Gas Furnace Combustion Air and Venting in a Tight Closet
A 100,000 Btu/h induced-draft gas furnace is installed in a 10 ft by 12 ft closet with an 8 ft ceiling and a solid door. The closet also contains a gas water heater rated 40,000 Btu/h with a draft hood. The closet has two louvered openings to the conditioned space: one high and one low, each 12 in. by 12 in. The furnace vent runs vertically through the roof. A CO alarm in the adjacent hallway has sounded twice in the past month. The homeowner reports headaches when the furnace runs. A combustion analyzer shows 120 ppm CO in the flue at steady state, rising to 300 ppm when the closet door is closed.
1. Assess the combustion air provisions for the combined appliances and determine whether they are adequate.
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Total input = 100,000 + 40,000 = 140,000 Btu/h.
Standard combustion air sizing for two permanent openings connecting to an interior space uses 1 sq in. of free area per 1,000 Btu/h, with each opening sized for the full input. Required free area per opening = 140 sq in.
Each louvered opening is 12 in. x 12 in. = 144 sq in gross. Louver free area is typically 50-75 percent of gross, so free area is roughly 72-108 sq in - well below the 140 sq in required per opening.
The openings are undersized. Combined with a solid door and a tight closet, the appliances are starved for combustion air when the door is closed, which matches the CO rise from 120 to 300 ppm.
2. Explain the mechanism by which closing the door raises CO, and identify the safety hazard.
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With the door closed, the closet becomes a confined space. The induced-draft furnace and the draft-hood water heater both draw combustion air from the closet. As the appliances fire, the closet pressure drops below the adjacent space, and the undersized louvers cannot supply enough air.
Consequences:
- Reduced air supply causes incomplete combustion, raising CO.
- The draft-hood water heater is especially vulnerable: a negative closet pressure can spill flue gases from the draft hood into the closet instead of up the vent.
- Spilled CO and flue gases migrate into the conditioned space, triggering the hallway alarm and the homeowner's symptoms.
This is a life-safety hazard. The appliances should not be operated until the combustion air and venting are corrected.
3. List the corrective actions in priority order and the verification steps before returning the system to service.
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Priority order:
- Immediate safety: shut down the appliances or provide temporary ventilation, and advise the homeowner. Confirm the CO alarm is functional.
- Correct combustion air: enlarge or add openings so each opening provides at least 140 sq in of free area, or duct combustion air directly from outdoors per the code method for the combined input. Use louvers with adequate free area or size gross openings to yield the required free area.
- Verify the water heater draft hood does not spill under worst-case conditions (door closed, both appliances running, exhaust fans operating).
- Re-run a combustion analysis on both appliances after correction; confirm CO drops to acceptable levels and draft is within the manufacturer's range.
- Verify the furnace vent is properly sized and supported and that no joints are compromised.
Do not return the system to service until combustion analysis and draft verification confirm safe operation with the door closed and the house under normal negative-pressure conditions.