How to use the 2015 IRC Chapter 24, G2407.6.2 to determine combustion air?
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 &quot;less than the sum of the areas of all vent connectors in the space.&quot;
For example, If you had a water heater with a 3&quot; round exhaust duct, and a furnace with a 5&quot; 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 &quot;minimum dimension of air openings shall not be less than 3 inches&quot;. So the opening for your installation would have to be one of the following sizes.
Rectangular

3&quot; x 16&quot;
4&quot; x 12&quot;
5&quot; x 10&quot;
6&quot; x 8&quot;

Square

7&quot; x 7&quot;

Round

8&quot;