266 on a NIBE heat pump: Low refrigerant amount (generated by heat pump): Low refrigerant amount detected [other NIBE documents: Insufficient refrigerant detected at start-up in cooling operation (BT15 must drop 4 K vs BT3 within 1 min) / Not enough refrigerant detected on start-up in cooling mode]
Short answer
On a NIBE heat pump, 266 means: Low refrigerant amount (generated by heat pump): Low refrigerant amount detected [other NIBE documents: Insufficient refrigerant detected at start-up in cooling operation (BT15 must drop 4 K vs BT3 within 1 min) / Not enough refrigerant detected on start-up in cooling mode]. 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:
- Low refrigerant amount (generated by heat pump): Low refrigerant amount detected [other NIBE documents: Insufficient refrigerant detected at start-up in cooling operation (BT15 must drop 4 K vs BT3 within 1 min) / Not enough refrigerant detected on start-up in cooling mode].
- Com. error Acc.-EQ1: Permanent communication error ACS310.
Part involved: pressure switches/sensors, expansion valve; PCB / accessory board communication.
When it shows: Low refrigerant amount detected; Insufficient refrigerant detected at start-up in cooling operation (BT15 must drop 4 K vs BT3 within 1 min); Not enough refrigerant detected on start-up in cooling mode; Insufficient refrigerant detected on start-up in cooling mode Permanent communication error ACS310.
Likely causes:
- F2050: Service valve closed; loose or defective sensor BT15/BT3; too little refrigerant | F2040: Service valve closed; loose or defective sensor BT15/BT3; too little refrigerant.
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 breakers for the heat pump, indoor module and any accessories are on at the consumer unit.
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: Service valve closed; loose or defective sensor BT15/BT3; insufficient refrigerant; Service valve closed; loose or defective sensor BT15/BT3; too little refrigerant.
- Check communication cables and connectors between PCBs/accessory boards, DIP-switch addressing, power supply to the accessory board, and that accessories activated in the menu are actually fitted.
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); Electrical work to BS 7671 by a competent person.
What you might notice
Sources
- TECHNICAL INFORMATION - ALARM LIST SMO 40 (TIF EN 2042-1, M12658EN) ↗ · TIF EN 2042-1 M12658EN · page 7 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026