Home Heat Pump Guide

Air Source Heat Pump Problems: Common Issues and Fixes

Air source heat pumps are reliable technology — far more so than most people expect. But like any heating system, they can develop issues. The challenge is knowing which problems are genuine faults, which are normal behaviour being misunderstood, and which are the result of poor installation or incorrect settings.

This guide covers the most common air source heat pump problems reported by UK homeowners, explains what is actually going on, and tells you when to call an engineer and when to adjust your expectations.

Problem 1: The House Is Not Warm Enough

This is the most common complaint, and it has several possible causes.

Cause: The Heat Pump Is Undersized

If the heat pump is too small for the building's heat loss, it simply cannot produce enough heat to keep the house comfortable in cold weather. This is a design and installation issue — a proper heat loss survey should have determined the correct size.

Fix: A heat loss survey needs to be carried out (or redone) and the system design reviewed. In some cases, the flow temperature can be increased to squeeze more output from the unit. In the worst case, the unit may need to be replaced with a larger one — this should be covered by the installer's workmanship warranty if the original sizing was wrong.

Cause: Flow Temperature Is Set Too Low

Heat pumps are most efficient at low flow temperatures, so installers sometimes set the flow temperature lower than the building needs. The result is radiators that feel lukewarm and rooms that never quite reach the desired temperature.

Fix: Increase the flow temperature gradually — try raising it by 2-3°C and see if comfort improves. A well-designed system should achieve comfort at flow temperatures between 35°C and 50°C, depending on the building's insulation and radiator sizes.

Cause: Heating Schedule Is Wrong

Heat pumps work differently from gas boilers. They produce a lower level of heat over a longer period, rather than a blast of heat for an hour. If you are running your heat pump on a short schedule (like a gas boiler — on at 6am, off at 8am, on at 5pm, off at 10pm), the house may not reach a comfortable temperature before the system switches off.

Fix: Run the heat pump for longer periods at a lower setpoint. Many homeowners find the best approach is to let the heat pump run continuously (or nearly continuously) during cold weather, with a small setback at night. The weather compensation function, if fitted, should handle this automatically.

For more on how heat pumps work and why the operating approach matters, see our complete guide to air source heat pumps.

Problem 2: Electricity Bills Are Higher Than Expected

Some heat pump owners report electricity bills that seem too high — sometimes close to what they were spending on gas. This undermines the financial case for the heat pump and understandably causes concern.

Cause: Poor Efficiency (Low COP)

The heat pump's efficiency, measured as the Coefficient of Performance (COP), depends heavily on the flow temperature. At 35°C flow temperature, a good ASHP achieves a COP of 3.5-4.0. At 55°C, it drops to 2.0-2.5. If the flow temperature is set higher than necessary, the system uses more electricity than it should.

Fix: Reduce the flow temperature as low as possible while maintaining comfort. Use weather compensation to automatically adjust flow temperature based on outdoor conditions. This is the single most impactful thing you can do to reduce running costs.

Cause: Supplementary Heating Running Unnecessarily

Most heat pumps have a built-in electric immersion heater or backup heater that activates when the heat pump cannot meet demand. If this is running frequently — due to incorrect settings, undersizing, or a fault — it uses electricity at a COP of 1.0 (pure resistance heating), which is three to four times less efficient than the heat pump itself.

Fix: Check whether the immersion heater or backup heater is activating. Many heat pump controllers show this information. If it is running regularly, the underlying cause needs to be identified — often it is a settings issue that can be easily corrected.

Cause: Legionella Cycle Running Too Often

Heat pumps periodically raise the hot water temperature to 60°C or above to prevent Legionella bacteria growing in the cylinder. This is necessary and important, but the immersion heater usually assists because many heat pumps cannot reach 60°C efficiently on their own. If this cycle is set to run daily rather than weekly, it adds unnecessary electricity consumption.

