Type the alarm number (for example 220) or a keyword like pressure, sensor or inverter to find your code, then expand the row for checks and next steps.
Description: The BT3 sensor (incoming/flow water sensor) is showing an open or short circuit.
Likely causes: Damaged sensor, loose plug, broken cable, or faulty control board input.
Checks you can do:
- Power-cycle the heat pump and see if the alarm returns.
- Visually check the BT3 sensor wiring and plug at the unit (no visible damage, firmly connected).
- Check for recent work near the pipe sensor that might have disturbed the cable.
Call an engineer if: The alarm reappears after reset or you can see damaged sensor cabling.
Operation: Heating may be limited or blocked until the sensor fault is cleared.
Description: The BT12 sensor (outgoing/return water) is not giving a valid reading.
Checks you can do:
- Reset the alarm from the controller.
- Check BT12 wiring and plug for damage or loose connection.
Call an engineer if: The alarm returns or you suspect damage to the sensor or control board.
Operation: Heat pump may run in a protective mode or stop heating.
Description: The BT15 sensor that monitors the refrigerant/pipe temperature is faulty or not connected.
Checks you can do:
- Try a reset from the alarm menu.
- Listen for abnormal noises; leave the outdoor unit covers in place.
Call an engineer: This sensor sits inside the refrigerant section; service access is required.
Operation: Compressor will usually be blocked until the sensor issue is fixed.
Description: The flow water leaving the condenser has reached the maximum permitted temperature, so the unit stops or ramps down to protect itself.
Common causes: Low system flow, closed or partially closed valves, dirty strainers/filters, undersized radiators, or an overly steep heat curve.
Checks you can do:
- Confirm all isolation valves around the heat pump and cylinder coil are open.
- Check system pressure is around the installer’s recommended value (often ~1.0–1.5 bar when cold).
- Bleed obvious air from radiators and any air vents on the system.
- If safe to do so, clean any user-serviceable strainers/filters your installer has shown you.
Call an engineer if: The alarm reoccurs after basic flow/pressure checks or pressure drops repeatedly.
Operation: Compressor is temporarily blocked until conditions are safe.
Description: The return water entering the condenser has reached its maximum allowed temperature.
Checks you can do: Same as for alarm 162 – check valves, pressure, air in the system, and filters.
Call an engineer if: Alarms 162/163 keep coming back or radiators/underfloor loops don’t heat evenly.
Operation: Compressor may be blocked until temperatures fall.
Description: The pressure sensor on the heating circuit has detected pressure below the minimum set level.
Checks you can do:
- Read the system pressure gauge – typically the target is around 1.0–1.5 bar when cold (check your installer’s label).
- If trained to do so, top up the system via the filling loop to the labelled pressure.
- Inspect for obvious leaks on radiators, valves and safety discharge pipes.
Call an engineer if: Pressure keeps dropping, you see leaks, or you’re unsure how to safely top up.
Operation: Heat pump may stop to protect itself from running dry.
Description: The heat pump has temporarily reversed to defrost the outdoor coil. Heating is reduced for a few minutes while ice is melted.
Checks you can do:
- Make sure the outdoor unit has good drainage so meltwater can run away freely.
- Keep snow and debris clear of the base and air inlet/outlet grilles.
Call an engineer if: The unit spends long periods defrosting and never seems to catch up, or you see heavy ice build-up that doesn’t clear.
Operation: Normal – the message clears automatically once defrost is finished.
Description: The controller has detected conditions consistent with a blocked system filter and has raised a filter alarm.
Checks you can do:
- If you’ve been shown how, isolate and clean the system filters/strainers.
- Bleed air from radiators and manifolds.
Call an engineer if: You’re not comfortable cleaning filters, or the alarm returns quickly.
Operation: Heat output may be reduced; persistent filter alarms can lead to high-pressure shut-downs (e.g. alarm 220).
Description: The high-pressure protection has tripped repeatedly. The heat pump shuts down to prevent damage.
Very common causes:
- Low system water pressure.
- Closed or partially closed isolation valves or zone valves.
- Blocked or dirty strainers/filters in the heat pump or system.
- Failed or seized circulation/charge pump.
- Badly balanced or undersized heating emitters causing very low flow.
Checks you can do:
- Verify system pressure and top up to the installer’s marked value if you know how.
- Ensure all isolation and zone valves that should be open are fully open.
- Bleed obvious air from radiators and manifolds.
- Clean any user-serviceable strainers your installer has identified.
- Reset the alarm and observe whether it returns quickly.
Call an engineer urgently if: The alarm comes back after basic checks, or you suspect pump failure or heavy system sludge.
