Chapter 8:                      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.      Heat pump 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. No weatherization work may be done until a non-operational primary heating unit is repaired or replaced. Fuel must be available (except for solid fuel units) to begin the inspection process. If inspecting a mobile home, ensure the heating unit is approved for use in mobile homes. The inspection may not pro­ceed if the heating unit is not approved for mobile home use.

8.1   HVAC-System Commissioning & Education

HVAC commissioning is the process of inspecting, testing, a system and educating occupants, landlords, and building opera­tors to achieve the following goals.

8.1.1   HVAC-System Commissioning

✓     Verify that the HVAC system works as the manufacturer, designer, and installer understand that it should work, based on plans, specifications, and manufacturers’ litera­ture.

✓     Take appropriate measurements to verify that the HVAC system works safely and efficiently.

✓     Verify that the building owner or building operator under­stands the HVAC system’s operation and has the necessary system documentation.

✓     Verify that the building owner or building operator under­stand the procedures and schedule for routine mainte­nance.

There are three (3) types of commissioning.

1.      Retro-commissioning, is commissioning implemented on existing HVAC equipment in an existing building.

2.      Initial commissioning occurs during installation of a new HVAC system.

3.      Re-commissioning is commissioning HVAC systems, that were already commissioned during original HVAC-system installation.

This chapter strives to provide the essential information for commissioning HCVAC systems. However, this information isn’t a substitute for plans, specifications, and manufacturers’ lit­erature that should guide all HVAC installations. Searching for the HVAC system’s documentation is an essential first step in retro-commissioning or recommissioning.

8.1.2   HVAC-System Education

Homes and multifamily buildings are complex systems of build­ing envelopes and mechanical systems that harbor a variety of hazards. Educate occupants, landlords, and building operators about the health and safety hazards and the improvements that you make to mitigate these hazards.

✓     Explain equipment operation and maintenance (O&M).

✓     Provide a O&M procedures manuals and manufacturers’ equipment specifications. Encourage occupants or staff to store important documents in a safe and obvious location.

✓     Instruct occupants or staff to remove combustible materi­als from near ignition sources.

✓     Inform occupants and staff about smoke alarms, carbon monoxide (CO) alarms, and combination alarms, and explain their functioning.

✓     For complex mechanical systems in multifamily buildings, provide signs to inform occupants and building operators about operations, maintenance, and emergency proce­dures.

8.2   Combustion-Safety Evaluation

Evaluate the combustion safety prior to beginning weatheriza­tion work and at the completion of the job.

8.2.1   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.

✓     CO detection or warning equipment will be installed in accordance with ASHRAE 62.2-2016.

8.2.2   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. Remember that natural gas rises from a leak and propane falls, so position the sensor accordingly. Do the following when leak-testing.

✓     Sniff all valves and joints with the gas sniffer.

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

✓     Repair all gas leaks.

✓     Replace copper, brass or kinked 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. If a date ring isn’t present, and you believe the gas line predates 1973, then replace the flexible gas line.

✓     If a gas leak is detected and verified with a commercially available leak detection solution, inform the occupant and have the problem corrected. No weatherization work may be done until major gas leaks are corrected.

8.2.3   Carbon Monoxide (CO) Testing

CO testing is essential for evaluating the safety of combustion and venting. Measure CO in the vent 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.

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 Building Performance Institute (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. See Carbon Monoxide Limits

•       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 requires the monitoring of CO levels 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 of greater than 9 ppm to find their cause. See Causes of Carbon Monoxide (CO)

8.2.4   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 depressurization testing uses the home’s exhaust fans, air handler, and chimneys to create worst-case depressur­ization in the CAZ. During this worst-case testing, you measure the CAZ pressure difference with reference to (WRT) 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 combustion-zone depressurization.

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

Analyzing CAZ Depressurization

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

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 we’re trying to avoid spillage, 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.

