Nibe 221 Fault Code: Causes, Fixes & Repair Costs
What does the Nibe 221 fault code mean?
Alarm 221 on a NIBE system is a low pressure alarm: the outdoor unit has told the indoor controller that refrigerant pressure on the suction side fell below its safe limit, and the compressor is blocked. Low suction pressure means the evaporator is not absorbing enough heat — the classic causes being refrigerant loss (a leak reducing the charge), severe evaporator icing or airflow restriction stopping heat getting in, or expansion valve and sensor faults. Because the leading suspect involves the sealed circuit, alarm 221 is more engineer-bound than its high-pressure sibling 220: leak-testing, repairing and recharging refrigerant is strictly F-Gas registered work.
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
- Refrigerant loss Common
A leak lowers the charge until suction pressure trips the switch — the leading cause of recurring low-pressure alarms, remedied by leak-test, repair and a weighed recharge.
- Evaporator starved of heat Common
A coil blocked with ice, snow or debris — or a failed fan — stops the evaporator absorbing heat and drags suction pressure down; the homeowner-visible version.
- Expansion valve or sensor fault Sometimes
A faulty expansion valve or a lying pressure sensor produces the same trip with a correct charge — the engineer's differential diagnosis.
How to fix it
- Look at the outdoor unit DIY safe
Check the coil and grilles for ice, snow or debris and confirm the fan spins when running. A visibly blocked unit is the one cause you can fix — clear it gently, never chipping ice from the fins.
- 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 check airflow and the fan, then measure the refrigerant pressures — and if the charge is low, leak-test, repair and recharge to specification, which is legally F-Gas work. 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
- Refrigerant leak repair and recharge · from £250
- Low pressure switch/sensor · from £60
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 £120–£600, depending on the underlying cause.
Frequently asked questions
What does alarm 221 mean on my NIBE?
The outdoor unit's refrigerant pressure fell too low on the suction side and the compressor is blocked. The evaporator was not absorbing enough heat — commonly a low refrigerant charge (a leak) or a blocked/iced coil. Clear any visible blockage; the refrigerant side needs an F-Gas registered engineer.
Does low pressure mean the refrigerant has run out?
Often it means some has leaked — heat pumps do not consume refrigerant, so a low charge implies a leak that must be found and fixed, not just topped up. Proper practice is leak-test, repair, then recharge to the prescribed weight.
It only happens in very cold weather — significant?
Yes — cold weather lowers suction pressure naturally, so a marginally low charge or restricted coil that passes in autumn trips in January. The winter-only pattern usually still means a real underlying cause; mention it to the engineer.