Heat Pump Diagnostic Troubleshooting and Service
Service questions test a repeatable diagnostic sequence: confirm the complaint, verify power and settings, measure airflow, then interpret pressures and temperatures. The exam favors evidence over guesswork.
Diagnostic sequence
- Confirm thermostat mode, setpoint, and configuration. Check line voltage, low-voltage transformer output, fuses, and error codes.
- Measure temperature split and external static pressure before attaching gauges. Low airflow mimics low charge and can cause icing.
- Calculate superheat for fixed-orifice systems and subcooling for TXV/EEV systems. Compare to the manufacturer's target at the measured indoor/outdoor conditions.
- Check compressor amp draw against nameplate, capacitor microfarads, contactor pitting, and winding resistance to ground.
- For reversing valves, compare temperatures on the three pipes; a valve stuck mid-position gives abnormal temperatures and poor capacity.
- A temperature drop across a filter drier indicates a restriction. Frost at the metering device suggests starvation, not necessarily undercharge.
Worked example
A system in cooling has low suction pressure, high superheat, low subcooling, and a 12°F temperature drop across the liquid-line filter drier. The pressure drop and temperature drop point to a restricted drier, not simply low refrigerant. Replacing the drier and verifying subcooling is the correct repair; adding charge first would overcharge the system once the restriction is removed.
Traps
Do not add refrigerant before checking airflow, filter, coil, and metering device. Do not use a single pressure to condemn a compressor. Do not confuse a frosted suction line from airflow with a refrigerant overcharge, and do not overlook thermostat configuration.