AL08 on a MasterTherm heat pump: Compressor Thermal - compressor overheating or circuit breaker activation
Short answer
On a MasterTherm heat pump, AL08 means: Compressor Thermal - compressor overheating or circuit breaker activation. The heat pump stops until the fault clears or is reset. An engineer is needed to fix the cause. The wording differs by model range — each is shown below.
Reviewed 23 September 2026 · 1 source
What it means
On the pCO5/pGD controller alarms - air/water units (BoxAir, BoxAir Inverter, BoxAir Inverter P, AirMaster, EasyMaster)
The manual says: Compressor Thermal - compressor overheating or circuit breaker activation.
Part involved: compressor thermal protection / compressor circuit breaker Q1.
When it shows: Control action: Switches off the compressor (and fan). Operator guidance: Reset the compressor thermal protection/circuit breaker; contact installer if it persists.
Likely causes:
- Too high heating water temperature.
- Power supply problem - missing phase, contactor fault.
- Incorrect compressor thermal protection setting.
- In extreme case compressor winding fault.
On the pCO5/pGD controller alarms - brine/water and water/water units (AquaMaster, AquaMaster Inverter, Inverter P, Combi)
The manual says: Compressor Thermal Protection.
Part involved: compressor thermal protection / circuit breaker.
When it shows: Control action: Switches off compressor and brine circulator. Operator guidance: Switch on compressor circuit breaker, call service if problem returns.
Likely causes:
- Too high water temperature.
- Power supply problem - missing phase.
- Compressor motor malfunction.
Is it safe?
| Does the heat pump stop? | Yes |
|---|---|
| Severity | Fault — clears by itself when the condition goes |
| Engineer needed? | Yes |
The heat pump stops until the fault clears or is reset.
Resetting itself: Resets automatically after the minimum compressor off time (~6 min) if the same alarm occurs fewer than 3 times per compressor operating hour.
Manual reset: After 3 occurrences in 1 compressor operating hour the unit is blocked (ALARM key lit steadily); manual reset by pressing ALARM key on the alarm display (pGD) or RESET on pGDx Alarm Management.
What you can check
On the pCO5/pGD controller alarms - air/water units (BoxAir, BoxAir Inverter, BoxAir Inverter P, AirMaster, EasyMaster)
- Read and note the alarm (ALx code/text) and the alarm counter shown on the pGD/pGDx display.
- Check the heat pump breaker/isolator in the consumer unit is on.
- Check the supply breaker in the consumer unit is on (do not open the heat pump switchboard).
On the pCO5/pGD controller alarms - brine/water and water/water units (AquaMaster, AquaMaster Inverter, Inverter P, Combi)
- Read and note the alarm (ALx code/text) and the alarm counter shown on the pGD/pGDx display.
- Check the heat pump breaker/isolator in the consumer unit is on.
- Check the supply breaker in the consumer unit is on.
What the engineer will check
On the pCO5/pGD controller alarms - air/water units (BoxAir, BoxAir Inverter, BoxAir Inverter P, AirMaster, EasyMaster)
- Reset compressor/DC drive circuit breaker in switchboard (power isolated first).
- Check phases/supply voltage and contactor.
- Check compressor windings.
Who to ask for: heat pump service engineer (refrigeration/controls).
Certification: F-gas certification (refrigerant circuit work); electrical competence for switchboard work.
On the pCO5/pGD controller alarms - brine/water and water/water units (AquaMaster, AquaMaster Inverter, Inverter P, Combi)
- Reset compressor circuit breaker inside switchboard (isolate first).
- Check phases, contactor, windings.
Who to ask for: ground source heat pump engineer (refrigeration/controls).
Certification: F-gas certification (refrigerant circuit work); electrical competence for switchboard work.
What you might notice
Sources
- MasterTherm pCO5/pGD digital controller - OEM instruction manual, heat pumps AirMaster, AquaMaster, EasyMaster, BoxAir (Documentation PGD1-MANUAL, Edition 41, sw_11) ↗ · Edition 41 / sw_11 · pages 31, 42-43, 31, 40-41 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026