Where gas appliances are located inside the thermal envelope, a sufficient amount of combustion air must be provided for the safe and complete combustion of natural gas fuels. As new buildings become tighter, it is imperative for design professionals to know that drawing combustion air from inside tightly sealed conditioned spaces can lead to life-threatening conditions.

Take for example, the following case where several new (tightly sealed) apartment building projects used low BTU/hour input water heaters for both hot water and space heating. Based on the International Fuel Gas Code (IFGC), Section 304.5 – Indoor Combustion air, the design professionals determined that the HVAC/water heater system (with a single low input ~40,000 BTU/hour) would be able to draw the air needed for combustion from inside the conditioned space because the air demand needed was low.  The IFGC does allow using indoor air for combustion air where the building is known to have a higher leakage rate, typical of older construction. If the building is tighter however, with a natural known air leakage of less than 0.40 Air Changers per Hour (ACH), then it is necessary to use a calculation to identify the required interior volume, based on the known leakage rate. The key consideration is, does the thermal envelope leak enough outside air back into the dwelling to replace the air used in the combustion process?

In the example above, the new apartment units were experiencing frequent carbon monoxide alarms and soot buildup in the heat exchangers, both of which are indications of incomplete combustion. It would seem that the design professionals, jurisdictions, and contractors recognized that the code now allows indoor combustion air under specific conditions but failed to carefully read and correctly apply the code section.

The background on the evolution of the energy and fuel gas codes below is provided to help design professionals recognize issues and prevent future, potentially fatal, outcomes.

 Unusually Tight Construction

Back in the mid 1980’s, mechanical and fuel gas codes introduced the term unusually tight construction to the building construction industry and codes. Through testing and experience, it was determined home building practices and energy efficiency improvements to the codes produced structures significantly tighter than earlier construction. Homes built of unusually tight construction, as defined in such codes, could not use inside air for combustion air. There was simply insufficient leakage through the building thermal envelope to safely provide the air required for gas appliance combustion. Existing homes and those built at a level less than unusually tight could continue to use inside combustion air, based on the well-known requirement of 50 ft3 of interior open volume per 1000 BTH/hour appliance input capacity. In the opinion of many, an over-reaction occurred, resulting in the  extremely excessive two duct combustion air (CA) requirements mandated for all newly constructed homes.

Consider an example of this two duct CA duct, where we have a 100,000 BTU/hr furnace with a 40,000 BTU/hr water heater, both open combustion appliances requiring CA from outside. Using the horizontal CA duct option, with a required 2 sq. in. per 1,000 BTU/hr. per opening:140 ÷ 2 = 70 sq. in. or two 10” pipes/holes through the thermal envelope. The question long debated, why is the energy code focusing on air sealing these homes if we have to cut two 10” holes through the exterior wall?

All Combustion Air from Outdoors Through Horizontal Ducts

Several years later in the early 1990s, the fuel gas codes introduced the 1 sq. in. per 3,000 BTU/hr single opening option, an obvious improvement, allowing a single CA opening within 12” of the ceiling, and a smaller total opening requirement. With this single duct alternative, the same furnace and water heater required a single 47 opening. A single 8” pipe at 50 sq. in. is the smallest compliant standard sized pipe.

 For decades, the codes continued to include this major conflict, where the energy code envelope air sealing requirements continued to become more stringent, requiring the sealing of all penetrations, while the mechanical/fuel gas codes continued to mandate cutting holes through the envelope for combustion air.

 Known Infiltration Rate

Jump forward to 2003, the International Fuel Gas Code re-introduced/modified indoor combustion air options for any building – See IFGC 404.5, IRC G2407.5. The code recognizes if we know the air infiltration rate into a building (air changes per hour (ACH)), calculations can be made to identify an interior volume sufficient to satisfy the combustion air needs of a gas appliance. The key here is knowing the natural infiltration rates. FYI, the building codes no longer include the term unusually tight construction, rather they now address the infiltration rate, a more accurate method to identify building air leakage. This code modification identifies how in some cases, a small gas appliance may obtain CA from the interior space, based on the known air infiltration (ACH) into the building, and the input (I) for the gas appliance using this, IFGC Equation 3-1:

2015 IECC Residential and 2018 IECC Commercial Update

The 2015 update to the International Energy Conservation Code included a new clarification resolving the conflict between efforts to reduce air leakage and cutting combustion air openings through the building thermal envelope for fuel-burning appliances. This code change simply requires isolating the open combustion gas appliances and the combustion air open from the interior conditioned living space. This is accomplished by enclosing the gas appliances and CA duct in a sealed, insulated room. Current Utah adopted Energy Codes are:

IECC R402.4.4 (IRC 1102.4.4) Rooms containing fuel-burning appliances.

