devices called sequencers that bring the 240 volt heating elements on in stages when the thermostat calls for heat. The variable speed fan switches to a higher speed as more elements engage to keep the air temperature stable.

9.19.3   Central Heat-Pump Energy Efficiency

An air-source heat pump is almost identical to an air condi­tioner, except for a reversing valve that allows refrigerant to fol­low two different paths, one for heating and one for cooling. Heat pumps move heat with refrigeration rather than converting it from the chemical energy of a fuel.

Like air conditioners, air-source heat pumps are available as centralized units with ducts or as room units. Heat pumps are 1.5 to 3 times more efficient than electric furnaces. Heat pumps can provide competitive comfort and value with combustion furnaces, but they must be installed with great care and plan­ning.

Heat pumps are also equipped with auxiliary electric resistance heat, called strip heat. The energy efficiency of a heat pump depends on how much of the heating load the compressor pro­vides without using the strip heat.

Evaluating Heat Pumps During the Heating Season

Heat pumps should have two-stage thermostats designed for use with heat pumps. The first stage is compressor heating and the second stage is the inefficient strip heat. Evaluating heat pumps in the winter is more difficult than a summer evaluation.

Although we can generally evaluate the heat pumps refrigerant charge in the winter, it may be necessary to return in warm weather to more accurately charge the system. This summer ver­ification is required with new heat-pump installations.

Consider these steps to evaluate heat pumps during the winter.

✓     Measure the airflow of the air handler by temperature rise method, flow plate or flow hood. Heat pumps must have 400-450 CFM per ton.

✓     Look for a temperature rise of 20°F when the outdoor tem­perature is 32°F. Add or subtract 1° of temperature rise for every 3° it is over or under 32°F outdoor.

✓     Check for operation of strip heat by measuring amperage. Then use the chart shown here to find out if strip heat is operating.

✓     External static pressure should be 0.5 IWC (125 pascals) or less for older, fixed-speed blowers and less than 0.8 IWC (200 pascals) for variable-speed blowers. Lower external static pressure promotes higher airflow.

✓     Seal supply and return ducts and insulate them after you’ve verified the airflow as adequate. Measure airflow again after duct sealing.

Most residential central heat pumps are split systems with the indoor coil and air handler indoors and outdoor coil and com­pressor outdoors. Individual room heat pumps are more effi­cient because they don’t have ducts, and are factory-charged with refrigerant. The illustrations show features of an energy-efficient heat pump installation.

In the summer, use the same procedures to evaluate central heat pumps as to evaluate central air conditioners, described on page 400.

The illustration shows features of an energy-efficient heat pump installation.

9.19.4   Room or Unitary Heat Pumps

Room heat pumps can provide all or part of the heating and cooling needs for small homes. These one-piece room systems (also known as terminal systems) look like a room air condi­tioner, but provide both heating and cooling. They can also pro­vide ventilation air when neither heating nor cooling are required. They mount in a window or through a framed open­ing in a wall.

Room (or unitary) heat pumps can be a good choice for replac­ing existing unvented gas space heaters. Their fuel costs may be somewhat higher than gas furnaces. However, they are safer a than combustion appliances.

Room heat pumps are more efficient than ducted units because they heat a single zone and don’t have duct losses. If they replace electric resistance heat, they consume only one-half to one-third the electricity to produce the same amount of heat.

Selection

Room heat pumps draw a substantial electrical load, and may require 240-volt wiring.

✓     Select an ENERGY STAR® qualified model with Energy-Saver Mode or better.

✓     Size the new unit according to Manual J, assuming design temperatures or 75 degrees for cooling and 70 degrees for heating.

✓     Select a room heat pump, thats input matches available voltage and doesn’t exceed the dedicated circuit’s ampacity.

Installation

✓     Before installing through-the-wall heat pump, seal all adjacent framing and provide a sealed opening.

✓     Pipe condensate away from the building or to a sanitary drain.

✓     Insulate condensate drain to a minimum of R-3 if the pos­sibility of freezing or condensation exists.

✓     Remove old room heat pump or air conditioner from job site and recycle or dispose of it, according to local and fed­eral law.

✓     Provide occupants with a user's manual, warranty infor­mation, installation instructions, and installer contact information

Caution: Don’t operate room heat pumps with extension cords or plug adapters.

✓     Insufficient wiring capacity can result in dangerous over­heating, tripped circuit breakers, blown fuses, or motor-damaging voltage drops.

9.19.5   Ductless Minisplit Heat Pumps

Ductless minisplit heat pumps contain an outdoor condenser and one or more indoor fan-coil units that heat or cool the rooms. Mini-split heat pumps are among the most efficient heating and cooling systems available, providing 2-to-4 watt hours of heating or cooling for each watt hour of electricity they use.

Select a system that is ENERGY STAR® certified or equivalent.

Specify minisplits heat pumps as replacement HVAC solutions when they are appropriate, for example.

•       Homes currently having no ducts.

•       Homes with poorly designed or deteriorating ducts outside the thermal boundary or located in inaccessible areas, such as floor cavities.

•       Isolated part of a building such as an addition or a bonus room.

•       Very well-insulated, airtight, and shaded homes.

•       Bedrooms needing cooling in homes with no central air conditioning.

•       Masonry buildings being retrofitted to replace obsolete central space-conditioning systems (often steam).

