Nibe 51 Fault Code: Causes, Fixes & Repair Costs
What does the Nibe 51 fault code mean?
Fault code 51 indicates that the refrigerant circuit pressure has dropped below an acceptable threshold. On the F1145, this is detected by a low-pressure sensor reading that falls beneath its cut-off value; on other Nibe models, a low-pressure switch triggers the alarm directly. In both cases the result is the same: the compressor locks out and will not restart until the fault has been manually reset from the controller. This is a refrigerant-side alarm and should be taken seriously — running a heat pump with low refrigerant pressure risks damaging the compressor, which is the most expensive component in the system.
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 leak or low charge Common
The most frequent cause across all Nibe heat pump types. If refrigerant has escaped — even a small amount over time — circuit pressure drops and the low-pressure switch or sensor trips. A leak can develop at brazed joints, service valves or within the heat exchanger. Only an F-Gas registered engineer can locate a leak, carry out a repair and recharge the system legally.
- Blocked or restricted ground loop filter valve (GSHP only) Common
On ground-source systems, a partially or fully blocked filter valve on the brine (ground loop) side restricts flow enough to cause suction pressure to fall. These filter valves should be inspected and cleaned as part of an annual service but are often overlooked. Check whether the valve has been serviced recently; if not, this is a likely culprit.
- Low glycol level or ground loop pressure loss (GSHP only) Sometimes
Ground-source systems typically run the brine loop at around 1 bar. If the glycol/brine mixture level in the expansion vessel has dropped — due to a slow leak in the ground loop pipework or inadequate filling — heat transfer to the refrigerant circuit is reduced and low pressure can result. If you find yourself topping up the ground loop regularly, this points to a pipework leak that needs tracing.
- Inadequate airflow across the outdoor unit (ASHP only) Sometimes
A severely restricted outdoor unit — blocked by compacted snow, ice, accumulated leaf debris or a nearby obstruction — can starve the evaporator of air, causing the refrigerant suction pressure to fall rapidly, especially in colder weather. Clearing the obstruction and resetting once is a safe first step for the homeowner.
- Defrost system fault or sensor error (exhaust air systems) Rare
On exhaust-air heat pumps, if the defrost cycle is not operating correctly — perhaps due to a faulty defrost sensor or a disconnected pressure switch — ice can build up on the evaporator, restricting refrigerant evaporation and causing pressure to drop. An engineer will need to inspect the defrost controls and associated sensors.
- Faulty low-pressure switch or sensor Rare
The switch or sensor itself can fail in a tripped position, generating a spurious alarm even when refrigerant charge and system pressures are normal. This is a component-level diagnosis that requires an engineer to verify pressures with calibrated gauges before condemning the sensor.
How to fix it
- Do not repeatedly reset the fault DIY safe
A single manual reset from the Nibe controller or display panel is reasonable as a first step. However, if the alarm returns immediately or within a short time, do not keep resetting — repeated cycling with low refrigerant pressure accelerates compressor wear and can turn a moderate repair into a very costly one.
- Check for obstructions around the outdoor unit (ASHP) DIY safe
Walk around the outdoor unit and clear any snow, ice, leaves or debris that may be blocking the inlet or outlet grilles. Ensure there is adequate clearance on all sides. Do not use sharp tools to chip away ice — allow it to melt naturally or pour lukewarm (not boiling) water carefully. Do not touch the fan blades or reach inside the unit.
- Check ground loop pressure and glycol level (GSHP) DIY safe
Locate the brine circuit pressure gauge and expansion vessel — usually in the plant room or utility space near the indoor unit. If pressure is noticeably below 1 bar, note this and report it to your engineer. Do not attempt to top up the brine loop yourself unless you have been specifically trained and instructed to do so by your installer, as the mixture ratio must be correct.