Fix: Check the legionella cycle frequency. Once or twice per week is sufficient for most households. Your installer should have set this appropriately during commissioning.

For detailed running cost information, see our heat pump running costs guide.

Problem 3: The Heat Pump Is Noisy

Noise is a common concern, though what constitutes "noisy" varies enormously between people.

Normal Operating Noise

All air source heat pumps produce some noise. The fan and compressor create a low hum, typically 40-55 dB at one metre — roughly comparable to a fridge or a quiet conversation. This is normal and unavoidable.

At night, when background noise levels drop, the heat pump may seem louder even though the actual sound level has not changed. Some people are more sensitive to low-frequency hum than others.

Abnormal Noise: When to Worry

The following noises are not normal and indicate a potential problem:

  • Grinding or rattling: Could indicate a failing fan bearing, loose panel, or debris caught in the fan
  • Loud clicking or banging: May be a faulty reversing valve (used during defrost cycles) or a refrigerant issue
  • High-pitched whistling: Could indicate a refrigerant leak or a restriction in the refrigerant circuit
  • Vibration transmitted through the building: The unit may not be properly isolated from the building structure. Anti-vibration mounts should be fitted between the unit and its base

Fix: For abnormal noises, call your installer or a qualified heat pump engineer. Do not ignore grinding or whistling sounds — they can indicate issues that worsen over time if left untreated.

Problem 4: Ice Building Up on the Outdoor Unit

Seeing ice on your heat pump can be alarming, but in most cases it is completely normal.

Normal: Frost and Light Ice During Cold Weather

When the outdoor temperature drops below about 5°C, moisture in the air can freeze on the heat pump's evaporator coil, forming a layer of frost or light ice. This is expected. The heat pump has a built-in defrost cycle that periodically reverses the refrigerant flow to melt the ice. You will hear the fan stop and may see steam rising from the unit during defrost — this is all normal.

Abnormal: Heavy Ice That Does Not Clear

If the outdoor unit becomes heavily encased in ice that does not clear between defrost cycles, there is a problem. Possible causes include:

  • Defrost cycle not working: A faulty sensor or control issue may prevent the defrost cycle from activating
  • Restricted airflow: Leaves, debris, or snow blocking airflow around the unit can cause excessive icing
  • Low refrigerant: A refrigerant leak reduces the system's ability to absorb and release heat, leading to ice build-up
  • Faulty reversing valve: If the valve that reverses refrigerant flow for defrosting is stuck or broken, the defrost cycle cannot work

Fix: If ice is building up and not clearing, call your installer. In the meantime, clear any snow or debris from around the unit to maximise airflow. Do not try to chip ice off the coil — you risk damaging the fins.

Problem 5: No Hot Water or Slow Hot Water Recovery

Heat pumps heat water more slowly than gas boilers. A gas combi boiler delivers hot water on demand. A heat pump heats a cylinder of water over a period of time — typically 1-3 hours to fully reheat from cold.

Normal: Longer Recovery Times

If you use a lot of hot water in quick succession (everyone showering in the morning, then running a bath), the cylinder may run out and take an hour or two to recover. This is normal behaviour, not a fault.

Fix: Consider a larger hot water cylinder (250L instead of 200L) if your household regularly runs out. Also, schedule the heat pump to prioritise hot water reheating during off-peak electricity times if you are on a time-of-use tariff.

Abnormal: No Hot Water At All

If the heat pump is not producing any hot water, check the obvious first: is the system set to heat water? Is the cylinder thermostat working? Is the motorised valve opening? If these are all fine, the issue may be a refrigerant problem, a failed compressor, or a control fault — all of which need an engineer.

Problem 6: The Heat Pump Keeps Turning On and Off (Short Cycling)

Short cycling — where the heat pump starts, runs for a few minutes, stops, and then starts again — is a sign that something is wrong with the system design or settings.

