Something has gone wrong with your heat pump. The house is not warming up, or the outdoor unit is making a noise you have not heard before, or the radiators feel barely warm. You need answers quickly.
Most heat pump faults fall into a small number of categories. Some you can sort yourself in five minutes. Others need a qualified engineer. This guide goes through each problem plainly, tells you what to check before calling anyone, what repair costs look like in 2026, and when it makes more sense to replace the system rather than fix it.
Before anything else: the quick checks
Run through these before assuming there is a serious fault. A surprising number of call-outs turn out to be one of these:
- Thermostat set to the wrong mode. Check it is set to ‘heat’ and the temperature is above the current room temperature.
- Circuit breaker tripped. Check the consumer unit. The heat pump and outdoor unit run on separate breakers, check both.
- Radiator valves closed. If individual rooms are cold, check the thermostatic valve on each radiator is open.
- Outdoor unit blocked. Leaves, debris, or overgrown plants can restrict airflow. Clear anything within a metre of the unit.
- System needs a reset. After a power cut, some units do not restart automatically. Try switching the heat pump off at the isolator for two minutes, then back on.
If none of those fix it, read on.
Problem 1: the heat pump is not heating the house
This is the most common complaint, especially during cold weather. The system is running, you can hear it, but the house is not warming up properly.
Radiators feel lukewarm rather than hot
This is not necessarily a fault. Heat pumps run at lower flow temperatures than gas boilers, typically 35 to 50 degrees rather than 65 to 75. Radiators will feel warm to the touch rather than hot. As long as the rooms are reaching the set temperature, the system is working as designed.
If rooms are not reaching temperature and it is taking much longer than usual, the likely causes are:
- Radiators undersized for the lower flow temperature, common when a heat pump has been retrofitted without upgrading the radiators
- The heating curve is set incorrectly on the system controller, an engineer can adjust this
- The property has poor insulation and is losing heat faster than the pump can replace it
The heat pump turns on but nothing happens
Check the thermostat first. If that is fine, check the breaker. If the unit is completely silent with no lights on the controller, there is likely a power supply issue. Call an engineer.
If the unit starts and the outdoor fan runs but there is still no heat output, the refrigerant may be low. You will sometimes hear a hissing sound near the outdoor unit. That is a refrigerant leak. Do not continue running the system and call a qualified F-Gas engineer.
The reversing valve is stuck
If the system heats in summer (cooling mode) but will not heat in winter, or vice versa, the reversing valve is the likely culprit. This valve switches the direction of refrigerant flow to change between heating and cooling modes. When it sticks, the system runs in the wrong mode.
A stuck reversing valve needs a professional. It is not a DIY repair.
Problem 2: the outdoor unit is icing up
Some frost on the outdoor unit in winter is completely normal. The system enters a defrost cycle periodically — you may hear it switch modes briefly, and the unit may produce a little steam. That is fine.
Ice that does not clear after a defrost cycle, or heavy ice building up on the coils, is a problem. Possible causes:
- Airflow restricted around the unit, clear any debris and check nothing is placed against it
- Defrost sensor or defrost timer fault, the system is not triggering defrost when it should
- Fan in the outdoor unit running slowly or not at all
- Low refrigerant causing the coil temperature to drop too far
Do not attempt to chip or melt ice from the outdoor unit with hot water. This can cause damage. If the ice is not clearing after a defrost cycle, switch the system off and call an engineer.
Problem 3: the heat pump is running constantly
Heat pumps run differently from gas boilers. A boiler fires up, heats the water quickly, and shuts off. A heat pump runs longer at lower output. Longer run times are normal and actually more efficient than short cycles.
Constant running becomes a problem when the house still is not reaching temperature despite the system never switching off. Common reasons:
- The system is undersized for the property, it was never designed to keep up with the heat loss on cold days
- Insulation is poor and the property is losing heat faster than the pump can add it
- Thermostat set too high, heat pumps work best set between 19 and 21 degrees, not 24 or 25
- Refrigerant leak causing efficiency to drop
If it is just a cold day and the system is running more than usual, that is expected behaviour. If it never stops even in mild weather and rooms are still cold, get it checked.
Problem 4: strange noises
Heat pumps make some noise, the outdoor fan, compressor hum, and the occasional click or gurgle during defrost or startup are all normal. New or unusually loud sounds are worth paying attention to.
Rattling or vibrating
Usually means something is loose. Check that the outdoor unit is still sitting level on its pad and that the fixings are secure. A loose panel or mounting bracket vibrates against the unit casing. If tightening everything does not stop it, the noise is coming from inside and needs an engineer.
Grinding or shrieking
Switch the system off immediately. This usually indicates a problem with the fan motor or compressor. Running it further risks causing more damage. Call an engineer.
Hissing or gurgling
Gurgling can mean low refrigerant. Hissing almost always means a refrigerant leak. Turn the system off and call an F-Gas certified engineer. Refrigerant is under pressure and potentially toxic, this is not something to investigate yourself.
Clicking that will not stop
A single click at startup and shutdown is normal. Repeated clicking when the system is trying to start can indicate a faulty starter capacitor or contactor. Both need a professional.
Problem 5: high electricity bills
If bills have gone up sharply since installation or after a period of normal running, a few things could be happening.
- The heating curve is set too high, the system is producing water hotter than it needs to, which reduces efficiency
- The thermostat is set too high, the pump is running longer than needed
- Refrigerant low, the system is working harder to produce the same output
- The system is older and efficiency has degraded, a service will identify this
- Radiators are too small and the system has to run at higher flow temperatures to compensate
Worth noting: electricity is currently more expensive per unit than gas. If you switched from gas heating, your electricity bill will be higher even with a working, efficient heat pump. The question is whether the total energy spend is lower. That depends on the COP of your system and your electricity tariff.
