Nibe 40 Fault Code: Causes, Fixes & Repair Costs
What does the Nibe 40 fault code mean?
Nibe fault code 40 has two distinct meanings depending on your model. On ground-source and older systems, code [40] indicates that the voltage on compressor phase 1 has dropped below 160 V for more than 30 minutes — the compressor is blocked and will not run until the phase supply is restored and the alarm is reset. On air-source models using the EB101 outdoor unit (e.g. the F2040 range), the same number appears as part of alarm 220 displayed as E:40, which is a high-pressure alarm caused by restricted water flow through the heat exchanger. Both variants lock out the compressor to prevent damage and need to be investigated before you attempt a reset.
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
- Low or unstable mains supply voltage (ground-source / older models) Common
The electrical supply to the compressor has fallen below 160 V and stayed there for over 30 minutes. This can happen during periods of low grid voltage, after a brownout, or because the supply cable to the unit is undersized or degraded.
- Restricted water flow causing high refrigerant pressure (air-source EB101 models) Common
Insufficient flow through the waterside of the plate heat exchanger causes refrigerant pressure to build until the high-pressure switch trips. Typical culprits are a blocked strainer or magnetic filter, a closed or partially closed isolation valve, a seized or failing circulation pump, or an airlock in the pipework.
- Low system water pressure Common
If the heating circuit pressure has dropped below around 1 bar, flow through the heat exchanger becomes inadequate and high-pressure conditions can develop quickly. Pressure loss is often caused by a slow leak or insufficient inhibitor maintenance causing corrosion.
- Loose, corroded or damaged wiring to the compressor Sometimes
Vibration over time can loosen terminal connections inside the unit, and moisture ingress can corrode contacts. Either can cause intermittent or complete loss of phase voltage at the compressor terminals even when the incoming supply is healthy.
- Tripped MCB or blown fuse in the consumer unit or unit isolator Sometimes
A momentary overload or a developing fault on the compressor can cause the relevant overcurrent device to trip. The compressor then sees a missing phase until the MCB is reset and the underlying cause identified.
- Faulty zone valve not opening fully Sometimes
On zoned systems a zone valve that sticks closed or only opens partially will prevent adequate flow to the heat exchanger, triggering the E:40 high-pressure alarm even though everything else is functioning correctly.
- Failing compressor winding Rare
A compressor with a deteriorating winding can draw unbalanced current across phases, causing the protection circuit to detect a phase as effectively missing. This is a more serious cause and typically accompanies other symptoms such as noise or slow heating.
How to fix it
- Check the system pressure gauge on the heat pump or buffer vessel DIY safe
The pressure should read between 1.0 and 1.5 bar when the system is cold. If it is below 1 bar, locate the filling loop (usually a braided hose with a small valve near the unit or an underfloor manifold) and top up slowly until the gauge reads approximately 1.2 bar, then close the valve. Re-pressurising the system yourself is a homeowner-safe task.
- Inspect the outdoor unit and clear any obstructions DIY safe
Walk around the outdoor unit and remove any leaves, debris or snow that may be blocking the air intake or discharge grille. Ensure there is at least 30 cm of clear space on all sides. Do not attempt to remove any panels or touch the fan blades.
- Check that all isolation valves and zone valves are fully open DIY safe
Trace the pipework from the heat pump to the buffer vessel and around the heating circuit. Any lever-style isolation valve should have its handle in line with the pipe (open). If you find a valve that was accidentally turned off, open it slowly and allow the system a few minutes to equalise before attempting a reset.
- Bleed the system to remove airlocks DIY safe
Use a standard radiator bleed key on each radiator or towel rail, starting with those on the ground floor and working upward. If underfloor heating manifolds are fitted, use the bleed screws on the manifold. After bleeding, re-check the system pressure and top up if it has dropped.
- Check the consumer unit and the outdoor unit isolator for tripped MCBs DIY safe
Locate your consumer unit (fuse board) and look for any breaker that has tripped to the middle or OFF position. A heat pump typically has its own dedicated MCB. If one has tripped, switch it fully off then firmly back on. Also check the local isolator switch adjacent to the outdoor unit. Do not repeatedly reset a breaker that trips again immediately — call an engineer.
- Attempt an alarm reset from the controller DIY safe
Navigate to the alarm or information menu on the Nibe display module. Select the active alarm, choose 'I want to see more information about this alarm' and then select 'Reset alarm and try again.' The unit will restart and attempt to resume normal operation. If the fault reappears within a short time, do not continue resetting — the underlying cause must be found first.
- Have an MCS-certified engineer inspect the electrical supply and compressor wiring heat pump engineer
If the fault code [40] relating to a missing compressor phase persists after basic checks, a qualified engineer needs to measure the incoming supply voltage at the compressor terminals, inspect all wiring connections for corrosion or looseness, and verify the integrity of the compressor windings. This is not homeowner-safe work.
- Have an MCS-certified engineer inspect the circulation pump, strainer and plate heat exchanger heat pump engineer
For the E:40 high-pressure variant on air-source models, if re-pressurising and opening valves does not resolve the fault, an engineer should remove and clean the strainer and magnetic filter, verify the circulation pump is running at the correct speed and flow rate, and check whether the plate heat exchanger requires descaling. Any work involving the refrigerant circuit requires an F-Gas registered engineer.
Parts you may need
- Magnetic filter / strainer (22 mm or 28 mm) · from £55
- Circulation pump (Grundfos UPM3 or equivalent) · from £180
- Zone valve (motorised, 22 mm) · from £65
- High-pressure switch · from £45
- Expansion vessel (heating circuit) · 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–£500, depending on the underlying cause.
Frequently asked questions
Can I reset Nibe fault code 40 myself?
You can attempt a single reset from the controller menu after carrying out the basic homeowner checks — verifying system pressure is above 1 bar, ensuring all valves are open, bleeding any airlocks, and checking no MCB has tripped. If the fault returns immediately or comes back within a few hours, stop resetting and call an MCS-certified engineer. Repeatedly resetting without finding the cause risks damaging the compressor.
What is the difference between fault code [40] and alarm 220 E:40 on a Nibe heat pump?
They share the same number but are different faults. Code [40] on its own (common on ground-source and older models) means the compressor has detected a missing or very low electrical phase in its supply. Alarm 220 displayed as E:40 (common on air-source models with the EB101 outdoor unit, such as the F2040) is a high-pressure alarm triggered by poor water flow through the heat exchanger. Check your controller display carefully — the full alarm description will tell you which situation you are dealing with.
Could a blocked magnetic filter cause Nibe fault code 40 on my air-source heat pump?
Yes. On air-source models the E:40 alarm is fundamentally a water-flow problem. A heavily blocked magnetic filter or strainer reduces the flow rate through the plate heat exchanger, causing refrigerant pressure to rise until the high-pressure switch trips. Cleaning or replacing the filter is one of the first things an engineer will check and is one of the more affordable repairs. This is why an annual service — during which the filter is cleaned — is so valuable for preventing this fault.
How much does it cost to fix Nibe fault code 40 in the UK?
Most repairs land in the £120–£500 range. A straightforward fix such as cleaning a blocked strainer, bleeding the system or tightening a loose electrical connection typically costs £120–£250 including the engineer's callout. Replacing a circulation pump is usually £300–£500 all-in. If the fault turns out to be a failed compressor, the cost rises significantly — often £1,500–£2,500 — but this is an uncommon outcome. Bear in mind that Nibe is a Swedish brand and parts are not always held in UK stock, so there can occasionally be a short wait for courier delivery if a component needs ordering.