How much combustion air does a gas furnace or water heater need indoors?
Under the standard method, the appliance space must contain at least 50 cubic feet of volume per 1,000 Btu/h of total appliance input rating (IRC G2407.5).
For gas-fired appliances, the IRC standard method requires the space to provide at least 50 cubic feet of volume for every 1,000 Btu/h of the combined input rating of all appliances in that space. A small closet or utility room often cannot meet this on its own.
When the space is too tight, combustion air must instead be brought in from outdoors through properly sized openings or ducts, or the space can be opened up to adjoining rooms to borrow their volume.
Solid-fuel and oil-fired appliances are not covered by this figure — they follow the appliance manufacturer's installation instructions (and NFPA 31 for oil), per IRC Chapter 17.
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Related answers
- When does a house need a carbon monoxide alarm? — IRC §R315.2
- Does a water heater or furnace in a garage have to be raised off the floor? — IRC §M1307.3
- Does a water heater need a temperature and pressure relief valve? — IRC §P2804
- Can HVAC return air be pulled from a bathroom, kitchen, or garage? — IRC §M1602.2
- When does a water heater need a drain pan, and what size is the drain? — IRC §P2801.6
- Does every plumbing fixture need a P-trap? — IRC §P3201.6
- How much insulation do HVAC ducts in the attic need? — IRC §N1103.2.1
- What is the maximum length of a clothes dryer exhaust duct? — IRC §M1502.4.4
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Ask Donnie →General information based on the International Residential Code (a model code). This is AI assistance to verify — not legal advice. Confirm the adopted edition and any local amendments with your local building department (AHJ) and a licensed professional before you build.