Air-to-water heat pumps

Outdoor units feeding wet central heating and hot water.

An air-to-water heat pump takes heat from the outside air and transfers it into water for a wet heating system. That water can feed radiators, underfloor heating or fan convectors. In most homes, the heat pump also heats stored domestic hot water through a suitable cylinder.

It is not simply an outdoor boiler. A boiler can deliver very hot water in short bursts and is often much larger than the home’s actual heat loss. A heat pump is most effective when it supplies the building steadily at the lowest practical water temperature. The outdoor unit, emitters, pipework, cylinder, controls and electrical supply therefore need to be designed as one system.

Start with the building’s heat loss

The design begins with room-by-room heat-loss calculations at the relevant internal and external design temperatures. These calculations establish both the building load and the output required from each emitter. The heat pump is then selected using its declared output at the proposed outdoor and water temperatures, not just the model number or a nominal rating taken from the front of a brochure.

This distinction matters because available output and efficiency change with outdoor temperature and leaving-water temperature. Defrost also interrupts useful heat delivery for short periods in cold, damp conditions. The design has to account for those behaviours while maintaining comfort.

Copying the old boiler’s output is not a sizing method. An oversized unit can cycle excessively in milder weather; an undersized unit may depend on direct-electric backup more than the proposal suggests.

The emitters set the water temperature

Every radiator, underfloor loop or fan convector must provide the room’s required heat at the proposed mean water temperature. Existing radiators may be adequate in some rooms and too small in others. Their condition, connections and actual output matter, so a rule such as “all radiators need replacing” or “the existing ones will be fine” is not credible without the room calculations.

Lower water temperatures generally improve heat-pump efficiency, but the target should follow from a workable emitter design. If the proposed flow temperature is pushed upwards to compensate for small emitters, the seasonal performance estimate should reflect that choice.

The distribution system also needs enough flow with acceptable resistance. Pipe sizes, pumps, valves, strainers and hydraulic separation all affect what reaches the emitters. A buffer tank is not automatically required, and fitting one does not correct undersized pipework or poor control logic.

Domestic hot water is stored

Most domestic air-to-water systems use a cylinder because they do not heat tap water instantaneously like a combi boiler. The cylinder needs a heat exchanger and controls suitable for the selected heat pump. Its volume, reheat performance and backup heater should be chosen around the household’s demand, available space and the heat pump’s hot-water performance.

Space-heating and cylinder-heating temperatures are different design questions. A periodic higher-temperature cycle may use the heat pump, an immersion heater or both, depending on the product and control strategy. Cylinder sizing must account for the available heat source, recovery time and operation when the heat pump is unavailable.

Monobloc and split arrangements

In a monobloc, the working refrigerant circuit is contained in the outdoor unit and water pipes cross the building wall. Those external water sections need a deliberate frost-protection strategy.

In a split system, refrigerant pipes connect the outdoor unit to an indoor hydraulic module. This reduces the amount of heating water exposed outdoors but introduces field-installed refrigerant joints and the qualification requirements that go with them.

Neither layout is inherently the better heating system. The relevant questions are product suitability, frost risk, refrigerant safety, pipe route, indoor and outdoor space, service support and how the complete system performs at the design condition.

Controls and normal operation

Weather compensation adjusts the target water temperature as outdoor conditions change. Correctly commissioned, it allows longer, steadier operation and avoids producing hotter water than the building needs. Room influence, thermostatic valves and zones must be arranged so they work with that strategy rather than repeatedly stopping flow through the heat pump.

The system needs a minimum water volume and flow rate specified by its manufacturer. How that is achieved depends on the design. It may use open emitters, a volumiser, a buffer or another hydraulic arrangement. There is no universal vessel size or flow rate that applies to every heat pump.

During defrost, water and vapour around the outdoor unit are normal. The base and drain route should manage this without allowing ice to create a slip hazard, obstruct the unit or damage the building.

Electrical supply, siting and noise

The electrical assessment must use the manufacturer’s maximum current and protective-device requirements, including any immersion or backup heater. The installer should check the service head, consumer unit, earthing and the effect of other large loads. A generic cable or breaker size is not enough.

The outdoor unit needs unobstructed airflow, service access, structurally suitable support and a compliant condensate route. Sound data from the selected operating condition feeds the planning or acoustic assessment. Moving the unit after design can invalidate both the noise calculation and the pipework plan.

Planning rules and incentive conditions are live matters and differ across the UK. They should be checked for the address and scheme in question. This component article therefore does not reproduce grant values or permitted-development limits that can change independently of the technology.

Complete heating-system design

Sizing the complete heating system requires:

the room-by-room and whole-building heat loss
heat-pump output at the chosen external and water design temperatures
emitter outputs and required design flow temperature
the seasonal performance estimate and its assumptions
the hydraulic layout, minimum flow and minimum system-volume strategy
hot-water cylinder sizing, reheat assumptions and backup arrangement
electrical maximum demand and any supply work
the outdoor-unit position, sound assessment, drainage and frost protection
commissioning settings and the handover information the owner will receive

Running cost cannot be reduced to “heat pumps are cheaper” or “electricity costs more than gas”. It depends on the home’s heat demand, achieved seasonal performance, electricity tariff and the cost and efficiency of the system being replaced.

Applies to

Heat

Last reviewed

22 Jul 2026