During a combustion safety inspection, a technician suspects a cracked heat exchanger in a gas furnace. Which diagnostic method provides the most reliable confirmation of this hazard?
- AUsing a combustion analyzer or CO detector to check for elevated CO levels in the supply air stream while the blower operates
- BListening for unusual blower motor noise during the heating cycle
- CMeasuring the static pressure across the furnace filter
- DChecking the thermostat's calibration against a reference thermometer
Show answer & explanationAnswer & explanation
Correct answer: A. Using a combustion analyzer or CO detector to check for elevated CO levels in the supply air stream while the blower operates
A cracked heat exchanger allows flue gases, including carbon monoxide, to mix into the supply air stream, so testing supply air for elevated CO with the blower running (which can pull gases through a crack) is the most reliable field confirmation method. Blower noise, thermostat calibration, and filter static pressure do not directly indicate combustion gas leakage.
Why the other options are wrong
- B. Blower noise could indicate a mechanical issue but not confirm gas leakage.
- C. Static pressure across the filter indicates airflow restriction, not combustion gas leaks.
- D. Thermostat calibration relates to temperature control, not combustion safety.
Cracked Heat Exchanger Detection
A cracked heat exchanger allows flue gases like CO to enter the supply air; testing supply air CO levels with the blower running is the most reliable confirmation method.
- Blower operation can pull gases through the crack (negative pressure effect)
- Visual inspection alone often misses hairline cracks
- Elevated supply-air CO confirms exchanger leakage
- Furnace should be shut down and tagged if confirmed
Memory trick: Blower on, CO detector out — that's how cracks get found.