In Climate Zones 3 through 8, where open combustion air ducts provide combustion air to open combustion fuel-burning appliances, the appliances, and combustion air opening shall be located outside the building thermal envelope or enclosed in a room, isolated from inside the thermal envelope. Such rooms shall be sealed and insulated in accordance with the envelope requirements of Table R402.1.2, where the walls, floors and ceilings shall meet not less than the basement wall R-value requirement. The door into the room shall be fully gasketed and any water lines and ducts in the room insulated in accordance with Section R403. The combustion air duct shall be insulated where it passes through conditioned space to a minimum of R-8.


  1. Direct vent appliances with both intake and exhaust pipes are installed continuously to the outside.
  2. Fireplaces and stoves complying with Section R402.4.2 and Section R1006 of the International

Residential Code.

 The 2018 IECC Commercial Provisions, currently adopted in Utah:

C402.5.3 Rooms containing fuel-burning appliances. In Climate Zones 3 through 8, where combustion air is supplied through openings in an exterior wall to a room or space containing a space-conditioning fuel-burning appliance, one of the following shall apply:

  1. The room or space containing the appliance shall be located outside of the building’s thermal envelope.
  2. The room or space containing the appliance shall be enclosed and isolated from conditioned spaces inside the building’s thermal envelope. Such rooms shall comply with all the following:

2.1. The walls, floors, and ceilings that separate the enclosed room or space from conditioned spaces shall be insulated to be not less than equivalent to the insulation requirement of below-grade walls as specified in Table C402.1.3 or C402.1.4.

2.2. The walls, floors, and ceilings that separate the enclosed room or space from conditioned spaces shall be sealed in accordance with Section C402.5.1.1.

2.3. The doors into the enclosed room or space shall be fully gasketed.

2.4. Water lines and ducts in the enclosed room or space shall be insulated in accordance with Section C403.

2.5. Where an air duct supplying combustion air to the enclosed room or space passes through conditioned space, the duct shall be insulated to an R-value of not less than R-8.

Exception: Fireplaces and stoves complying with Sections 901 through 905 of the International Mechanical Code, and Section 2111.14 of the International Building Code.

 Please note, if the gas appliances are sealed/direct vent, an opened CA duct is not required at the appliance and these code sections do not apply, as combustion air openings into the conditioned space do not exist.

Identifying Known Infiltration Rates

One method for approximating infiltration rates prior to construction is found in ACCA/ANSI Manual J Table 5B, Simplified Infiltration Rates for One or Two Exposures, Default Air Change Values for Townhouses and Condominiums. From this Table we find:

1000 sq. ft. apartment unit of tight construction; Heating ACH = 0.18 and Cooling ACH = 0.09

Obviously, these ACH numbers are significantly lower than 0.40, the trigger for Volume Calculations.

Please notice during heating seasons the air infiltration rate is higher due to stack action or heat rising through ceilings, pulling outside air into the building to replace ceiling leakage. Water heaters operate year-round and, in the summer, the ACH is lowest. Let’s consider a calculation for the required interior volume if the indoor combustion air option is selected.

IFGC Equation 3-1 for nat. draft, where I = 40,000 input BTU/hr. water heater, ACH cooling = 0.09: 

Required Volume = 21 ÷ 0.09 X 40 = 9333 cubic feet.  Divide by 8’ ceiling hgt. = 1167 sq. ft.

Recalling the volume used in any indoor CA calculation is based on open areas that cannot be closed off with doors. The total volume of the apartment is simply too small to use indoor combustion air. Considering bedroom, bathroom, and laundry room doors may be closed, typically no more than half the volume is available for combustion air. Volume is insufficient for both heating and cooling conditions.

Other conditions which might compound the problem include continuously operating fans, dryer and range hood operations, and extra-ordinary sealing practices resulting in even lower ACH rates.

Conclusion: It is not safe to use combustion air from inside tightly air-sealed buildings. Combustion air for gas furnaces, boilers, and water heaters, for virtually all residential dwelling units, air sealed in compliance with current codes must come from outside. Either by providing an outside combustion air duct, to a gas furnace/water heater room sealed/separated from the living space, or by installing direct vent appliances, appliances sealed from the interior environment, with combustion air and exhaust piping connected directly to the gas appliance. The latter option is strongly recommended. Please feel free to respond for further information or if you have other questions.

Direct Vent Gas Appliance


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