221 on a NIBE heat pump: Low pressure alarm: Heat pump (outdoor unit) sends low pressure fault message to the controller [other NIBE documents: Too low value on low pressure sensor 3 times within 60 min / Low-pressure sensor BP8 (PSL) detects abnormally low pressure]
Short answer
On a NIBE heat pump, 221 means: Low pressure alarm: Heat pump (outdoor unit) sends low pressure fault message to the controller [other NIBE documents: Too low value on low pressure sensor 3 times within 60 min / Low-pressure sensor BP8 (PSL) detects abnormally low pressure]. 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:
- Low pressure alarm: Heat pump (outdoor unit) sends low pressure fault message to the controller [other NIBE documents: Too low value on low pressure sensor 3 times within 60 min / Low-pressure sensor BP8 (PSL) detects abnormally low pressure].
- Defrost: The heat pump is defrosting (status alarm) [other NIBE documents: Not an alarm but an operating status / Defrosting in progress].
Part involved: pressure switches/sensors, expansion valve; defrost cycle.
When it shows: Heat pump (outdoor unit) sends low pressure fault message to the controller; Too low value on low pressure sensor 3 times within 60 min; Low-pressure sensor BP8 (PSL) detects abnormally low pressure; Too low value on the low pressure sensor 3 times within 60 minutes The heat pump is defrosting (status alarm); Not an alarm but an operating status; Defrosting in progress.
Likely causes:
- S2060: Open/short circuit on LP sensor input; defective LP sensor; defective baseboard AA2; open/short circuit or defective suction gas sensor BT17 (Tho-S); lack of refrigerant | F2050: Open/short circuit on LP sensor input; defective LP sensor; defective PWB1; open/short circuit or defective suction gas sensor Tho-S | F2040: Open/short circuit on LP sensor input; defective LP sensor; defective control board; open/short circuit or defective suction gas sensor Tho-S.
- S2060: Activated when the heat pump runs its defrosting procedure | F2050: Set when the heat pump runs the defrosting procedure.
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. No action needed: this is a normal operating status. Vapour/steam from the outdoor unit during defrost is normal.
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: Open/short circuit on LP sensor input; defective LP sensor; defective control board in outdoor unit; open/short circuit on suction gas sensor (Tho-S) input; Open/short circuit on LP sensor input; defective LP sensor; defective baseboard AA2; open/short circuit or defective suction gas sensor BT17 (Tho-S); lack of refrigerant; Open/short circuit on LP sensor input; defective LP sensor; defective PWB1; open/short circuit or defective suction gas sensor Tho-S; Open/short circuit on LP sensor input; defective LP sensor; defective control board; open/short circuit or defective suction gas sensor Tho-S.
- None unless defrosts are excessive; then check airflow and evaporator sensor. NIBE lists: Activated when the heat pump runs its defrosting procedure; Set when the heat pump runs the defrosting procedure.
Who to ask for: Heat pump engineer with F-gas qualification (NIBE-trained / NIBE PRO installer); NIBE-trained heat pump installer (NIBE PRO / VIP).
Certification: F-gas certification for refrigerant circuit work (plus flammable-refrigerant competence for R290/R32 models); 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 5 · Reviewed 23 September 2026
- Installer manual Air/water heat pump NIBE S2060 UK 1x230V IHB EN 2623-3 (831281) ↗ · IHB EN 2623-3 · page 30 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026