High CO After Furnace Cleaning
An 80% efficient natural-gas furnace is being checked after a routine cleaning. The technician's combustion analyzer, calibrated that morning, samples the flue before dilution: O2 4.2%, CO 420 ppm, CO2 9.6%, flue temperature 410°F, ambient CO 1 ppm, and draft -0.03 in w.c. Manifold gas pressure is 3.8 in w.c.; the rating plate calls for 3.5 in w.c. The flame is blue with orange tips. Burners are clean and aligned. A visual inspection of the heat exchanger with a flashlight shows some discoloration but no obvious crack. The customer reports headaches and has a CO alarm in the hallway that has not sounded. The technician wants to adjust manifold pressure to 3.5 in w.c., note that CO should drop, and leave the furnace running.
1. Interpret the readings. Is the furnace safe to leave operating while you adjust gas pressure? Explain.
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No. CO at 420 ppm in an undiluted flue sample is high, and O2 4.2% is low, meaning little excess air. CO2 9.6% is not unusually high; the CO/CO2 ratio is about 0.0044, which is a useful red flag. Draft is weak but present; ambient CO is low, but that does not prove the heat exchanger is intact.
Overfiring from 3.8 vs 3.5 in w.c. can contribute, but it does not explain a 420 ppm reading by itself. The headache report and discoloration raise concern for a compromised heat exchanger or blocked secondary passage. I would not leave it running while making an adjustment and hoping. I would shut it down, verify the analyzer and sample point, and test supply-air CO.
2. Outline a diagnostic sequence to find the cause before replacing parts.
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- Confirm analyzer calibration and sample location; repeat the flue test. 2) Measure supply-air CO and ambient CO while the blower runs. 3) Set manifold pressure to nameplate and verify input by clocking the meter. 4) Check orifice size and altitude, burner alignment, primary air, and crossover. 5) Inspect the heat exchanger more thoroughly using limited visual, mirror, or manufacturer-approved methods; check for flue blockage or recirculation. 6) Re-test CO, O2, CO2, and temperature rise after each change.
If CO remains high after gas pressure and air are correct, shut down and red-tag the furnace. A cracked heat exchanger or blocked flue cannot be corrected by a pressure adjustment.
3. What would you tell the customer and document if the heat exchanger is found cracked?
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Explain in plain language that the furnace is unsafe, that CO can enter the home, and that it must not be operated. Offer to shut off the gas and electric supply to the furnace, tag it, and arrange repair or replacement. Advise the customer to keep CO alarms operating and to seek medical advice if symptoms persist.
Document the CO readings, draft, gas pressure, visual findings, test method, photos if allowed, actions taken, and the customer's decision. Do not reconnect or leave the appliance operating because the customer wants heat.