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For a 100,000 BTU/hr furnace in a closet, you're in confined-space territory and need either two openings using the traditional method or one opening if you go with the known air infiltration rate method. Sizing depends on whether the air comes from inside the building, directly outdoors, or via a vertical or horizontal duct. Quick sizing under IFGC 304 (traditional method, two-opening rule, one high and one low within 12 inches of ceiling and floor): - **All air from inside the same building**: 1 sq in per 1,000 BTU/hr per opening, so 100 sq in each. Only valid if the connected interior space itself qualifies as unconfined (50 cu ft per 1,000 BTU/hr of all appliances combined). - **Direct through exterior wall (two openings to outdoors)**: 1 sq in per 4,000 BTU/hr per opening, so 25 sq in each. - **Vertical ducts to outdoors**: 1 sq in per 4,000 BTU/hr per opening, so 25 sq in each. - **Horizontal ducts to outdoors**: 1 sq in per 2,000 BTU/hr per opening, so 50 sq in each. - **Single-opening method (outdoors)**: 1 sq in per 3,000 BTU/hr AND not less than the sum of the appliance draft hood/vent connector areas, so minimum 34 sq in, located within 12 inches of the ceiling with clearance around the appliance. Add any other fuel-burning appliance in that closet (water heater, etc.) to the BTU total before sizing. Louver free area matters: derate 25% for metal louvers and 75% for wood louvers unless the actual free area is known. A few things worth pinning down before you cut holes: - Is this a Category I atmospheric/80% furnace, or a 90%+ condensing unit? A sealed-combustion 90%+ with a concentric or two-pipe intake pulling combustion air directly from outside doesn't need closet combustion air openings at all, only the appliance clearances and service access from IFGC 306 and the manufacturer's instructions. - Is it gas or oil? Oil-fired follows NFPA 31, not IFGC, and the numbers differ. - Any other appliances sharing the closet? Reference: IFGC 2021 Section 304 (Combustion, Ventilation and Dilution Air), specifically 304.5 for the standard method and 304.6 for the known air infiltration rate method. IMC 701 mirrors this for mechanical code jurisdictions.
