344 on a NIBE heat pump: Recurring low pressure: Alarm from outdoor unit (repeated low pressure) [other NIBE documents: BP8 below limit, 5 low pressure alarms within 4 hours / 5 repeated low pressure alarms within 4 hours]
Short answer
On a NIBE heat pump, 344 means: Recurring low pressure: Alarm from outdoor unit (repeated low pressure) [other NIBE documents: BP8 below limit, 5 low pressure alarms within 4 hours / 5 repeated low pressure alarms within 4 hours]. 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 manuals for this range word it in different ways:
- Recurring low pressure: Alarm from outdoor unit (repeated low pressure) [other NIBE documents: BP8 below limit, 5 low pressure alarms within 4 hours / 5 repeated low pressure alarms within 4 hours].
- Inverter alarm type III: Not used function (false alarm).
Part involved: pressure switches/sensors, expansion valve; inverter and compressor drive.
When it shows: Alarm from outdoor unit (repeated low pressure); BP8 below limit, 5 low pressure alarms within 4 hours; 5 repeated low pressure alarms within 4 hours Not used function (false alarm).
Likely causes:
- F2120: Wind effect; refrigerant leak; defective expansion valve | F2120: Lack of refrigerant; blocked expansion valve; disruption in refrigerant circuit | S2125: Lack of refrigerant; blocked expansion valve; disruption in refrigerant circuit.
Is it safe?
| Does the heat pump stop? | Partly |
|---|---|
| Severity | Safety lockout |
| 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. Air source systems: check the outdoor unit is clear of leaves/snow/ice. Read the heating system pressure gauge and note the reading for 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 heating medium flow, particle filter and circulation pump; check pressure switches/sensors, expansion valve, service valves and refrigerant charge (F-gas qualified). NIBE lists: See outdoor unit manual; Wind effect; refrigerant leak; defective expansion valve; Lack of refrigerant; blocked expansion valve; disruption in refrigerant circuit.
- 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: Heat pump engineer with F-gas qualification (NIBE-trained / NIBE PRO installer); NIBE-trained heat pump engineer and/or qualified electrician.
Certification: F-gas certification for refrigerant circuit work (plus flammable-refrigerant competence for R290/R32 models); 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 9 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026