234 on a NIBE heat pump: Flow fault: Return is hotter than flow [other NIBE documents: Heat pump (selected outdoor unit) sends fault message to controller]
Short answer
On a NIBE heat pump, 234 means: Flow fault: Return is hotter than flow [other NIBE documents: Heat pump (selected outdoor unit) sends fault message to controller]. The heat pump keeps running with some functions stopped. An engineer is needed to fix the cause.
Reviewed 23 September 2026 · 1 source
What it means
The manual says: Flow fault: Return is hotter than flow [other NIBE documents: Heat pump (selected outdoor unit) sends fault message to controller].
Part involved: heating medium flow, charge pump, particle filter.
When it shows: Return is hotter than flow; Heat pump (selected outdoor unit) sends fault message to controller.
Likely causes:
- S2125: Supply and return line connections switched around | F2120: Supply and return line connections switched around.
Is it safe?
| Does the heat pump stop? | Partly |
|---|---|
| Engineer needed? | Yes |
The heat pump keeps running with some functions stopped.
Resetting itself: no/unknown - Compressor blocked.
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.
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: Supply and return line connections switched around.
Who to ask for: NIBE-trained heat pump installer (NIBE PRO / VIP).
Certification: None specific beyond competent heat pump installer; electrical work to BS 7671.
What you might notice
Sources
- Installer manual Air/water heat pump NIBE S2125 UK 1x230V IHB EN 2525-1 (831874) ↗ · IHB EN 2525-1 · page 41 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026