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.