Nibe 159 Fault Code: Causes, Fixes & Repair Costs
What does the Nibe 159 fault code mean?
Alarm 159 on a NIBE heat pump means high evaporator temperature: sensor BT16 has read the evaporator above 50°C — far beyond anything normal operation produces — and the unit has stopped the compressor and discontinued any defrost in progress. An evaporator that hot points at one of two stories: something genuinely abnormal in the refrigerant circuit or its operating mode (a defrost or reversing-valve situation gone wrong, since defrosting deliberately runs the evaporator warm), or a BT16 sensor lying. Because the honest-fault possibilities involve the sealed refrigerant circuit, diagnosis belongs with an engineer; the sensor check comes first, being quick and cheap.
General guidance only — not a substitute for professional advice, and not certified by an heat pump engineer. Always have your specific appliance assessed by a qualified professional. Any heat pump work must be carried out by an MCS-certified heat pump engineer. If you notice a refrigerant leak, a burning smell or water leaking from the unit, switch it off at the isolator and call an MCS-certified heat pump engineer. Refrigerant work must be carried out by an F-Gas registered engineer.
Common causes
- BT16 sensor misreading Common
A drifted or failing evaporator sensor reporting phantom heat is the cheap explanation, and checked first against the resistance table.
- Defrost or reversing valve anomaly Sometimes
Defrosting intentionally warms the evaporator; a reversing valve sticking or a defrost failing to terminate can push it past the 50°C limit.
- Refrigerant circuit fault Sometimes
Abnormal pressures or flows heating the evaporator beyond design — F-Gas engineer territory once the sensor and valve are proven.
How to fix it
- Acknowledge the alarm once and observe DIY safe
On the display, select the alarm and choose to reset/acknowledge it (or use 'aux. operation' if offered, which keeps some heating running). One reset is legitimate; if the alarm returns, leave it for an engineer rather than resetting repeatedly.
- Have a suitably qualified heat pump engineer complete the documented checks heat pump engineer
The engineer will verify BT16's accuracy first, then watch the unit through a defrost cycle for reversing-valve behaviour, and investigate the refrigerant circuit only if the sensor and valve are proven — the correct cheap-to-expensive order. Any work on the sealed refrigerant circuit legally requires an F-Gas registered engineer; a NIBE-accredited (NIBE Pro) installer is the natural choice.
Parts you may need
- BT16 evaporator temperature sensor · from £30
- Reversing (4-way) valve · from £200
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £100–£450, depending on the underlying cause.
Frequently asked questions
What does alarm 159 mean on my NIBE?
The evaporator sensor (BT16) read above 50°C — abnormally hot — so the compressor stopped and any defrost was abandoned. Either the sensor is misreading (checked first, cheap) or something in the defrost/refrigerant side genuinely overheated the evaporator (the engineer's investigation).
How is this different from alarm 16?
Alarm 16 means BT16 has failed electrically — no valid reading at all. Alarm 159 means BT16 gave a reading, and it was alarmingly high. Confusingly, a drifting sensor can cause either; the resistance check tells them apart.
Does it happen around defrost cycles?
That timing is diagnostic gold — defrosting deliberately warms the evaporator, so a 159 during or just after defrost points at the reversing valve or defrost termination rather than the sensor. Note when it happens and tell the engineer.