Spillage and Elevated CO from a Closet Furnace
A 100,000 Btu/h, 80% AFUE induced-draft furnace is in a 6 ft x 8 ft x 8 ft closet with a solid door. The door has one 18 in. x 12 in. louvered grille with 75% free area. The return-air duct passes through the closet and has a loose joint at the plenum. Combustion analysis shows 5.2% O2 and 210 ppm CO (air-free) in the flue; draft in the vent connector is -0.01 in. w.c. The vent connector is 4 in. single-wall, 12 ft long with two 90° elbows, and it slopes downward toward the furnace from the chimney. After five minutes of operation, a mirror fogs at the draft hood. The homeowner reports headaches but no CO alarm.
1. Is the closet combustion air adequate? Show the calculation and explain the code logic.
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The closet is a confined space, and the single existing grille does not satisfy the standard combustion-air methods.
- Closet volume: 6 x 8 x 8 = 384 cu ft. At 50 cu ft per 1,000 Btu/h, a 100,000 Btu/h appliance would need 5,000 cu ft to use indoor air from an unconfined space. The closet is confined.
- Two-permanent-opening method: each opening requires 1 sq in. free area per 1,000 Btu/h, or 100 sq in. free area each. The existing grille is 18 x 12 = 216 sq in. gross, and at 75% free area it provides about 162 sq in. free. That is enough area for one opening, but there is only one opening and its location cannot serve as both the high and low opening.
- One-permanent-opening method: 1 sq in. free area per 3,000 Btu/h gives about 33 sq in. free, but that method requires the opening to communicate with a space having adequate volume, not a depressurized closet with a leaking return.
- The loose return joint is the larger problem: it can depressurize the closet and reverse draft at the vent hood even if the grille area looks adequate.
2. What is the immediate diagnosis and action?
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This is an unsafe venting condition, not a tuning issue.
- Spillage at the draft hood, 210 ppm CO air-free, weak draft, and a vent connector sloping the wrong way together indicate products of combustion are entering the home.
- Shut off the furnace, tag it as unsafe, and do not relight it or tell the homeowner to 'open a door.' Ventilate the space and advise the occupant to leave if symptoms persist or if CO is present in the living space.
- The downward slope toward the furnace is a code violation; the vent connector must slope upward toward the chimney at about 1/4 in. per ft. The two elbows and 12 ft run also need to be checked against the vent tables and the appliance's venting instructions.
- Check the heat exchanger for cracks or holes before returning the appliance to service, because elevated CO can also come from a failed heat exchanger.
3. What repairs and verification steps are required before the furnace is placed back in service?
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Correct both the venting and the pressure boundary, then prove performance with instruments.
- Seal the return-air joint and any other return leaks in the closet so the furnace room is not depressurized by the air handler.
- Correct the vent connector slope to 1/4 in. per ft upward toward the chimney, secure joints, and verify the connector and chimney are sized and lined properly for the appliance.
- Provide combustion air by one of the code-recognized methods: two permanent openings with the required free area, or a direct outside-air duct sized per the appliance instructions. A single louvered door grille is not enough.
- After repairs, run the furnace through a full cycle and perform combustion analysis: record O2, CO air-free, draft, and temperature rise. Verify no spillage at the draft hood after five minutes and confirm the CO reading is within the manufacturer's limit and the vent draft is within the range listed on the rating plate.
- Document the before-and-after readings and the red-tag clearance.