9.20   Evaluating Ducted Central Air-Conditioning Systems

An energy-efficient home shouldn’t need more than a ton of air-conditioning capacity for every 1000 square feet of floor space. Evaluate window shading, attic insulation, and airtightness together with air-conditioner performance.

The following four installation-related problems are characteris­tic of central air conditioning systems.

1.      Inadequate airflow

2.      Duct air leakage

3.      Incorrect charge

4.      Oversizing

Refrigerant-charge tests and adjustment come after airflow mea­surement and improvement, and after duct testing and sealing. Manufacturers recommend that you verify adequate airflow before checking and adjusting the refrigerant charge.

The recommended airflow rate for central air-conditioning sys­tems is between 350 CFM and 450 CFM per ton of refrigeration capacity. Heat pump airflow rate should be between 400 CFM and 450 CFM per ton.

9.20.1   Central Air-Conditioner Inspection

Verify proper function and safety of the following system ele­ments: fan motor, compressor, outdoor temperature sensors, bearings, safety devices, electrical disconnect, electrical wiring, contactors, capacitors, fan blades, refrigerant access ports.

On the equipment or in a conspicuous location, post a list of all systems and components inspected and services performed. Include in this list: service technician’s name, contact informa­tion, and service date.

Cleaning the Air Handler

Air conditioners move a lot of air, and that air contains dust. The filter in the air handler catches most large dust. However some dust travels around or through the filter, depending on the type of filter and its mounting assembly.

✓     Check the filter for dirt and replace it if dirty.

✓     Check the filter-mounting hardware for a close fit to avoid dirt moving around the filter and on to the blower and heat exchanger. Repair if necessary.

✓     Consider providing a supply of filters for occupants to change.

✓     Inspect the blower and clean it if dirty.

✓     Clean the blower compartment.

See also "Ducted Air Distribution" on page 348.

Cleaning the Condenser Coil

The condenser coil outdoors isn’t protected by a filter and is usually quite dirty.The goal of this procedure is to drive the dirt out by spraying inside to outside. With high-pressure water, however, you can drive the dirt through the coil and into the cabinet where it drains out through drain holes.

✓     Clear foliage, grass, and other debris from within 3 feet of the unit.

✓     Inspect the condenser coil and know that it is probably dirty even if it looks clean on the outside. Take a flat tooth­pick and scrape between the fins. Can you scrape dirt out from between the fins?

✓     Apply a biodegradable coil cleaner to the outside of the coil. Then spray cold water through the coil, preferably from inside the cabinet. Many coils can tolerate a high-pressure spray but others require low-pressure spray to avoid bending the fins.

✓     Lubricate the blower motor.

✓     Straighten bent fins with a fin comb.

Cleaning the Evaporator Coil

Dirt enters the filter, blower, and coil from the return plenum.

✓     Inspect the filter slot in the air handler or the filter grille in the return air registers. Do the filters completely fill their opening? Are the filters dirty?

✓     Inspect the blower in the air handler after disconnecting power to the unit. Can you remove significant dirt from one of the blades with your finger? If the blower is dirty, then the evaporator coil is also dirty.

✓     Clean the blower and evaporator. Rake surface dirt and dust off the coil with a brush. Then use an indoor coil cleaner and water to clean between the fins.

✓     Straighten bent fins with a fin comb.

9.20.2   Air-Conditioner Sizing

Calculate the correct size of an air conditioner before purchas­ing or installing it. The number of square feet of floor space that can be cooled by one ton of refrigeration capacity is determined by the home’s energy efficiency. Air-conditioners provide cool­ing most cost-effectively when they are sized accurately and run for long cycles.

The cooling-cost reduction strategy should focus on making the home more energy efficient and making the air conditioner work more efficiently. Making the home more efficient involves shading, insulation, and air-leakage reduction. Making the air conditioner more efficient involves duct sealing, duct insulation, and a quality installation.

9.20.3   Duct Leakage and System Airflow

Unfortunately, duct leakage and poor airflow afflict most air-conditioning systems. The testing and mitigation of these prob­lems was covered earlier in this chapter.

1.      See “Evaluating Duct Air Leakage” on page 362.

2.      See “Ducted Air Distribution” on page 348.

9.20.4   Evaluating Air-Conditioner Charge

Air-conditioning replacement or service includes refrigerant charge-checking. The efficiency of the air-conditioning system is directly related to the amount of refrigerant. HVAC techni­cians evaluate refrigerant charge by two methods depending on what type of expansion valve the air conditioner has.

1.      If the expansion valve has a fixed orifice, perform a superheat test.

2.      If the valve is a thermostatic expansion valve (TXV), perform a subcooling test.

Checking and Correcting Charge

Superheat and subcooling tests indicate whether the amount of refrigerant in the system is correct, or whether there is too much or too little refrigerant. In the refrigerant is low, test for refriger­ant leaks.

Perform charge-checking after you complete the airflow tests, airflow adjustments, and duct-sealing. Do charge-checking while the air conditioner operates during the cooling season.

✓     In the refrigerant is low, test for refrigerant leaks.

✓     Verify that indoor and outdoor temperatures are in the allowable testing range when you test.

✓     Add or remove refrigerant as necessary.

✓     Weigh in calculated refrigerant charge if outdoor condi­tions prevent accurate charge-checking according to man­ufacturer’s refrigerant-weight specifications.

✓     Document your charge-checking and charge-correction and post the document on or near the equipment.

9.21   SWS Alignment