Natural-Draft Furnace Spillage and CO
A 100,000 BTU/h natural-gas, 80% AFUE furnace is in a basement. It vents through a 6 in. single-wall connector into an exterior masonry chimney. The water heater also connects to the same chimney. On a cold day the furnace runs, but the draft hood spills for several minutes after startup. A combustion analyzer reads 150 ppm CO in the flue before dilution; the flame is yellow and lifting. The basement has a clothes dryer and a bath fan. The homeowner reports headaches when the furnace runs.
1. What immediate actions and unsafe-condition determination are appropriate?
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Treat this as a potential CO exposure, not a tune-up. Shut off the furnace, do not leave it running while diagnosing, ventilate the basement, and check CO in the occupied space. Advise the occupants about symptoms and, if anyone is symptomatic, tell them to leave and seek medical evaluation. Tag the furnace as unsafe for continued operation until the spillage and CO cause is corrected. A yellow, lifting flame plus draft-hood spillage plus elevated flue CO is enough to red-tag the appliance even if the heat exchanger has not yet been inspected.
2. What tests and inspections would you perform to find the cause?
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Check the vent connector rise, slope, length, and supports; inspect the chimney for blockage, liner condition, and sizing. Measure draft at the connector and at the chimney, and observe draft-hood spillage with the furnace and water heater operating together. Test combustion air availability with the dryer and bath fan running, because exhaust devices can depressurize the basement. Verify gas pressure and flame characteristics, inspect the heat exchanger, and use a combustion analyzer to compare CO before and after any correction. The shared chimney and the orphaned water heater are prime suspects for reduced draft.
3. What would justify clearing the furnace versus requiring repairs?
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Clear the furnace only after the venting is corrected and verified: no spillage at the draft hood during steady operation, stable draft with other exhaust devices running, normal CO and oxygen readings, proper gas pressure, and a sound heat exchanger. If spillage persists, the chimney is blocked or undersized, the liner is deteriorated, or the heat exchanger is cracked, the furnace remains unsafe until repaired or replaced. Document the before-and-after readings and the reason for the clearance decision.