8.2.5   Mitigating CAZ Depressurization and Spillage

If you find problems with CAZ depressurization or spillage, consider the following improvements 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 duct improvements may solve spillage problems detected during the previous tests on a forced air heating sys­tem.

•       Seal all return-duct leaks near the furnace.

•       Isolate the CAZ from return registers and exhaust fans by air-sealing the CAZ from the depressurizing zones and providing combustion air to the sealed CAZ.

•       Reduce depressurization from the exhaust appliances.

•       Return air duct work must always be connected to the fur­nace.

These two suggestions may reduce depressurization caused by the home’s exhaust appliances.

1.      Isolate combustion appliances from exhaust fans and clothes dryers by air sealing between the CAZ and zones containing these exhaust devices as described on Zone Isolation for Atmospherically Vented Appliances.

2.      Provide make-up air for dryers and exhaust fans and/or provide combustion air inlet(s) to the CAZ. See Combustion Air.

Chimney Improvements to Mitigate Spillage Problems

Use 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 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 corroded, install a new chimney liner.

•       Increase the pitch of horizontal sections of vent.

8.2.6   Zone Isolation for Atmospherically Vented Appliances

An isolated CAZ improves the safety of atmospherically vented appliances. The CAZ is isolated if it receives combustion air only from outdoors. Perform worst case draft and spillage test and inspect the CAZ for connections with the home’s main zone and make sure it is isolated. Do the following.

1.      Look for connections between the isolated CAZ and the home. Examples include joist spaces, transfer grilles, leaky doors, and holes for ducts or pipes.

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

3.      Perform 50-pascal blower door depressurization test with the combustion appliances disabled. The CAZ-to-outdoors pressure should not change more than 5 pas­cals during the blower door test.

4.      If the CAZ-to-outdoors pressure changed more than 5 pascals, air-seal the zone, and retest as described in steps 2 and 3.

5.      If you can’t air-seal the CAZ adequately to isolate the zone, solve worst-case depressurization and spillage problems as described in Mitigating CAZ Depressurization and Spillage.

8.3   Electronic Combustion Analysis

The goal of a combustion analysis is to quickly determine com­bustion safety and efficiency. When the combustion heater reaches steady-state efficiency (SSE), you can measure its most critical combustion parameters. This information helps deter­mine required service and installation adjustments.

Modern flue-gas analyzers measure O₂, CO, and flue-gas tem­perature. Some models also measure draft. Flue-gas analyzers also calculate combustion efficiency or steady-state efficiency (SSE), which are synonymous. See SSE Testing Locations for test locations on common furnace types.

8.3.1   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 a normal flue gas and air measurement (as measured) or a corrected measurement that calculates the concentration in theoretical air-free flue gases. Adjusting combustion to produce less than 200 ppm as measured or 400 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.

8.4   Heating System Replacement

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

8.4.1   Combustion Furnace Replacement

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

Preparation

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

✓     Disconnect and remove the furnace or heat pump, 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.

Air-Handler Installation

✓     Install MERV 6 or higher filter in the new furnace.

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

✓     The filter must be easy to replace.

✓     The filter must provide complete coverage of blower intake or return register. 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

✓     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 a non-combustible mate­rial 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.

8.4.2   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.

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. Follow these recommendations when installing gas-fired heat­ing systems.

•       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.

•       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

•       Check clearances of the heating unit and its vent connector to nearby combustibles, according to NFPA 54. See Vent Connectors.

•       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 Carbon Monoxide (CO) Testing and Gas Burner Safety & Efficiency Service.

✓     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.

8.4.3   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.

Design

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

✓     Determine the correct size of the boiler, using ACCA Manual J and considering the installed radiation surface connected to the boiler.

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

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

✓     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. See page 322. 

✓     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 Distribution System Improvements

✓     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

b.  Minimum Oil Burner Combustion Standards

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. Steam boilers should have a low-water cut-off to shut the gas off if the water level gets too low. See Steam Heating and Distribution

8.4.4   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.

