In unconfined spaces in buildings of conventional frame, brick, or stone construction, infiltration is normally adequate to provide air requirements. If the unconfined space is within a building of unusually tight construction, the air supply shall be obtained from outdoors or from spaces freely communicating with the outdoors. Under these conditions, a permanent opening or openings having a total free area of not less than one square inch per 5,000 Btu/hr of total input rating of all appliances shall be provided.
A 100,000 Btu/hr furnace and a 50,000 Btu/hr water heater.
150,000 Btu/hr x (1 square inch / 5,000 Btu/hr) = 30 square inches
A duct having a minimum free area of 30 square inches shall be provided.
Ducts used to convey make-up air from the outdoors shall be of the same cross-sectional area as the free area of the openings to which they connect. Ducts connected to the outside air only, may be connected to the cold air return of the heating system.
This is not a recommended procedure in cold climates for mid-efficient, fan-assisted furnaces. Consult the manufacturer’s installation instructions.
The minimum cross-sectional dimensions of air ducts shall be not less than 3 inches. (NFPA 54, 9.3.3).