343 on a NIBE heat pump: Low temp water out: Low temperature out during cooling operation [other NIBE documents: Low temperature out / Supply temperature BT12 out of condenser below minimum]
Short answer
On a NIBE heat pump, 343 means: Low temp water out: Low temperature out during cooling operation [other NIBE documents: Low temperature out / Supply temperature BT12 out of condenser below minimum]. The heat pump keeps running with some functions stopped.
Reviewed 23 September 2026 · 1 source
What it means
The manuals for this range word it in different ways:
- Low temp water out: Low temperature out during cooling operation [other NIBE documents: Low temperature out / Supply temperature BT12 out of condenser below minimum].
- Inverter alarm type III: Not used function (false alarm).
Part involved: heating medium flow, charge pump, particle filter; inverter and compressor drive.
When it shows: Low temperature out during cooling operation; Low temperature out; Supply temperature BT12 out of condenser below minimum Not used function (false alarm).
Likely causes:
- F2120: Low system temperature; defective return sensor | S2060: Low flow; incorrect set values during cooling operation.
Is it safe?
| Does the heat pump stop? | Partly |
|---|---|
| Severity | Fault — clears by itself when the condition goes |
The heat pump keeps running with some functions stopped.
Resetting itself: yes - Temporarily stops compressor.
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 heating system pressure gauge and check that radiator/underfloor thermostats are not all closed; check heating and hot water temperature settings have not been changed.
- 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 heating medium flow (pump speed/settings, particle filter, air in system), heating curve and hot water settings, correct sensor placement and flow/return connections. NIBE lists: Flow-related; incorrect settings; Low system temperature; defective return sensor; Low flow; incorrect set values during cooling operation.
- Check main and group fuses and cable connections, supply voltage under load, inverter terminal blocks, inverter mounting/cooling paste, compressor wiring and compressor.
Who to ask for: NIBE-trained heat pump installer (NIBE PRO / VIP); NIBE-trained heat pump engineer and/or qualified electrician.
Certification: None specific beyond competent heat pump installer; electrical work to BS 7671; Electrical work to BS 7671 by a competent electrician (notifiable work under Part P in England & Wales).
What you might notice
Sources
- TECHNICAL INFORMATION - ALARM LIST SMO 40 (TIF EN 2042-1, M12658EN) ↗ · TIF EN 2042-1 M12658EN · page 17 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026