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

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

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

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.

8.4.5   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 also NFPA 31 Chapter 7 Fuel Oil Tanks.

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.

8.5   Combustion Space Heater Replacement

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

Weatherization agencies replace space heaters as an energy-con­servation measure or for health and safety reasons. Inspect existing space heaters for health and safety problems. When replacing a space heater, choose a sealed-combustion unit if possible. Use the following guidelines when replacing or install­ing a space heater.

✓     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 Table 8-3:   . or PMI

✓     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.

8.5.1   Space Heater Operation

Inform the customer of the following operating instructions.

✓     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 can cause corrosion to the space heater’s heat exchanger.

8.5.2   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.

In dwellings where an unvented, secondary heating unit is pres­ent, inform the occupant of the potential health hazards of oper­ating an unvented appliance in the post-weatherized dwelling. Document in the client file.

DOE forbids unvented space heaters as primary heating units in weatherized homes. However, unvented space heaters may be used as secondary heaters, under these five 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.      The unit is being operated in compliance with ANSI Z21.11.2

6.      Home must have adequate ventilation: See ASHRAE Standard 62.2–2016 Ventilation

8.6   Gas Burner Safety & Efficiency Service

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

8.6.1   Combustion Efficiency Test for Furnaces

Perform the following procedures at steady-state efficiency (SSE) 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.

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

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

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

Know the airflow through the furnace from measurements described in Ducted Air Distribution.

8.6.2   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 a shut-off switch 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 Inspecting Venting Systems.

✓     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.

8.6.3   Testing and Adjustment

The goal of these measures is to test CO, draft, and spillage, and to verify the operation of safety controls. Do the following when testing and adjusting combustion appliances. This includes gas furnaces, boilers, water heaters and space heaters.

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

✓     Test for spillage and measure draft. Take action to improve the draft if it is inadequate because of improper venting, obstructed chimney, leaky chimney, or depressurization. See Mitigating CAZ Depressurization and Spillage.

✓     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 Combustion Air.

✓     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 Inspecting Venting Systems.

•       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.

8.7   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 adjustment and mainte­nance. Use test equipment as discussed to measure other essen­tial operating parameters and to make adjustments as necessary.

8.7.1   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. Use the fol­lowing procedures when testing an oil-fired appliance.

✓     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 greater than 1, 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 per PMI at a test plug in the heating unit.

✓     Adjust the air shutter to achieve the oxygen and smoke values, specified in Table Table 8-5:   .

✓     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.

8.7.2   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 Minimum Oil Burner Combustion Standards  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

✓     Verify that each oil furnace or boiler has a dedicated elec­trical circuit. Assure that all 120-volt wiring connections are enclosed in covered electrical boxes.

✓     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 PMI for hot-water boilers.

Oil Burner Maintenance

After evaluating the oil burner’s operation, some or all of these maintenance tasks may be 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.

8.8   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, flu-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 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.

8.9   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.

8.9.1   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.

8.9.2   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 and Stove Clearances.

8.9.3   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. Have the chimney cleaned if it contains creosote build-up, soot, scale or other debris.

✓     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.

8.10   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.

If the chimney is the primary vent stack for the dwelling and not in sound condition, it must be repaired or replaced with an approved chimney liner or double-walled, metal vent material as specified by NFPA codes.

National Fire Protection Association Codes

The National Fire Protection Association (NFPA) publishes authoritative information on material-choice, sizing, and clear­ances for chimneys and vent connectors, as well as for combus­tion air. The information in this venting section is based on the following NFPA documents.

•       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

8.10.1   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 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 seven times the area of the smallest vent. For specific vent sizes, see the NFPA codes listed on National Fire Protection Association Codes.

•       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 should enter the chimney above the vent for the larger appliance.

8.11   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.

8.11.1   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 fireclay 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 be bonded 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. Installed liners must 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 collapsing during an earthquake and damaging the building.

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 90+ sealed-combustion furnace with combustion air through a dedicated pipe from outdoors.

