Troubleshooting, Servicing, and Maintenance
Domain 7 tests systematic troubleshooting, servicing, and maintenance. The exam rewards a measured sequence: gather information, verify the complaint, test in order, repair, and confirm with combustion analysis and temperature rise.
Troubleshooting sequence
Check the thermostat call, power, and error code. Verify gas pressure at the meter and manifold. Confirm the vent and combustion air are unobstructed. Measure temperature rise and compare to the rating plate. Use a combustion analyzer to check O2, CO2, CO, and stack temperature. Do not parts-swap.
Servicing and maintenance
Clean burners and orifices, inspect the heat exchanger for cracks, check the flue connector, lubricate motors where permitted, replace filters, and verify safety switches. Document before-and-after readings. Use the correct orifice for the gas and altitude.
Worked example
An 80,000 Btu/h furnace has a rated temperature rise of 40-70 F but measures 85 F. Output is about 64,000 Btu/h at 80% efficiency. Using CFM = output / (1.08 x delta T), CFM = 64,000 / (1.08 x 85) = 697 CFM. The blower is rated for 1200 CFM, so airflow is low. You find a clogged filter and a collapsed duct. After repair, delta T falls to 55 F.
Traps
- Adjusting gas pressure to fix a temperature-rise problem.
- Ignoring a cracked heat exchanger because the flame looks fine.
- Using a manifold pressure reading without checking inlet pressure.
- Failing to verify operation under all safety conditions.
- Leaving the appliance without documenting final readings.