Diagnose a carbon monoxide callback on a solid-fuel or gas appliance using spillage and worst-case depressurization testing

Steps

1. Treat any CO alarm or CO-symptom callback as a combustion-safety emergency: verify occupant safety first (ventilate, confirm no one is symptomatic, involve emergency services if there is an active CO reading in occupied space)
2. Inspect the appliance, venting/chimney system, and combustion air supply for obvious defects (blocked flue, disconnected vent, cracked heat exchanger, backdrafting) before running diagnostic tests
3. Perform a spillage test at the draft hood/appliance under normal operating conditions to check whether combustion gases are properly exiting the flue rather than spilling into the room
4. Perform worst-case depressurization testing by operating exhaust fans, the clothes dryer, and other air-moving equipment simultaneously to see whether depressurization induces backdrafting or spillage that would not appear under normal conditions — following the general combustion-safety testing approach used in building-performance/BPI-aligned protocols
5. Measure ambient and flue-gas CO levels with a calibrated CO analyzer at multiple points (near appliance, in occupied space) rather than relying on a single spot reading
6. Do not return a failing appliance to service — document findings, red-tag or advise shutdown of the appliance if unsafe, and refer for repair (venting correction, makeup air, appliance service) before re-testing to confirm resolution