Calculate combustion air requirements for gas appliances from Btu/h input: the room volume needed for indoor air and the opening size for each method.

Add the input ratings from the rating plates of the appliances with a draft hood, such as many tank water heaters. Blank counts none.

Add the input ratings of the appliances with a built-in combustion fan and no draft hood, such as many furnaces. Blank counts none.

The room the appliances are in. Leave the room size blank to get only the required volume and opening sizes.

Typical 8 ft ceiling. Change it to match the room.

Tight house or louvers (optional)
Air changes per hour from a blower door test. If it is 0.40 or less, the code requires the volume to be worked out from it. Blank uses the standard 50 ft³ per 1,000 Btu/h.

Louvers block part of an opening, so the opening must be larger than its free area. Use the maker’s free area when it is known.

Calculator Functions

The calculator works through the combustion air requirements for gas appliances in a room: the room volume that lets indoor air supply them, whether your room is large enough, and the free area of the openings needed when it is not.

FunctionUse it whenYou enter
Combustion air requirementsA furnace, boiler or water heater draws its air from the room it sits inTotal Btu/h input of the appliances, and the room size if you want it checked

Enter the input ratings from the appliance rating plates, with draft hood appliances and fan-assisted appliances in their own fields. The room size is optional: leave it blank to see only the required volume and the opening sizes. In the optional section, a known air infiltration rate switches the volume to the method for tight houses, and choosing metal or wood louvers adds the larger opening that gives the same free area. For the air a burner uses per unit of fuel, see the Combustion Air Calculator.

Combustion Air Formulas

V = 50 × Q ÷ 1,000 [1]
VRoom volume needed for indoor air to supply the appliances (ft³)
QTotal input rating of all appliances in the room (Btu/h)

This is the standard method: 50 ft³ of room for every 1,000 Btu/h. A 50,000 Btu/h water heater needs 2,500 ft³, so a 20 × 16 × 8 ft room of 2,560 ft³ is large enough [1]. Rooms joined by the openings below count together toward V.

Known air infiltration rate

V = (21 × D + 15 × F) ÷
ACH [1]

D is the input of draft hood appliances and F the input of fan-assisted appliances, both in thousands of Btu/h. Fan-assisted appliances need less air because they have no draft hood to dilute. ACH is the building’s air changes per hour. This method must be used when the rate is known to be 0.40 or less, and a rate above 0.60 is not allowed in it [1]. A 75,000 Btu/h draft hood water heater and a 100,000 Btu/h fan-assisted furnace at 0.40 ACH need 3,938 + 3,750 = 7,688 ft³.

Opening sizes

Air comes fromOpeningsFree area of each
Rooms on the same story [1]Two1 in² per 1,000 Btu/h, at least 100 in²
Rooms on another story [1]One or more2 in² per 1,000 Btu/h in total
Outdoors, direct or by vertical ducts [2]Two1 in² per 4,000 Btu/h
Outdoors, by horizontal ducts [2]Two1 in² per 2,000 Btu/h
Outdoors, single opening [2]One1 in² per 3,000 Btu/h

Every ratio uses the total input Q, and the calculator rounds each area up to the next whole square inch. Where two openings are required, one starts within 12 in of the top of the room and the other within 12 in of the bottom, and no opening may be narrower than 3 in. A single outdoor opening goes within 12 in of the top and must also be at least as large as the combined area of the vent connectors in the room [2]. For 135,000 Btu/h, openings to a room on the same story are 135 in² each.

Louvers and grilles

Agross = Afree ÷ F

F is the share of the louver that is open. Use the maker’s free area when it is known. Otherwise F is 0.75 for metal louvers and grilles and 0.25 for wood louvers, so 135 in² of free area takes a 180 in² metal louver [1]. The Louver Free Area Calculator checks a louver of a given size.

Limits of This Estimate

  • The ratios are those of the 2021 International Fuel Gas Code and Chapter 24 of the International Residential Code [1]. States and cities adopt their own editions and amendments, so the local code and the inspector have the final word.
  • They apply to gas appliances that take combustion air from the room. Direct-vent appliances and appliances with power burners are supplied with air as the manufacturer’s instructions say [3].
  • The calculator does not check the single opening against the vent connector area, the opening positions or the duct sizes.
  • Combustion air is a safety requirement for fuel-burning appliances. Have a licensed installer confirm the sizes before relying on them.

Sources

  1. International Code Council, CodeNotes: Gas Appliance Combustion, Ventilation and Dilution Air, Part 2, Indoor Combustion Air Methods: room volume methods, openings between indoor spaces and louver free area
  2. International Code Council, CodeNotes: Gas Appliance Combustion, Ventilation and Dilution Air, Part 1, Outdoor Combustion Air Methods: sizes and positions of openings to the outdoors
  3. Minnesota Rules, part 1346.5304, Section 304 (IFGC) Combustion, Ventilation and Dilution Air: appliances that follow the manufacturer’s instructions instead