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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

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  • ignore, since it would need to be replenished
    – Traveler
    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.
    – crip659
    Dec 19, 2022 at 22:30

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