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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"?

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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.

Rectangular

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

Square

  • 7" x 7"

Round

  • 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 '18 at 2:12

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