Heating Equipment: Furnaces, Boilers, Combustion Air, and Venting
Categories and venting
Furnaces fall into four categories by flue pressure and temperature. Category I - the common 80 percent AFUE fan-assisted furnace - vents under negative pressure in Type B vent or a listed liner. Category IV condensing units reach 90 percent AFUE or more, vent in PVC or CPVC per the manufacturer, and must slope back to the unit about 1/4 inch per foot with a proper condensate trap. Never vent a condensing furnace in B-vent, and never common-vent across categories.
Combustion air
NFPA 54 and the mechanical code treat a space as confined when its volume is under 50 cubic feet per 1,000 Btu/h of input. Two-opening method: one opening within 12 inches of the top, one within 12 inches of the bottom, each at 1 square inch per 1,000 Btu/h; sizing improves to 1 per 2,000 with horizontal ducts and 1 per 4,000 with vertical ducts. A single opening needs 1 square inch per 3,000 Btu/h. Louver free area must be corrected.
Worked example
A 100,000 Btu/h furnace sits in an 8 by 10 by 8 foot closet, 640 cubic feet. Required volume = 50 x 100 = 5,000 cubic feet, so the space is confined. Two openings of 100 square inches net free area are needed; with 75 percent free-area louvers, each grille must be 133 square inches.
Traps
Assuming a louvered door counts as outdoor air; forgetting free-area correction; no condensate trap or neutralizer; and ignoring the gas derate of roughly 4 percent per 1,000 feet above sea level, which matters at Albuquerque and Santa Fe elevations.