253 on a NIBE heat pump: Sensor fault QZ1-BT70: Sensor not connected/defective (hot water flow, hot water comfort)
Short answer
On a NIBE heat pump, 253 means: Sensor fault QZ1-BT70: Sensor not connected/defective (hot water flow, hot water comfort). The heat pump keeps running with some functions stopped. An engineer is needed to fix the cause.
Reviewed 23 September 2026 · 2 sources
What it means
The manuals for this range word it in different ways:
- Sensor fault QZ1-BT70: Sensor not connected/defective (hot water flow, hot water comfort).
- The inverter has detected an abnormally high current: Abnormally high current detected by the inverter [other NIBE documents: Voltage from the inverter outside parameters four times within 30 minutes / Incorrect voltage from the inverter].
Part involved: temperature/pressure sensor; inverter and compressor drive.
When it shows: Sensor not connected/defective (hot water flow, hot water comfort) Abnormally high current detected by the inverter; Voltage from the inverter outside parameters four times within 30 minutes; Incorrect voltage from the inverter.
Likely causes:
- S2060: Communication faults between inverter and control unit | F2050: Incoming power supply interference; service valve closed; insufficient refrigerant; compressor fault; defective inverter board.
Is it safe?
| Does the heat pump stop? | Partly |
|---|---|
| Severity | Fault — needs a manual reset once the cause is fixed |
| Engineer needed? | Yes |
The heat pump keeps running with some functions stopped.
Resetting itself: no/unknown - Mixing valve closes.
Manual reset: Reset from the controller alarm menu ('reset alarm' / 'Reset alarm and try again'); alarm recurs if the cause remains.
What you can check
- Read the alarm text and the 'info/action' help on the controller display (VVM/SMO or S-series smartguide). NIBE's manuals let the homeowner select 'reset alarm' ('Reset alarm and try again' on S-series); if the lamp returns to normal the alarm has cleared. If it comes back, do not keep resetting: select 'aid mode' ('Auxiliary operation' on S-series) if offered, so any electric additional heat keeps heat/hot water going, and contact your installer.
- Read the alarm text and the 'info/action' help on the controller display (VVM/SMO or S-series smartguide). NIBE's manuals let the homeowner select 'reset alarm' ('Reset alarm and try again' on S-series); if the lamp returns to normal the alarm has cleared. If it comes back, do not keep resetting: select 'aid mode' ('Auxiliary operation' on S-series) if offered, so any electric additional heat keeps heat/hot water going, and contact your installer. Check the heat pump and indoor unit breakers/RCD at the consumer unit are on; if one trips again, leave it off and call an engineer.
What the engineer will check
- Check sensor and its connections/wiring against the wiring diagram; measure sensor resistance against the NIBE temperature/resistance table; check the relevant PCB input.
- Check main and group fuses and cable connections, supply voltage under load, inverter terminal blocks, inverter mounting/cooling paste, compressor wiring and compressor. NIBE lists: Communication faults between inverter and control unit; Incoming power supply interference; service valve closed; insufficient refrigerant; compressor fault; defective inverter board.
Who to ask for: NIBE-trained heat pump installer (NIBE PRO / VIP); NIBE-trained heat pump engineer and/or qualified electrician.
Certification: Electrical work to BS 7671 by a competent person; Electrical work to BS 7671 by a competent electrician (notifiable work under Part P in England & Wales).
What you might notice
Sources
- ALARM LIST - NIBE products with Emmy display, Version 1747-1 (updated 2017-11-23) ↗ · 1747-1 · page 7 · Reviewed 23 September 2026
- Installer manual Air/water heat pump NIBE S2060 UK 1x230V IHB EN 2623-3 (831281) ↗ · IHB EN 2623-3 · page 31 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026