Playbook: Heat Pump Faults

Adapted from HVAC FDD Reference v1.0, Remediation Playbooks (pp. 169–170).

Step 1 — Verify the fault

1. Compressor proof (HP-0007): first compare the final command with independent run proof for the same compressor or explicitly documented compressor group. Review OEM lockouts, defrost state, anti-cycle logic, and safety status. If any of those states can withhold operation, they must be reflected in the final command or make the rule NO_EVAL; do not use an upstream thermostat demand or fleet request as the command.
2. Auxiliary heat above lockout (HP-0008): prove the point represents an explicitly classified auxiliary source actively producing space heat, not availability, demand, crankcase/base-pan heat, or defrost heat. Verify heating and defrost/emergency state, the installed lockout/balance point or dual-fuel switchover, compressor proof, and whether concurrent operation is actually prohibited by the OEM/site strategy.
3. COP degradation (HP-0001): calculate measured COP as thermal output divided by electrical input and compare it to the baseline regression model (COP vs. OAT). A 15% or greater drop below the baseline curve indicates degradation. Evaluate heating and cooling modes separately — degradation may appear in only one mode. Ensure the baseline R² > 0.6 before trusting the comparison.
4. Defrost anomaly (HP-0002): count defrost cycles per hour — more than 4/hr is excessive. Check individual defrost duration — more than 15 minutes per cycle is abnormal. Check for defrost initiating when OAT is above 7 °C (45 °F); defrost should not be needed at mild temperatures.
5. Reversing valve (HP-0003): after a mode change command, wait 10 minutes for the system to settle. In cooling mode, SAT should be well below RAT — if SAT > RAT, the valve has not switched. In heating mode, SAT should be well above RAT — if SAT < RAT, the valve has not switched. This is a Severity 2 (high) fault: the unit is actively working against its intended purpose.

Step 2 — Remote triage

1. Confirm command and proof timestamps are fresh, aligned, and scoped to the same physical compressor. An aggregate OR can hide a failed lag compressor.
2. Review controller and VFD/OEM fault histories, local/remote state, defrost, pressure and temperature safeties, anti-cycle timing, and recent service.
3. Correct only verified BAS binding or sequence defects. Never bypass smoke, freeze, condensate, high/low-pressure, electrical, or OEM safeties, and do not repeatedly reset a compressor lockout.
4. For HP-0008, inspect thermostat/OEM staging, site OAT lockout, recovery and demand-response modes, and OAT sensor quality. Never disable backup heat until load, equipment safety, and the installed sequence have been verified.

Step 3 — On-site service

Only qualified HVAC/refrigeration personnel may open electrical panels, enter OEM service mode, or work on a refrigerant circuit. Follow the manufacturer’s procedure, lockout/tagout requirements, and applicable refrigerant-recovery rules before approaching capacitors, contactors, motors, or compressors.

1. Compressor proof:
   1. Verify the final output at the controller and the independent proof at the same compressor without forcing or bypassing an interlock.
   2. Inspect approved terminals, contactors, overloads, current/speed proof, and wiring under the manufacturer’s de-energized test procedure.
   3. Diagnose any active OEM safety or lockout before attempting a single manufacturer-authorized reset.
2. COP degradation:
   1. Check refrigerant charge — undercharge is the most common fault per Barandier (2023). Measure subcooling and superheat.
   2. Check for refrigerant overcharge — also degrades COP, but less common.
   3. Inspect the condenser and evaporator coils for fouling (see the rtu-compressor-refrigerant playbook).
   4. Check compressor amp draw against nameplate — elevated amps suggest mechanical degradation.
   5. Check for non-condensable gases in the refrigerant circuit.
3. Defrost anomaly:
   1. Inspect the outdoor coil for heavy ice or frost buildup.
   2. Check the defrost temperature sensor — a failed sensor can trigger continuous defrost.
   3. Check the defrost control board for fault codes.
   4. If the unit uses time-temperature defrost, verify the timer settings match the manufacturer’s recommendation.
4. Reversing valve:
   1. Check the reversing valve solenoid — listen for a click when the mode changes. No click indicates a failed solenoid ($100–$300 to replace).
   2. Check the wiring between the thermostat/controller and the reversing valve solenoid.
   3. If the solenoid energizes but the valve doesn’t shift, the valve body is stuck. Low refrigerant charge can prevent the valve from shifting — check charge first.
   4. If the valve body has failed, replace the reversing valve ($500–$2,000 plus refrigerant recovery).
5. Auxiliary heat: with the unit under normal OEM control, verify the auxiliary contactor/fuel valve and independent production proof, OAT input, and configured lockout/switchover. Qualified personnel should correct only the confirmed sensor, staging, or configuration defect; do not bypass high/low-pressure, electrical, temperature, or defrost safeties.

Step 4 — Confirm resolution

1. Compressor proof: through a normal OEM-controlled cycle, verify command and independent proof agree after the commissioned pickup/dropout allowances. Do not force a compressor start or bypass anti-cycle and safety logic.
2. COP: monitor measured COP for 7 days — it should return to within 10% of the baseline curve.
3. Defrost: monitor defrost frequency and duration for 48 hours. Target: fewer than 4 cycles/hr, less than 10 minutes each.
4. Reversing valve: use manufacturer-approved operation to verify multiple mode changes and confirm SAT responds correctly each time.
5. Auxiliary heat: monitor normal heating through representative OAT/load conditions. Confirm required backup heat remains available below the commissioned strategy and prohibited concurrent operation stays clear above it.