Problem 6: the radiators feel lukewarm and the house feels damp
This is something competitors rarely discuss, but it has become one of the most common complaints from UK homeowners in 2025 and 2026.
Heat pumps run at lower temperatures than gas boilers. When a heat pump is installed in a home without upgrading the radiators or improving insulation, the house heats up more slowly, some rooms may not reach temperature, and condensation can build up on cold surfaces, particularly on north-facing walls and in rooms with poor airflow.
The technology is not at fault. The installation is. A heat pump retrofitted into a poorly insulated home without a proper heat loss calculation will underperform. The fix involves:
- Upgrading undersized radiators to larger panels that work efficiently at lower flow temperatures
- Improving loft and cavity wall insulation to reduce heat loss
- Ensuring the heating curve on the controller is set correctly for the property
- Checking that all rooms have adequate heat emitters and no zones are being locked out
If this is the situation you are in, Wunergy can carry out a system assessment across the West Midlands. It involves checking the heat pump setup, flow temperatures, and whether the radiators are appropriate for the load.
Problem 7: odd smells
A musty smell from the outdoor unit is usually mould or algae growth on the coil, common and not a serious problem. Clean it with a damp cloth.
A burning, smoky, or fishy smell from any part of the system, indoor controller, outdoor unit, or pipework, means electrical components are overheating. Switch the system off at the breaker and do not turn it back on until an engineer has looked at it. Running it further risks a fire.
A rotten smell from the outdoor unit that does not improve with cleaning sometimes means a small animal has got into the casing. Open it and check.
What you can check yourself vs when to call an engineer
| You can do this yourself | Always call an engineer |
| Check thermostat settings and mode | Refrigerant leak suspected |
| Reset a tripped circuit breaker (once) | Grinding or shrieking noise |
| Clear leaves/debris from outdoor unit | Burning or fishy smell |
| Check if radiator valves are open | Ice that does not clear after defrost cycle |
| Clean accessible filters | Breaker keeps tripping repeatedly |
| Restart the system after a power cut | System over 15 years old and failing often |
| Check hot water cylinder settings | Reversing valve fault suspected |
What repairs cost in 2026
Most heat pump repairs in the UK fall between £150 and £600 according to iHeat’s 2026 repair data. More complex faults involving the compressor or major refrigerant work cost more.
| Fault type | Typical repair cost UK (2026) |
| Thermostat replacement | £100 to £250 |
| Filter clean or replacement | £50 to £150 |
| Refrigerant recharge (minor leak) | £200 to £500 |
| Refrigerant recharge (major leak + fix) | £400 to £800 |
| Fan motor replacement | £200 to £450 |
| Reversing valve replacement | £300 to £600 |
| Compressor replacement | £800 to £2,000+ |
| Control board fault | £200 to £500 |
| Annual service (prevention) | £150 to £300 |
Costs vary by region, engineer call-out rates, and parts availability. Always get a written estimate before authorising work. An annual service (£150 to £300) prevents most of these faults from developing.
When to repair and when to replace
Most faults are worth repairing if the system is under 12 years old and the repair cost is under 30 to 40 percent of what a replacement would cost. Beyond that, the calculation shifts.
Consider replacement when:
- The system is over 15 years old and developing repeated faults
- The compressor needs replacing on an old system, a new system often makes more financial sense
- The existing system was badly sized or installed and has never performed correctly
- A newer system would qualify for the £7,500 Boiler Upgrade Scheme, which replaces an underperforming older system
Wunergy can assess whether your current system is worth repairing or whether a replacement installation, potentially funded by the Boiler Upgrade Scheme, makes more sense.
How to prevent most of these problems
The majority of heat pump faults are avoidable with basic annual maintenance. Most manufacturers require an annual service to keep the warranty valid.
A standard service covers:
- Checking and cleaning filters
- Inspecting refrigerant levels and pressure
- Checking electrical connections
- Testing the defrost cycle
- Verifying flow rates and temperatures
- Checking the controller settings and heating curve
Beyond the annual service, keep the outdoor unit clear of debris year-round, check that nothing has been placed against it, and do not ignore unusual sounds or performance changes when they first appear. Small issues caught early are cheap to fix.
Common questions
Yes, in cold weather. The system defrosts itself automatically. If ice is building up and not clearing after the defrost cycle, or if it is covering large areas of the unit, there is a fault, likely the defrost sensor, a fan issue, or low refrigerant.
Yes, in cold weather. The system defrosts itself automatically. If ice is building up and not clearing after the defrost cycle, or if it is covering large areas of the unit, there is a fault, likely the defrost sensor, a fan issue, or low refrigerant.
Longer run times are normal for heat pumps, they are designed to run more continuously at lower output. If the house is still not reaching temperature despite constant running, the system may be undersized, insulation may be inadequate, or there is a refrigerant issue.
Most repairs fall between £150 and £600 in 2026. Refrigerant work, fan motor replacements, and control board faults are the most common. Compressor replacement is the most expensive at £800 or more, and often prompts a replacement decision on older systems.
You can do basic checks: thermostat settings, circuit breakers, clearing debris from the outdoor unit, and resetting after a power cut. Anything involving refrigerant, electrical components, or internal parts needs an F-Gas certified or qualified heating engineer. Refrigerant work specifically is illegal without the right certification.
Typically 20 to 25 years with regular servicing. Ground source systems can last longer, with underground pipework rated for 50 or more years. Without annual servicing, lifespan shortens and faults become more frequent.
