Troubleshooting, Service, and Maintenance
Domain 8, Troubleshooting, Service, and Maintenance, tests systematic diagnosis, safety judgment, and preventive maintenance. Expect symptom-based questions about lockout, short cycling, noise, leaks, and CO.
Core rules
- Diagnose in sequence: thermostat call, safety chain, inducer/proof, ignition, gas valve, flame sense, then heat delivery. Do not replace parts without a proven failure.
- Lockout after ignition attempt often involves flame sensing, gas pressure, air in the gas line, or a dirty flame rod. Repeated lockout requires combustion and vent checks.
- Short cycling can come from air binding, a failing circulator, a waterlogged expansion tank, a high limit set too low, or a blocked heat exchanger. Check flow and temperature difference first.
- Annual maintenance includes burner and heat-exchanger cleaning, vent inspection, condensate trap flush, relief-valve exercise, expansion-tank charge, LWCO test, and combustion analysis.
- CO alarms and spillage are urgent safety findings; stop operation and correct the cause.
Worked example
A boiler runs for five minutes, the high limit opens, and the burner shuts off. The supply pipe is hot but the return is cool. Reason: low flow, likely air or a failed circulator. Purge air, verify pump operation, and check the expansion tank and valves before adjusting the high limit.
Traps
- Shotgunning parts instead of following the sequence.
- Adjusting the high limit to mask a flow problem.
- Ignoring a relief-valve discharge or a failed expansion tank.
- Skipping combustion analysis after service.