523 on a NIBE heat pump: Low flow defrosting: Low flow during defrosting [other NIBE documents: The flow is low. Check particle filter and pump / Low flow for a certain time triggers alarm to cancel defrosting]
Short answer
On a NIBE heat pump, 523 means: Low flow defrosting: Low flow during defrosting [other NIBE documents: The flow is low. Check particle filter and pump / Low flow for a certain time triggers alarm to cancel defrosting]. 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: Low flow defrosting: Low flow during defrosting [other NIBE documents: The flow is low. Check particle filter and pump / Low flow for a certain time triggers alarm to cancel defrosting].
Part involved: evaporator defrost.
When it shows: Low flow during defrosting; The flow is low. Check particle filter and pump; Low flow for a certain time triggers alarm to cancel defrosting.
Likely causes:
- S2125: Clogged particle filter; defective circulation (charge) pump; pressure drop in climate system too large | S2060: Low temperature in the hot water tank; dirt in filters.
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 - Defrosting stops.
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. Check the outdoor unit is not blocked by leaves, snow or ice and that nothing obstructs the airflow.
What the engineer will check
- Check system volume and flow during defrost, particle filter, circulation pump, evaporator airflow, evaporator/defrost sensors and refrigerant circuit. NIBE lists: Check particle filter and circulation pump; Clogged particle filter; defective circulation (charge) pump; pressure drop in climate system too large; Low temperature in the hot water tank; dirt in filters.
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
- TECHNICAL INFORMATION - ALARM LIST SMO 40 (TIF EN 2042-1, M12658EN) ↗ · TIF EN 2042-1 M12658EN · page 20 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026