- Attempt a single manual reset from the controller DIY safe
Navigate to the alarm or info menu on your Nibe display, acknowledge the fault code 51 and attempt a reset. Allow the system a few minutes to restart. If the alarm clears and the system runs stably, monitor it closely over the next 24–48 hours. If the fault returns, move on to the next step without resetting again.
- Inspect and clean the ground loop filter valve (GSHP — engineer or competent installer) heat pump engineer
If you have a ground-source system and the filter valve has not been serviced recently, an engineer or your original installer should isolate, inspect and clean this component. It is a mechanical task on the brine side rather than the refrigerant circuit, but it should still be carried out by someone familiar with the system to avoid air ingress or pressure loss.
- Have an F-Gas registered engineer check refrigerant pressure and inspect for leaks heat pump engineer
An F-Gas registered engineer will connect calibrated manifold gauges to the refrigerant circuit to measure actual suction and discharge pressures. If pressures confirm a low charge, they will perform a leak search using electronic detection equipment or UV dye, repair any identified leak, pressure-test the repair and then recharge the system with the correct refrigerant type and quantity. This work is a legal requirement for qualified personnel only — it cannot be performed by a homeowner or an unqualified tradesperson.
- Have the low-pressure switch or sensor tested and replaced if faulty heat pump engineer
If refrigerant pressures are found to be within normal limits, the engineer will assess whether the low-pressure switch or sensor has failed electrically. They will check wiring connections, continuity and sensor readings. A replacement switch or sensor is a relatively straightforward repair once the root cause is confirmed.
- Contact Nibe or an MCS-certified Nibe Approved Repair Partner heat pump engineer
If you are unsure where to start, Nibe operates a dedicated homeowner care line (0330 311 2201, option 1) and can sometimes assist with diagnosis over the phone or direct you to an approved repair partner. MCS-certified engineers who are Nibe-trained will have access to the correct technical documentation and genuine parts for your model.
Parts you may need
- Low-pressure switch (refrigerant circuit) · from £35
- Low-pressure sensor (F1145 and similar models) · from £55
- Brine circuit filter valve / strainer · from £45
- Refrigerant recharge (R410A or R32, typical quantity) · from £120
- Brine circuit expansion vessel · from £80
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 £200–£600, depending on the underlying cause.
Frequently asked questions
Can I just keep resetting fault code 51 and carrying on?
No — and this is important. Each time the compressor attempts to start with insufficient refrigerant pressure, it runs under stress and risks overheating or seizing. One reset to see whether the fault is transient (for example, caused by a temporary freeze in very cold weather) is reasonable. If the alarm returns, further resets without an investigation will likely make the eventual repair significantly more expensive.
How much does it cost to fix a refrigerant leak on a Nibe heat pump in the UK?
For the majority of refrigerant-related low-pressure faults — a minor leak at a joint, a recharge and a leak repair — most UK homeowners pay in the region of £200 to £600 including the engineer's call-out and labour. If the leak has caused compressor damage, or if the heat exchanger itself has failed and needs replacing, costs can rise considerably beyond this range. Acting quickly when the alarm first appears gives you the best chance of a lower-cost outcome.
My Nibe is a ground-source heat pump — does fault code 51 mean the same thing?
The fault code means the same thing — low refrigerant pressure — but on a ground-source system there are additional causes to rule out first, particularly a blocked brine filter valve or a drop in glycol level on the ground loop. These are worth checking before assuming the refrigerant circuit is at fault, as they are simpler and cheaper to address. Your engineer should systematically check both the brine side and the refrigerant side.
How often should a Nibe heat pump be serviced to prevent faults like this?
Nibe recommends annual servicing by a qualified engineer. A proper annual service will include a check of refrigerant pressures, inspection of the brine circuit (on GSHP), cleaning of the filter valve, and a check of system pressure — all of which directly help to catch the conditions that lead to fault code 51 before they develop into a lockout. Annual service costs in the UK typically run from around £150 to £300 depending on the installer and system type.