Installing a 100,000 BTU/h 80% gas furnace in a utility closet that's 6 by 8 feet with a 9-foot ceiling.

Based on the 2020 International Residential Code (IRC), your 6×8×9 ft closet provides 432 cubic feet of air. For a gas appliance like your 80% efficient furnace, the IRC requires 50 cubic feet of air per 1,000 BTU/h of input rate.

Your furnace’s input rate is 100,000 BTU/h ÷ 0.80 efficiency = 125,000 BTU/h. Required combustion air: 125,000 ÷ 1,000 × 50 = 6,250 cubic feet.

Your closet only provides 432 cubic feet—far short of the requirement.

What You Need:

1. Permanent openings sized to provide the required air: - Total free area needed: 6,250 ÷ 60 minutes = 104 square inches of net free area. - Provide two openings: one within 12 inches of the floor and one within 12 inches of the ceiling. - Each opening must be at least 52 square inches (e.g., a 7"×7.5" vent or equivalent).

1. Alternative: If the closet shares air with the home, you may qualify for reduced requirements under IRC Section G2407.5.2, but this typically applies to direct-vent or sealed-combustion units—not standard 80% furnaces.

Bottom Line:

Install two properly sized permanent vents (one high, one low) connecting the closet to adjacent spaces or outdoors. Without adequate combustion air, the furnace risks incomplete combustion, carbon monoxide production, and safety hazards.

If unsure about venting paths or local code amendments, consult a licensed HVAC technician.

No, the closet is too small for unconfined combustion air.

The Math (IRC G2407.5.1 / IFGC 304.5):

• Required Volume: 100,000 BTU/h ÷ 1,000 BTU/ft³ = 100 ft³ required per 1,000 BTU/h? Wait. Standard method: 50 ft³ per 1,000 BTU/h.
• Calculation: 100 (MBH) × 50 ft³ = 5,000 ft³ required.
• Closet Volume: 6 ft × 8 ft × 9 ft = 432 ft³.
• Deficit: You have < 9% of the required volume.

You have two compliant options (IRC G2407.6 / IFGC 304.6):

Option 1: Two Permanent Openings (Vertical Ducts/Attic/Crawl)

• One high (≤12" from ceiling), one low (≤12" from floor).
• Sizing (Communicating with Attic/Crawl): 1 in² per 4,000 BTU/h each.
• 100,000 ÷ 4,000 = 25 in² each opening (min 5" round / 5x5).
• Sizing (Direct to Outdoors): 1 in² per 2,000 BTU/h each.
• 100,000 ÷ 2,000 = 50 in² each opening (min 8" round).
• Ducts must be same size as opening; max length per Table G2407.6.1.

Option 2: One Opening (Direct to Outdoors Only)

• Location: Within 12" of top of enclosure.
• Sizing: 1 in² per 3,000 BTU/h.
• 100,000 ÷ 3,000 = 33.3 in² (min 6.5" round).

Best Practice for 80% Furnace:

Install as Direct Vent (Sealed Combustion). Pipe combustion air and flue gas directly outdoors (PVC/CPVC per manufacturer). This isolates the burner from the room entirely.

• Code: IRC G2407.1 / IFGC 304.1 (Manufacturer’s instructions override).
• Benefit: Zero combustion air openings needed in closet; eliminates negative pressure/backdraft risk; highest efficiency/safety.

Ventilation Air (Separate): Still required for cooling equipment heat rejection (IRC M1305.1.2 / IMC 304.6). Provide louvered door or transfer grille sized per manufacturer (typically 1 in² per 1,000 BTU/h cooling).