8.11.2   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 connecting 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.

8.11.3   Chimney Terminations

Masonry chimneys and all-fuel metal chimneys should termi­nate at least 3 feet above the roof penetration and 2 feet above any obstacle within 10 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.

8.11.4   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.

8.12   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.

8.12.1   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 atmospheric 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.

8.13   Combustion Air

A combustion appliance zone (CAZ) is classified as either a space having adequate combustion air, or a space lacking ade­quate combustion air. A CAZ having adequate combustion air meets the NFPA-required amount of room volume that is assumed to provide enough combustion air. A confined space is a CAZ with less than the NFPA-required amount of volume.

A CAZ lacking adequate combustion air is defined by NFPA 54 as a room containing one or more combustion appliances that has less than 50 cubic feet of volume for every 1000 BTUH of appliance input.

For a CAZ lacking adequate combustion air, the NFPA 54 requires additional combustion air from outside the CAZ. Com­bustion air is supplied to the combustion appliance in three ways.

1.      To a CAZ lacking adequate combustion air through an intentional opening or openings between the CAZ and other indoor areas where air leaks replenish combus­tion air.

2.      To a CAZ lacking adequate combustion air through an intentional opening or openings between the CAZ and outdoors or ventilated intermediate zones like attics and crawl spaces.

3.      Directly from the outdoors to the appliance. Appliances with direct combustion air supply are called sealed-combustion or direct vent appliances.

8.13.1   CAZ Lacking Adequate Combustion Air

A confined space is defined by NFPA 54 as a room containing one or more combustion appliances that has less than 50 cubic feet of volume for every 1000 BTUH of appliance input.

When the home is relatively airtight (<0.40 ACHn), the CAZ may not have adequate combustion air, even when the combus­tion zone is larger than the minimum confined-space room vol­ume.

Combustion Air from Outdoors

In a CAZ that lacks adequate combustion air, or airtight homes where combustion appliances need outdoor combustion air, the NFPA provides several options to provide adequate combustion air.

For the intake, choose an outdoor location that is sheltered from prevailing winds, but not in an inside corner. Don’t choose an exterior wall that is parallel to prevailing winds. Wind blowing parallel to the exterior wall or at a right angle to the vent open­ing de-pressurizes both the vent and the CAZ connected to it.

Net free area is smaller than actual vent area and takes the blocking effect of louvers and grilles into account. Metal grilles and louvers provide 75% of their area as net-free area while wood louvers provide only 25%. Combustion-air vents should be no less than 3 inches in their smallest dimension.

Example Combustion Air Calculation

Here is an example of one indoor space providing combustion air to another indoor space. The furnace and water heater are located in a CAZ that lacks adequate combustion air. The fur­nace has an input rating of 100,000 BTUH. The water heater has an input rating of 40,000 BTUH. Therefore, there should be 140 in² of net free area of vent between the mechanical room and other rooms in the home.

([100,000 + 40,000] ÷ 1,000) = 140 x 1 in² = 140 in²

Each vent should therefore have a minimum of 140 in² net free area. If a metal grille covers 60% of the opening’s area, divide the 140 in² by 0.60.

140 in² / 0.6 = 233 in²

8.14   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

8.14.1   Sequence of Operations

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

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

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

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

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

8.14.2   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 gives 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.

8.14.3   Airflow Test 

Closing interior doors often separates supply registers from return registers in homes with central returns. A bedroom 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 each rooms’ pressure difference with reference to the central living area.

3.      No room will exceed +/- 4 pascals with reference to the main body of the home.

To estimate the amount of pressure relief needed, slowly open the bedroom door until the pressure difference drops below 4 pascals.

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 or installing transfer grilles or jumper ducts.

8.14.4   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.

✓     Check temperature rise after 5 minutes of operation or steady state efficiency. Refer to manufacturer’s nameplate for acceptable temperature rise (supply temperature minus return temperature). The temperature rise should be between 40°F and 70°F or PMI with the lower end of this scale being preferable for energy efficiency.

