Nibe 228 Fault Code: Causes, Fixes & Repair Costs
What does the Nibe 228 fault code mean?
Alarm 228 on a NIBE system is a defrost fault: the outdoor unit has reported to the indoor controller that its defrost function failed, and the compressor is blocked. Air source outdoor units frost their coil in normal UK winter operation and reverse their cycle periodically to melt it; when that process fails — cycles not completing, not achieving a clear coil, or faulting mid-way — the unit reports it rather than ice up progressively. Causes span the reversing (4-way) valve that redirects refrigerant for defrost, the defrost sensor timing the cycles, drainage failures letting meltwater refreeze, and refrigerant-side shortfalls robbing the defrost of heat. It is the split-system sibling of NIBE's alarm 83 (unsuccessful defrosting).
General guidance only — not a substitute for professional advice, and not certified by an heat pump engineer. Always have your specific appliance assessed by a qualified professional. Any heat pump work must be carried out by an MCS-certified heat pump engineer. If you notice a refrigerant leak, a burning smell or water leaking from the unit, switch it off at the isolator and call an MCS-certified heat pump engineer. Refrigerant work must be carried out by an F-Gas registered engineer.
Common causes
- Meltwater refreezing / drainage failure Common
Ice building beneath and into the coil because defrost water cannot drain away defeats every cycle — the homeowner-visible cause, common in prolonged freezes.
- Reversing valve or defrost sensor fault Sometimes
A sticking 4-way valve stops the cycle reversing properly; a drifted sensor mistimes it — the two classic mechanical causes.
- Low refrigerant charge Sometimes
A defrost cycle short of refrigerant lacks the heat to clear the coil — F-Gas engineer territory, often revealed by poor heating performance beforehand.
How to fix it
- Check the outdoor unit's ice and drainage DIY safe
Look for ice sheeting under the unit or encasing the coil, and clear snow or debris from around it. Do not chip ice off the fins — address drainage and let the repaired unit clear itself.
- Acknowledge the alarm once and observe DIY safe
Reset/acknowledge the alarm on the indoor display a single time. If it returns, stop resetting — the outdoor unit is reporting a genuine condition and repetition only stresses the compressor.
- Have a suitably qualified heat pump engineer complete the documented checks heat pump engineer
The engineer will watch a commanded defrost cycle end-to-end — reversing valve operation, sensor readings, termination — check the drainage arrangement, and investigate the refrigerant charge if cycles genuinely lack heat. Any work on the sealed refrigerant circuit legally requires an F-Gas registered engineer; a NIBE-accredited (NIBE Pro) installer is the natural choice.
Parts you may need
- Reversing (4-way) valve · from £200
- Defrost/coil sensor · from £30
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £100–£500, depending on the underlying cause.
Frequently asked questions
What does alarm 228 mean on my NIBE?
The outdoor unit's defrost function failed and the compressor is blocked. Check the unit is not iced into a block and that meltwater can drain; mechanically, the usual suspects are the reversing valve that redirects refrigerant for defrost and the sensor that times the cycles — an engineer proves each in turn.
Is frost on the coil the fault?
No — frosting and routine self-defrosting are normal winter behaviour. The alarm marks defrosting attempted and failed. Solid ice sheeting under or into the unit is the abnormal sign worth reporting.
Why does it always strike in the coldest week?
Defrost demand peaks exactly when marginal components are working hardest — a slightly sticky valve or slightly low charge that copes in November fails in a January freeze. The seasonal pattern is typical and still means a real underlying cause.