Troubleshooting, Service, and Safety
Troubleshooting, Service, and Safety
This domain tests systematic diagnosis and safe service of heat pumps. It covers electrical and refrigerant faults, defrost and reversing-valve problems, compressor failure, recovery, and personal and equipment safety. The exam favors a sequence: verify the complaint, inspect, measure, isolate, repair, and confirm.
Core rules
- Start with the thermostat call and safeties. Confirm the 24 V control signal, contactor pull-in, line voltage, and motor or compressor operation. A no-heat call may be a defrost lockout, tripped breaker, bad sequencer, or open high-pressure switch.
- In heating, low suction pressure and low head pressure can mean low charge, a restricted metering device, or reversing-valve bypass. A stuck reversing valve may show equalized pressures and no mode change.
- Defrost problems include a frozen outdoor coil, short defrost, or no defrost. Check sensor placement, the board, and termination temperature.
- Refrigerant service requires EPA 608 certification, recovery equipment, and never venting. Recover, evacuate, and weigh charge; do not guess.
- Safety: lockout/tagout, discharge capacitors, PPE, nitrogen regulator, no oxygen, and proper brazing shields. Do not pressurize with oxygen or air.
Worked example
A heat pump runs but supplies cool air in heating. The indoor fan and outdoor unit run. The suction line is not hot, and the reversing valve solenoid is not energized. Check the thermostat O/B terminal and wiring. If the valve is energized in heating on this model, a failed solenoid or board output explains the symptom. Verify coil temperature and pressures before condemning the valve.
Traps
Do not replace parts without verifying power and control signals. Do not assume all reversing valves energize in cooling. Do not overlook airflow and defrost. Do not vent refrigerant or use oxygen for pressure testing.