Nibe 55 Fault Code: Causes, Fixes & Repair Costs
What does the Nibe 55 fault code mean?
Fault code 55 on a Nibe heat pump is a Hot Gas Alarm. The unit's controls have detected that the discharge (hot gas) temperature leaving the compressor has climbed beyond its permitted limit. As a safety measure, the compressor is immediately shut down and locked out, and the alarm requires a manual reset before the system will attempt to run again. A single occurrence can sometimes be cleared with basic checks and a reset, but repeated triggering of this alarm — especially three or more times within a four-hour window — strongly indicates an underlying problem in the refrigerant circuit that needs professional diagnosis.
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 charge or circuit restriction Common
A low refrigerant charge caused by a slow leak, or a partial blockage elsewhere in the sealed refrigerant circuit, forces the compressor to work harder than intended. This causes discharge temperatures to spike rapidly and exceed the alarm threshold. This is the most frequently diagnosed root cause when fault 55 recurs.
- Faulty or sticking expansion valve Common
The expansion valve regulates refrigerant flow through the circuit. If it sticks, drifts out of calibration, or fails to modulate correctly, the refrigerant balance is disrupted and the compressor outlet temperature can rise sharply. This is a common finding on older or heavily-cycled Nibe systems.
- Low heating-system flow or water pressure Common
If the heating circuit pressure has dropped below around 1.0 bar, or if zone valves, isolation valves, or a blocked strainer are restricting water flow through the heat exchanger (condenser), heat cannot transfer efficiently. The compressor then overheats trying to push heat it cannot shed, triggering the hot gas alarm.
- Blocked or obstructed outdoor unit Sometimes
Leaves, compacted snow, ice, or other debris covering the outdoor coil or fan reduces the airflow needed for heat exchange. Restricted airflow causes operating pressures and temperatures to rise inside the refrigerant circuit, potentially pushing discharge temperatures to the alarm limit.
- Compressor deterioration Rare
A compressor that is mechanically worn or has developing electrical faults may run inefficiently, generating excessive heat during operation. If fault 55 appears alongside unusual noises, higher-than-normal energy consumption, or a gradual decline in heating output over time, compressor condition should be assessed by an engineer.
How to fix it
- Carry out a visual check of the outdoor unit DIY safe
Walk around the outdoor unit and check that the coil fins and fan are not blocked by leaves, garden debris, compacted snow or ice. If safe to do so, gently clear any obvious obstructions from around the unit — do not poke anything into the fan or touch the fins directly. Ensure there is at least one metre of clear space around the unit for airflow.
- Check heating-circuit pressure at the pressure gauge DIY safe
Locate the pressure gauge on the indoor pipework (usually near the heat pump or buffer vessel). The system should read between 1.0 and 1.5 bar when cold. If it reads below 1.0 bar, top up via the filling loop until the pressure is back in range, then close the filling loop valve. If the system cannot hold pressure or drops again quickly, report this to an engineer as it may indicate a leak.
- Confirm all valves and zone valves are fully open DIY safe
Check that all isolation valves on the flow and return pipework around the heat pump and cylinder coil are fully open (lever parallel to pipe). Ensure any zone valves or manifold valves serving the heating zones are in the open position. A partially closed valve is a common cause of restricted flow that can contribute to this alarm.
- Bleed radiators or underfloor manifolds if air is suspected DIY safe
Trapped air in the heating circuit reduces flow and heat transfer efficiency. Using a radiator bleed key, release any air from the highest radiators or bleed points on the manifold. Recheck system pressure after bleeding and top up if needed.
- Manually reset the alarm via the Nibe controller DIY safe
On the Nibe display module, navigate to the alarm message and select the option to view more information. Then choose 'Reset alarm and try again.' The controller will restart and the heat pump will attempt to return to normal operation. Monitor it closely over the next hour. If the alarm clears and does not return, continue to watch the system over the following days.
- If the alarm returns, do not continue resetting — call an MCS-certified, F-Gas registered engineer heat pump engineer
A hot gas alarm that recurs — particularly if it triggers multiple times within a few hours — means the root cause has not been resolved and the compressor is being exposed to damaging conditions. Continued resets without diagnosis risk causing permanent compressor damage. An F-Gas registered engineer must attend to carry out refrigerant circuit pressure testing, check the expansion valve operation, and diagnose the fault properly. Refrigerant top-up, expansion valve replacement, or compressor assessment can only be carried out by an F-Gas qualified technician.
Parts you may need
- Electronic expansion valve (EEV) assembly · from £280
- Refrigerant recharge (F-Gas labour and refrigerant) · from £180
- System pressure filling loop / Schrader valve · from £35
- Nibe display module / controller (SMO) · from £320
- Compressor (model-dependent) · from £1200
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 £180–£550, depending on the underlying cause.
Frequently asked questions
Can I reset Nibe fault code 55 myself, or do I always need an engineer?
You can attempt a single manual reset after completing basic checks — clearing the outdoor unit, confirming system pressure is correct, and ensuring all valves are open. If the alarm clears and does not come back, you may have resolved a temporary flow or pressure issue. However, if fault 55 returns — especially more than once in a short period — do not keep resetting it. Repeated compressor shutdowns without fixing the cause can cause permanent damage. At that point an F-Gas registered engineer must diagnose the refrigerant circuit.
What does the hot gas temperature actually measure, and why does it matter?
The hot gas temperature is the temperature of the refrigerant as it leaves the compressor on its way to the condenser (heat exchanger). In normal operation this temperature stays within a safe range. If it rises too high — due to a low refrigerant charge, restricted flow, or a stuck expansion valve — it means the compressor is under severe stress. Nibe's controls shut the compressor down automatically to prevent internal damage. Ignoring this alarm and forcing the system to run could result in a compressor failure, which is a significantly more expensive repair.
How much does it typically cost to fix Nibe fault code 55 in the UK?
For the most common causes — a refrigerant recharge or an expansion valve replacement — most UK homeowners pay in the range of £180 to £550 including parts, refrigerant, and labour. An engineer call-out and diagnosis alone typically costs £80–£150. If the root cause turns out to be compressor damage, costs rise considerably: compressor repair is typically £800–£2,500 and full compressor replacement can exceed £3,500 depending on the model. Always check your Nibe warranty status first — Nibe generally provides 24 months parts and labour cover from installation — as this may cover the repair at no cost to you.
Could a cold snap or very low outdoor temperatures cause fault code 55?
Extreme cold on its own rarely causes fault 55, but very low outdoor temperatures combined with another marginal issue — such as a slightly low refrigerant charge or a borderline expansion valve — can push the system over the threshold. If your unit triggered fault 55 during an unusually cold period and has not repeated since, it is still worth having an engineer check the refrigerant charge at the next service. Consistent reliable operation in cold weather is a sign of a correctly charged, well-maintained system.