7 Common Air Source Heat Pump Problems (And How to Avoid Them)

Air source heat pumps are mature, reliable technology: no combustion, few moving parts, and standard equipment in countries with far harsher winters than the UK's. But like any system with a compressor, a fan and a design phase, things can go wrong, and a handful of issues come up again and again in owner feedback.

The encouraging pattern: nearly all of them are design and installation problems, not hardware problems, which means they are preventable at quote stage. Here are the seven to know about before you buy, and what to do if you already own one.

The outdoor unit produces a low fan-and-compressor hum. Modern units are quiet, but at night, near a bedroom window or a neighbour's boundary, even a quiet hum can be noticed.

How to avoid it:

• Site the unit away from bedroom windows, yours and the neighbours', at quote stage. Moving it later is expensive.
• Use anti-vibration mounts so the hum does not transmit through walls or decking.
• Ask your installer about the unit's sound rating and about siting rules for your property; quieter premium units exist if placement is tight.
• Avoid boxing the unit into echoing corners; hard surfaces reflect sound as well as restricting airflow.

As air temperature falls there is less heat to harvest, so the unit works harder, and around freezing it periodically runs defrost cycles to clear ice off the coil. Both are normal physics, and both are already baked into the SCOP figures and the design.

How to avoid problems:

• Make sure the system was sized from a real heat loss survey using your local design temperature. A unit sized for Hampshire will struggle in Aberdeenshire; the survey accounts for that.
• In colder regions (Scotland, the North East, upland areas), expect more defrost activity in winter: that is the system working, not failing.
• Do not panic-switch to boost modes in a cold snap; steady running wins. If the house genuinely cannot hold temperature on design-cold days, call the installer: that is a design conversation.

This is the complaint behind most bad heat pump press, and it is almost always a design or operation issue:

• Radiators too small: a heat pump runs cooler water than a boiler, so it needs more radiator surface. If the survey skipped radiator checks, rooms with undersized emitters will lag.
• Run like a boiler: switching the system on for two hours in the morning and evening guarantees disappointment. Heat pumps heat gently and continuously; the house should be held at temperature, not sprinted to it.
• Flow temperature set too low or compensation misconfigured: a commissioning settings issue an installer can fix in a visit.

The fix: get the installer back with the heat loss survey in hand, check each underperforming room's radiator against its calculated loss, and review the control settings. If the installer will not engage, the Consumer Rights Act 2015 requires services to be carried out with reasonable care and skill, which includes system design that achieves what was promised.

Usually three culprits, in order of frequency:

• Wrong tariff. A heat pump moves a meaningful share of your bill from gas to electricity, and the default cap rate (24.67p/kWh, rising to 26.11p in July 2026) is the most expensive way to feed it. Heat pump time-of-use tariffs with cheaper windows exist; not being on one leaves money on the table monthly.
• Hot flow temperatures. Every degree hotter than needed costs efficiency. If someone "fixed" a comfort complaint by cranking the flow temperature instead of addressing radiators, the bills carry the cost.
• On/off operation. As above: heat pumps are most efficient running long and low. Aggressive schedules and big overnight setbacks force inefficient catch-up running.

Check the system's own consumption display against the design estimate before assuming the worst; the data usually identifies the issue. The full cost framework is in our running costs guide.

Hot water comes from the cylinder, so running out is a cylinder sizing or scheduling issue, not a heat pump fault.

How to avoid it:

• Size the cylinder for your real household at install time: the worst morning, not the average one. (Combi converts: this is part of the cylinder conversation in our replacement guide.)
• Check the heating schedule: the cylinder should reheat after the morning rush and before the evening one.
• Guests staying? Most controls have a one-off boost for high-demand days.
• Expect the periodic high-temperature sterilisation cycle to run automatically; it is a hygiene safeguard, not a fault.

Modern systems carry control boards with temperature, pressure and flow sensors. Occasionally one fails or a transient condition throws an error code and the system locks out.

How to handle it:

• Note the code and check the manual; some clear with a simple restart.
• Try one power cycle at the isolator or breaker (off for 30 seconds). If the code returns, stop and call rather than repeatedly resetting.
• Call the installer first (workmanship and commissioning issues are theirs), then the manufacturer's warranty line for hardware.
• Know your rights: the manufacturer's warranty applies, and the Consumer Rights Act 2015 requires goods to be of satisfactory quality and to last a reasonable time regardless of the warranty paperwork. Keep your invoice, MCS certificate and commissioning documents together.

Heat pumps need to breathe. A unit jammed into an enclosed corner, behind dense planting, or under a low deck cannot draw enough air, and efficiency falls sharply. Worse, if the chilled exhaust air recirculates into the intake, the unit tries to harvest heat from air it has just cooled.

How to avoid it:

• Maintain the manufacturer's clearance around the unit; your installer will know the figures for the model.
• Never fully box the unit in; if you want screening, use open slatted designs with generous airflow.
• Keep vegetation trimmed and the coil free of leaves; an annual visual check costs nothing.
• Think about winter: avoid spots where cold exhaust pools or where roof run-off drips onto the unit and freezes.

Placement is decided once, at install. It is one more reason the cheapest quote with the least site thought is rarely the cheapest system to own. Compare quotes and ask each installer where the unit goes and why.