Carbon Monoxide Reading on a No-Heat Call
A technician responds to a no-heat call at a 22-year-old rental house on a 28°F evening. The 80% induced-draft furnace lights and runs, but the technician notices rust streaks at the flue collar and light soot on the burner box. With a calibrated combustion analyzer he measures 9 ppm CO in the basement ambient air and 48 ppm in the supply airstream about 6 feet downstream of the evaporator coil. The tenants say three family members, including two children, have had headaches and nausea for two weeks. A portable kerosene heater is running in the hallway. The landlord, reached by phone, says the tenants are exaggerating and asks the technician to "clean the flame sensor, get heat back on, and not make a big deal of it because there is a home inspection tomorrow."
1. State the immediate actions the technician should take and explain the reasoning behind each one.
Read the worked answer
The reading of 48 ppm CO in the supply airstream, with only 9 ppm in the basement air, indicates combustion products are entering the conditioned air, which points to a breached heat exchanger or a severe venting failure. The technician should shut off the gas at the appliance valve rather than rely on the thermostat, because a call for heat can restart the burner. Do not run the blower to "clear" the air; it distributes CO through the house. Tag and lock the appliance out, open doors and windows, shut down the kerosene heater, and move the family outdoors or to fresh air. Advise medical evaluation for anyone with symptoms, and notify the gas utility and fire department as required by local protocol and company policy. Inform the landlord in writing that the appliance is unsafe and will remain off.
2. What follow-up testing and documentation should support the shutdown decision?
Read the worked answer
Run a combustion analysis at the flue before the appliance is condemned: CO, CO2 or O2, stack temperature, and draft. Compare undiluted flue CO with the diluted supply-air reading; a supply reading that climbs when the blower runs is strong evidence of a heat exchanger breach. Perform a visual inspection of the heat exchanger with a mirror and borescope, look for flame distortion or rollout, and check the vent for blockage, proper slope, and correct termination. Verify the induced-draft motor proves and that the pressure switch is not bypassed. Record analyzer make and model, calibration date, readings, ambient conditions, and photographs. Provide copies to the landlord and keep them in the file, because these readings are the evidence that justifies the red tag and protect the technician if the decision is challenged.
3. Why is cleaning the flame sensor and restoring heat not an acceptable response to the landlord's request?
Read the worked answer
A dirty flame sensor causes nuisance shutdowns; it does not produce CO in the supply air. Cleaning it would only allow a compromised appliance to keep running and could turn a complaint into an emergency. Any technician who restores an appliance with a known CO breach and documented symptoms assumes liability for the outcome, and most jurisdictions require unsafe appliances to be shut off and reported. The kerosene heater adds another CO source in the same occupied space. The correct judgment is to hold the shutdown, explain the evidence plainly, and offer a safe path forward: repair or replace the furnace and provide temporary safe heat only if it is vented and approved.