Operation: Compressor is blocked until the fault is cleared and alarm reset.
Description: The low-pressure switch or sensor has triggered. This is a refrigerant-side issue.
Possible causes: Refrigerant leak, restriction in the circuit, or very low outdoor air temperature combined with other issues.
User actions:
- Check outdoor coil and fan are not blocked by leaves, snow or debris.
- Reset once; do not keep repeatedly resetting.
Call an F-gas engineer: Low-pressure faults are usually refrigerant related and must be investigated professionally.
Operation: Compressor is blocked until an engineer clears the alarm.
Description: The indoor module (SMO/VVM etc.) cannot communicate with the outdoor unit, so the compressor is blocked.
Checks you can do:
- Confirm power is present to the outdoor unit (MCB on, no tripped breaker).
- Check any visible communication cable plugs at the indoor module haven’t been disturbed.
- Power off both indoor and outdoor units at isolators for a few minutes, then power back on.
Call an engineer if: The alarm returns; there may be a wiring, board or communication issue.
Operation: Compressor blocked until communication is restored.
Description: The outdoor fan has stopped or is drawing abnormal current, so the controller blocks the compressor.
Checks you can do (with power off):
- Visually check for objects stuck in the fan guard (branches, bags, ice).
- Do not remove covers or try to force the fan – this is engineer work.
Call an engineer: If nothing obvious is blocking the fan, there may be a motor or control fault.
Operation: Compressor blocked until the fan fault is resolved.
Description: The controller has detected abnormal conditions between flow and return, often pointing to low flow or a blocked circuit.
Checks you can do:
- Check system pressure, valves and filters as for alarm 220.
- Ensure all key heating zones are open so water can circulate.
Call an engineer if: The alarm persists or appears alongside 162/163/220.
Operation: Depending on model, compressor may be limited or blocked.
Description: A sensor fault has been detected in the outdoor unit (exact sensor is shown in the detailed alarm info or service menu).
User actions: Reset once; beyond that, sensor diagnosis is engineer-level work.
Call an engineer: If the alarm returns or you see several sensor alarms at once.
Description: The outdoor unit reports a problem completing defrost, often linked to sensors or refrigerant conditions.
Checks you can do: Keep the outdoor unit clear of snow/ice buildup and ensure drainage is not blocked.
Call an engineer: Repeated defrost faults need investigation of sensors, charge and expansion device.
Description: The compressor has tripped three times in a row shortly after starting, so the controller blocks it temporarily.
Possible causes: System flow issues, high-pressure trips, incorrect settings, or inverter problems.
Checks you can do: Review any associated alarms (162, 220 etc.), then follow their guidance.
Call an engineer: If 229 appears frequently – it’s usually a symptom of another underlying fault.
Description: The hot-gas/discharge temperature has remained above its limit and the compressor is blocked.
User checks: Same as for high-pressure alarms – confirm good system flow and clean filters.
Call an engineer: Persistent hot-gas alarms can indicate charge or expansion-valve issues.
Description: The heat pump has detected that the three-phase supply is in the wrong order. The compressor is blocked to prevent damage.
User action: Do not attempt to swap phases yourself. Contact the installer or electrician.
Description: The evaporating temperature is too low, often due to refrigerant charge or expansion issues.
User action: Keep the coil clear of snow/ice; then call an F-gas engineer.
Description: The display unit has lost communication with the base card; the compressor and charge pump usually stop.
User actions: Try a full power cycle at the isolators; if the alarm returns, contact an installer.
Description: In multi-unit systems, the master heat pump has lost communication with a slave.
User actions: Check power supplies and visible comms cables; then contact the installer if alarms persist.
Description: The controller reports a general communication problem, often between the indoor board and an accessory or inverter.
User actions: Power cycle; if it recurs, engineer investigation is needed.
Description: The heat exchanger has overheated and the inverter/controls have stopped the compressor.
User checks: Same as for alarm 220 – focus on water flow and filters.
Description: The inverter module has reported an over-temperature condition on its power electronics.
User actions: Ensure vents around the unit are clear and the area is well-ventilated; then call an engineer.
Description: The inverter drive has logged a permanent or repeated error. The detailed sub-code is usually visible in the alarm log and service manual.
User action: Do not repeatedly reset; contact an installer or NIBE service partner for inverter diagnostics.
Description: The controller has detected a mismatch or lost contact with the outdoor unit – commonly after a short power interruption.
Checks you can do:
- Confirm outdoor unit isolator and breakers are on.
- Power cycle both indoor and outdoor units if safe to do so.
Call an engineer: If the alarm persists there may be firmware, configuration or hardware compatibility issues.