The 2015 International Residential Code (IRC), Chapter 24 ("Fuel Gas"), Section G2407.6.2 allows for one permanent opening to be used to supply outdoor combustion air into a mechanical room for gas appliances. How is the area of opening calculated for a gas furnace and gas water heater with a total input of 140,000 Btu/h?

How do you check that the calculated area is "not less than the sum of the areas of all vent connectors in the space"?

1 Answer 1


2407.6.2 requires the opening to have a minimum free area of 1 in.2 per 3,000 Btu/h of total input.

140,000 Btu/h / 3,000 Btu/h = 46.667 in.²

It also says that the opening cannot be "less than the sum of the areas of all vent connectors in the space."

For example, If you had a water heater with a 3" round exhaust duct, and a furnace with a 5" round exhaust duct. The opening would have to be at least 26.704 in.².

A = πr²
A = π * 1.5²
A = π * 2.25
A = 7.069 in.²

A = π * 2.5²
A = π * 6.25
A = 19.635 in.²

Total Area = 7.069 in.² + 19.635 in.²
Total Area = 26.704 in.²

Section G2403 (202) General Definitions

Connector, Chimney or Vent. The pipe that connects an appliance to a chimney or vent.

It also says that the "minimum dimension of air openings shall not be less than 3 inches". So the opening for your installation would have to be one of the following sizes.


  • 3" x 16"
  • 4" x 12"
  • 5" x 10"
  • 6" x 8"


  • 7" x 7"


  • 8"
  • It's been a long time but this sounds close to me I don't remember all of it but there was a ceiling and floor level vent required to turn the air. When it was required . close to the vent size as I remember.
    – Ed Beal
    Oct 16, 2018 at 2:12

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.