✓     Verify that the fan-off temperature is between 95° and 105° F, or PMI. The lower end of this scale is preferable for maximum efficiency.

✓     Verify that the fan-on temperature is 120–140° F, or PMI. The lower the better.

✓     With time-activated fan controls, verify that the fan is switched on within two minutes of burner ignition and is switched off within 2.5 minutes of the end of the combus­tion cycle.

✓     Verify that the high limit controller shuts the burner off before the furnace temperature reaches PMI.

✓     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.

8.14.5   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 air-conditioning or heat pump coil.

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

✓     Increase blower speed.

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

✓     Lubricate 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 and return.

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

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

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

✓     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.

✓     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.

8.15   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.

8.15.1   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 or smoke generator. Use one of these four 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. Use your hand or smoke gen­erator to test for duct 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 –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 required.

If instead, the basement is unconditioned, open a window or door between the basement and outdoors. Close any door or hatch between conditioned spaces and basement during 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. If the basement isn’t conditioned living space, close the door between basement and upstairs, and open a base­ment window if the pressure differential is between the duct zone, and the main body of the home is less than
-50 pascals.

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

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

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

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

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

8.      With the blower door’s manometer reading –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.

9.      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.

10.  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 pressure pan reading is more than 1 pascal, a visual inspection of the boot, boot and floor intersection, and the duct will be needed to determine where the sealing will take place.

•       Following duct-sealing work, registers should not have a pressure pan reading greater than 1 pascal.

•       The reduction you achieve depends on your ability to find the leaks and whether you can access the leaky ducts.

•       Ducts located in unconditioned spaces must be sealed and insulated to an R-8.

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 the ducts have large holes.

8.15.2   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.

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

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

9.      Record duct-blower airflow.

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

11.  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. Use the following steps to test duct leakage to the outdoors.

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

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

8.15.3   Measuring House Pressure 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. The following test measures pressure differences between the house and outdoors, caused by duct leakage.

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.

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.

8.16   Sealing Duct Leaks

Ducts located outside the thermal boundary or in an intermedi­ate zone like a ventilated attic or crawl space should be sealed and insulated to a minimum R-8. 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.

8.16.1   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 three stan­dards.

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

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

3.      Overlap the mastic and mesh at least 0ne inch beyond the seams, repairs, and reinforced areas of the ducts.

8.16.2   Sealing Return Ducts

Use the following techniques to seal return ducts. All return duct leaks must be sealed for combustion safety and for energy efficiency.

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

✓     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.

✓     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 silicone caulking.

8.16.3   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 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.

✓     Supply plenum: Permanently seal all grilles and registers.

✓     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 plastic strap, tightening it with a strap ten­sioner. Attach the insulation and outer liner with another strap.

✓     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 permanently seal 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 4 feet or PMI 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.

8.16.4   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. Aluminum foil and cloth duct tape are prohibited for use in Ohio’s weatherization program.

8.17   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.

✓     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 straps.

✓     Cover the insulation’s joints with tape to stop air convec­tion. However, tape often falls off if the installer expects tape to support the insulation’s weight.

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

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 straps can help hold the insulation facing and tape together.

8.17.1   Spray Foam 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 particularly helpful in areas where the foam can seal seams and insulate in one application. However, the spray foam application is limited by space around the duct to a greater amount than wrapping with fiberglass blankets.

8.18   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 with the water tem­perature set too high. see Combustion Boiler Replacement.

8.18.1   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.

8.18.2   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-temperature (P&T) relief valve. The P&T relief valve should have a drain pipe that terminates 6 inches from the floor. Replace a mal­functioning valve or add a P&T relief valve if none exists. Look for signs of leakage or discharges. Find out why the relief valve is discharging.