Causes

  • Oversized heat pump: If the unit is too large for the building's heat demand, it reaches the target temperature very quickly and shuts off, only to restart when the temperature drops. This is hard on the compressor and reduces efficiency.
  • No buffer tank (or buffer tank too small): A buffer tank provides thermal mass that prevents rapid temperature fluctuations. Without one, the system temperature swings quickly and the heat pump cycles on and off.
  • Incorrect controls: The flow temperature differential (the difference between the on and off temperatures) may be set too tight, causing the unit to cycle frequently.

Fix: This is usually an installation or design issue. A buffer tank may need to be added, the controls adjusted, or — in the worst case — the heat pump downsized. Discuss with your installer.

Problem 7: Error Codes and Fault Lights

Modern heat pumps display error codes when something goes wrong. These can range from minor sensor faults to serious compressor issues.

What to do: Note the error code and check your user manual — some codes are informational (like "defrost in progress") and not actual faults. If the code indicates a genuine fault, contact your installer. Do not try to reset the system repeatedly without understanding the cause — persistent resetting can mask an underlying issue.

When Is It Normal Behaviour, Not a Problem?

Many "problems" reported by heat pump owners are actually normal operating behaviour that feels unfamiliar compared to a gas boiler:

  • Radiators feeling lukewarm: Normal at low flow temperatures. The room should be warm even if the radiators feel cooler than you are used to with gas.
  • The heat pump running for hours: Normal. Heat pumps are designed to run for long periods. Constant operation at low output is more efficient than cycling on and off.
  • Steam rising from the outdoor unit: Normal during a defrost cycle. It looks dramatic but is just ice melting.
  • The fan stopping periodically: Normal during defrost. The fan stops, the unit reverses to melt ice, then normal operation resumes.
  • Hot water being slightly cooler than with a gas boiler: Normal. Heat pumps typically store water at 48-52°C rather than the 60-65°C common with gas boilers. This is still hot enough for showers and baths.

How to Avoid Problems in the First Place

  • Choose an experienced installer: Most heat pump problems stem from poor installation. Use an MCS-certified installer with a strong track record. Read our installation guide for what to look for.
  • Insist on a proper heat loss survey: This ensures correct sizing — the foundation of a trouble-free system.
  • Understand how to operate your system: Ask your installer to walk you through the controls, settings, and what normal operation looks like.
  • Service annually: Preventive maintenance catches issues early.
  • Do not fiddle with settings unnecessarily: The most common cause of problems in the first year is homeowners changing settings without understanding the consequences. If in doubt, call the installer before adjusting anything.

Frequently Asked Questions

Are air source heat pumps unreliable?

No. Heat pumps are proven technology with decades of track record in Scandinavia and across Europe. The vast majority of UK installations work well. The problems that do occur are usually related to installation quality or incorrect settings, not the technology itself.

My heat pump is not heating my house properly — is it broken?

Not necessarily. Check the flow temperature settings, the heating schedule, and whether the weather compensation is configured correctly. Many "not heating properly" complaints are resolved by adjusting settings rather than repairing hardware. If adjustments do not help, call your installer for a system review.

Why is my heat pump using so much electricity?

The most common causes are the flow temperature being set too high, the backup immersion heater running unnecessarily, or the legionella cycle running too frequently. Check these settings first. If they are all correct and bills are still high, a system efficiency review by a qualified engineer is worthwhile.

Should I turn my heat pump off in summer?

You can turn off the space heating function in summer, but keep the hot water function running — you still need hot water year-round. Most modern heat pumps have a summer mode that does this automatically.

How often does a heat pump need servicing?

Annual servicing is recommended. This is less frequent and typically cheaper than a gas boiler service. Some manufacturers require annual servicing to maintain the warranty.

My installer has gone out of business — who do I call?

Contact the heat pump manufacturer directly — they can recommend qualified service engineers in your area. You can also search the MCS installer database for engineers certified to work on your brand of heat pump. Many heating engineers are now cross-training in heat pump servicing.