Heat Pump Basics: Air Source

An air source heat pump takes heat from outdoor air, even on a cold day, and uses a refrigerant circuit to lift that heat to a temperature useful indoors. A fan draws air across an outdoor coil, the refrigerant absorbs heat and is compressed, and the hotter refrigerant then warms water for radiators or underfloor pipes, or warms air directly in some designs. Because it moves heat instead of burning fuel, it can deliver more heat energy than the electrical energy it consumes under favourable conditions.

Performance falls as outdoor temperature drops and as the required flow temperature rises, which is why a mild autumn afternoon and a frosty January night give very different results. That variation is normal, and it is the reason sizing and emitter design matter more than any single headline figure.

The outdoor unit needs free airflow, a firm level base, drainage for condensate and defrost water, and clearance from walls, fences and planting. In winter it will periodically reverse briefly to melt frost from its coil, producing meltwater and a short change in sound. Indoor space is needed too, often for a cylinder if hot water is stored, plus pipework, controls and an installer access route for servicing.

Siting also affects neighbours. Sound travels across gardens and reflects off walls, so a spot that looks tidy on a plan can be noisy at a bedroom window. Planning rules, permitted development limits and local noise expectations vary, so siting should be discussed early rather than after the unit arrives.

Check the home heat loss, the existing emitter sizes and the coldest local conditions, not just an average winter figure. Ask how the proposed unit performs at low outdoor temperatures and at the flow temperature the emitters actually need, and whether a buffer or a larger cylinder is part of the design. Look at the proposed outdoor position in person, stand at the nearest neighbouring window, and consider prevailing wind and drifting snow or leaves.

Also check the electrical supply, including whether an upgrade is needed, and what happens during a power cut. Note the defrost drainage route so meltwater does not freeze across a path or soak a wall base.

In the EU, product efficiency labelling and national building rules govern air source heat pumps, and member states differ on planning permission, noise limits and support schemes. Some countries require notification or a permit for an outdoor unit, while others treat small units as permitted development with conditions on distance and sound.

In the UK, permitted development rules for air source heat pumps in England and the devolved nations have their own conditions, and MCS certification is commonly required for grant-supported work. Always check the current national and local rules for the exact property, since conservation areas and listed buildings add further controls.

One mistake is placing the outdoor unit in a cramped corner where it recirculates its own cold exhaust air, which lowers performance and can increase icing. Another is sizing from floor area alone, without a room-by-room heat loss calculation, which risks a unit that is too small on the coldest nights or too large and short-cycling the rest of the year.

A third mistake is ignoring the cylinder: a heat pump that heats rooms well can still disappoint if hot water runs out during back-to-back showers. Ask how hot water demand was estimated and how reheat time was allowed for. It is also worth asking how the unit behaves in fog, driving rain and light snow, since coil icing and defrost frequency vary with humidity as well as temperature, and a site that is damp and sheltered can ice more often than a colder but drier and more exposed position.

Keep the heat loss calculation, the product data sheet with performance at several outdoor temperatures, the siting drawing, the electrical assessment and the commissioning record. Those documents let later work be judged against the original design.

Ask a qualified installer: where exactly will the outdoor unit sit and drain, what sound level is expected at the nearest neighbouring window, what flow temperature was designed for, and what will defrost look and sound like in winter. Answers tied to this home beat generic reassurance.