AL15 on a MasterTherm heat pump: R290 refrigerant leak - alarm of the refrigerant leak or leak detector malfunction
Short answer
On a MasterTherm heat pump, AL15 means: R290 refrigerant leak - alarm of the refrigerant leak or leak detector malfunction. 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 · 2 sources
What it means
On the pCO5/pGD controller alarms - air/water units (BoxAir, BoxAir Inverter, BoxAir Inverter P, AirMaster, EasyMaster)
The manual says: R290 refrigerant leak - alarm of the refrigerant leak or leak detector malfunction.
Part involved: R290 leak detection sensor.
When it shows: R290 concentration reported via serial link; alarm at 15% LFL (safe level 25%).
Likely causes:
- Refrigerant leakage.
- Leak detector malfunction.
On the pCO5/pGD controller alarms - brine/water and water/water units (AquaMaster, AquaMaster Inverter, Inverter P, Combi)
The manual says: R290 Leakage Alarm - leak sensor detected alarm condition.
Part involved: R290 leak detection sensor.
When it shows: Leak sensor alarm.
Likely causes:
- R290 refrigerant leak detected.
Is it safe?
On the pCO5/pGD controller alarms - air/water units (BoxAir, BoxAir Inverter, BoxAir Inverter P, AirMaster, EasyMaster)
| Does the heat pump stop? | Yes |
|---|---|
| Severity | Safety lockout |
| Engineer needed? | Yes |
The heat pump stops until the fault clears or is reset.
Resetting itself: Alarm clears automatically when concentration drops; unit returns to operation on backup heaters.
Manual reset: Service required.
On the pCO5/pGD controller alarms - brine/water and water/water units (AquaMaster, AquaMaster Inverter, Inverter P, Combi)
| Does the heat pump stop? | Yes |
|---|---|
| Severity | Safety lockout |
| Engineer needed? | Yes |
The heat pump stops until the fault clears or is reset.
Resetting itself: Clears when concentration drops.
Manual reset: Service required.
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.
- Stay calm; do NOT approach the outdoor unit.
- Keep the unit powered ON (fan disperses leaked refrigerant) unless there is a fire.
- Keep ignition sources away; report the event to the service company.
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.
- Do not approach/open the unit; ventilate the plant room only if safe to do so without entering a gas-filled space.
- Keep ignition sources away and call the service company.
What the engineer will check
On the pCO5/pGD controller alarms - air/water units (BoxAir, BoxAir Inverter, BoxAir Inverter P, AirMaster, EasyMaster)
- Locate and repair R290 leak (flammable refrigerant procedures).
- Check leak sensor with R290 calibration gas.
Who to ask for: R290 / flammable refrigerant qualified engineer.
Certification: F-gas plus A3 flammable refrigerant (R290) handling competence.
On the pCO5/pGD controller alarms - brine/water and water/water units (AquaMaster, AquaMaster Inverter, Inverter P, Combi)
- Locate/repair leak; check ventilation box ducting and leak sensor with calibration gas.
Who to ask for: R290 / flammable refrigerant qualified engineer.
Certification: F-gas plus A3 (R290) competence.
What you might notice
Sources
- MasterTherm pCO5/pGD digital controller - final user's instruction manual, R290 BoxAir Inverter (BAxxIP), edition 30/08/2024 ↗ · PGD1-MANUAL 30/08/2024 · page 38, 40, 43, 48 · Reviewed 23 September 2026
- MasterTherm pGDx touch - End User Manual (doc. 4783, revision 010, SW 1118) ↗ · 4783 rev 010, SW1118 · page 70 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026