•       Verify that the expansion tank isn’t waterlogged or too small for the system. A non-operational expansion tank can cause the pressure-relief valve to discharge. Measure the expansion tank’s air pressure. The pressure should be 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 disengages the burner at a water temperature of 200°F or less.

•       Lubricate circulator pump(s) per PMI 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 (PRV) con­nected to the building’s water supply. If there is a shutoff and no PRV, open the shutoff during air-purging and close it afterwards. 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 foam pipe insulation, at least one-inch thick, rated for temperatures up to 200° F.

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 atmospheric gas- and oil-fired high-mass boilers.

8.19   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. Multi-family buildings, espe­cially multi-story buildings, may have little choice but to con­tinue with steam because of the high cost of switching systems.

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.

If the steam-heating system remains, operate it 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. Electric vent dampers reduce off-cycle losses for both gas- and oil-fired systems.

8.19.1   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 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.

✓     Verify that flush valves on low-water cutoffs are operable and don’t leak water.

✓     Ask owner about instituting a schedule of blow-down and chemical-level checks.

✓     Specify that technicians drain mud legs on return piping.

8.19.2   Steam System Energy Conservation

Specify the following efficiency checks and improvements for steam systems.

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.

✓     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.

✓     Clean fire side of heat exchanger of noticeable dirt.

✓     All steam piping that passes through unconditioned areas should be insulated to at least R-3 with fiberglass or spe­cially designed foam pipe insulation rated for steam pip­ing.

8.20   Thermostats

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

If you replace the existing thermostat, as a part of weatheriza­tion work, discuss programmable thermostats with occupants. If they can use a programmable thermostat effectively, then install one. Educate occupants on the use of the thermostat and leave a copy of manufacturer’s directions with them. Identify and dis­pose of any mercury-containing thermostats in accordance with EPA guidance.

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.

8.21   Electric Heat

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

8.21.1   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.

8.21.2   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.

8.22   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. Con­sider these steps to evaluate heat pumps during the winter.

•       Look for a temperature rise of around half the outdoor temperature in degrees Fahrenheit.

•       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 and two-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. Return ducts should be sealed too.

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 since 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 Central Air-Conditioner Inspection.

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

8.22.1   Room 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 heating as well as 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, though they are safer and require less maintenance than combustion appliances. Room heat pumps also gain some overall efficiency because they heat a single zone and don’t have the delivery losses associated with central furnaces or central boilers. If they replace electric resis­tance heat, they consume only one-half to one-third the electric­ity to produce the same amount of heat.

Room heat pumps draw a substantial electrical load, and may require 240-volt wiring. Provide a dedicated circuit that can supply the equipment’s rated electrical input. Insufficient wiring capacity can result in dangerous overheating, tripped circuit breakers, blown fuses, or motor-damaging voltage drops. In most cases a licensed electrician should confirm that the house wiring is sufficient. Don’t run portable heat pumps or any other appliance with extension cords or plug adapters.

8.22.2   Ductless Mini-split Pumps

Ductless mini-split 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.

Specify mini-split 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).

8.23   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. Window shading, attic insulation, and air leakage should be evaluated 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.

8.23.1   Central Air-Conditioner Inspection

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 filter and its mounting assembly. The condenser coil outdoors isn’t protected by a filter and is usually quite dirty.

Cleaning the Condenser Coil

Dirt enters the coil from the outside. 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.

✓     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 need low-pressure spray to avoid bending the fins.

✓     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. Technicians rake surface dirt and dust off the coil. Then they use an indoor coil cleaner and water for cleaning.

✓     Straighten bent fins with a fin comb.

8.23.2   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

2.      See Ducted Air Distribution

8.23.3   Evaluating Air-Conditioner Charge

Air-conditioning replacement or service includes refrigerant charge-checking. 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, the technician performs a superheat test.

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

These two tests indicate whether the amount of refrigerant in the system is correct, or whether there is too much or too little refrigerant. The amount of refrigerant is directly related to the efficiency of the air-conditioning system.

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

8.24   SWS Alignment