# When calculating the vent size for combustion air in a confined space, do I factor in the air available from the inside space?

Water heater + furnace in basement = 140,000 BTUs

Combustion air required = 7,000 cu. ft. (based on (140,000 BTUs / 1000) x 50 cu. ft.)

Basement = 2,800 cu. ft. so the appliances are considered to be in a "confined" space (7,000 cu. ft. required > 2,800 cu. ft. available) which requires a vent to outside.

The 2,800 cu ft basement covers up to 56,000 BTUs (2,800 cu. ft. / 50). So, when calculating the Net Opening Required for my vent to the outside, do you ignore the available inside air (e.g. 140,000 BTUs / 3,000 = 46.6 sq. in. required) or do I first subtract the area of the basement from the calculation (e.g. (140,000 - 56,000 BTUs) / 3,000 = 28 sq. in. required)?

Thanks!

References:

IRC G2407.5.1 (304.5.1) Indoor Combustion Air - Standard Method
IRC G2407.6.2 (304.6.2) Outdoor Combustion Air - One-permanent-opening method

• ignore, since it would need to be replenished Dec 19, 2022 at 22:29
• Would not count the basement, since when that air is used, it will create negative pressure causing the house to suck in cold outside air to make up for it. Dec 19, 2022 at 22:30