8.2   Combustion-Safety Evaluation

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.

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

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.

✓     Sniff all valves and joints with the gas sniffer at a rate of one inch per second. Joints should be tested all the way around.

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

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

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

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

✓     Ground all CSST gas lines that are not properly grounded.

8.2.3   Carbon Monoxide (CO) Testing

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

Vent Testing for CO

Testing for CO in the appliance vent is a part of combustion test­ing that happens under worst-case conditions. If the undiluted combustion byproducts exceed the following CO limits, the appliance fails the CO test under current BPI standards.

•       Space heaters and water heaters: 100 ppm as measured in flue or 400 ppm air-free

•       Furnaces or boilers: 100 ppm as measured or 400 ppm air-free

Ambient Air Monitoring for CO

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

8.2.4   Worst-Case CAZ Depressurization Testing

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

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

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

Worst-Case CAZ Depressurization Test

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

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

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

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

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

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

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

7.      Turn on the furnace air handler.

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

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

10.  Refer to “Maximum CAZ Depressurization” on page 517.

Analyzing CAZ Depressurization

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

Spillage and CO Testing

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

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

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

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

b.  A mirror fogs at the draft diverter

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

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

5.      Measure CO in the undiluted flue gases of each furnace or boiler after 2 minutes of operation at worst-case depressurization. If CO in undiluted flue gases is more than 100 ppm as measured or 400 ppm air-free mea­surement, take action to reduce CO level.

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

8.2.5   Evaluating Combustion Air at Worst-Case

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

Evaluating the CAZ Volume

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

Evaluating Combustion Air by Flue-Gas Analysis

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

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

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

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

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

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

5.      If O₂ is too low at the correct firing rate, open a door or a window connected to the CAZ. If opening the CAZ door, a nearby window, an exterior door, or any combi­nation of these increases the O₂ reading and decreases CO, then consider the options specified in “Combus­tion-Air-Related Solutions” on page 265.

8.2.6   Mitigating CAZ Depressurization and Spillage

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

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

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

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

Improvements to Mitigate CAZ Depressurization

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

✓     Seal all return-duct leaks in the CAZ.

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

✓     Provide make-up air for dryers and exhaust fans. See Ducted Air Distribution.

✓     Reduce the CFM of exhaust appliances.

Chimney Improvements to Mitigate Spillage Problems

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

•       Remove chimney obstructions.

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

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

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

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

•       Increase the pitch of horizontal sections of vent.

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

Fixing Persistent Depressurization and Spillage Problems

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

•       Replace open combustion appliances with sealed-combus­tion appliances.

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

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

8.2.7   Combustion-Air-Related Solutions

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

Installing Supplemental Combustion Air

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

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

•       Another indoor space

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

•       From outdoors into an isolated CAZ. See below.

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

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

Zone Isolation for Natural-Draft Vented Appliances

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

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

4.      Measure CO and O₂ at worst-case and evaluate com­bustion air as described in “Evaluating Combustion Air by Flue-Gas Analysis” on page 261.

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

6.      If you can’t air-seal the CAZ adequately to isolate the zone, solve worst-case depressurization and spillage problems as described in “Evaluating Combustion Air at Worst-Case” on page 260.

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