An air source heat pump is a low-carbon heating system that draws warmth from the outside air and uses it to heat your home and hot water. It works even when the air feels cold, running on electricity rather than burning gas or oil. For many Sussex homes — particularly those off the mains gas grid — it can replace an oil boiler, LPG system or older electric heating.
What an air source heat pump is
An air source heat pump is a unit that sits outside the property, usually against an external wall or on a small base in the garden. It looks broadly like an air-conditioning condenser and contains a fan, a compressor and a heat exchanger.
Rather than generating heat by combustion, it moves heat that already exists in the air indoors. Because it transfers heat instead of creating it, a well-designed system delivers more energy as heat than it consumes as electricity. That ratio is described by the coefficient of performance, or COP — a measure of useful heat out for each unit of power in.
Most domestic units are "air-to-water" systems. They heat water that circulates through radiators, underfloor heating and a hot water cylinder. A smaller number of "air-to-air" systems heat the rooms directly with fans, but these do not produce hot water for taps and baths.
How it pulls heat from outside air
An air source heat pump is a low-carbon heating system that draws warmth from the outside air and uses it to heat your home and hot water.
The process relies on a refrigerant — a fluid that boils and condenses at low temperatures. The cycle repeats continuously while the system is calling for heat.
- A fan draws outside air across an evaporator coil. Even cold air holds usable energy, so the refrigerant inside absorbs it and turns from liquid to gas.
- A compressor squeezes that gas, which raises its temperature sharply.
- The hot gas passes through a heat exchanger, giving up its warmth to the water in your heating system.
- As it loses heat the refrigerant condenses back to a liquid, then passes through an expansion valve and the cycle begins again.
This is heat extraction at work: the unit harvests low-grade warmth from the air and concentrates it into something useful. Manufacturers typically quote performance down to around -15°C or lower, though efficiency falls as outdoor temperatures drop and the unit has to work harder.
Heat pumps suit low-temperature heating. They are most efficient when the water leaving the unit is around 35°C to 45°C, rather than the 60°C–70°C an older boiler might use. Lower flow temperatures keep running costs down but mean radiators and pipework need to be sized to release enough heat at those gentler temperatures.
Where it works well in a Sussex home
A heat pump suits some properties more readily than others. Sussex has a wide mix of housing — from coastal flats and Victorian terraces to rural cottages and modern estates — so suitability is decided property by property.
The strongest candidates tend to be homes that:
- Already have reasonable insulation, or where loft and cavity insulation can be improved before installation.
- Have space outside for the unit, with clearance around it for airflow and a sensible distance from neighbouring windows to limit noise.
- Are off the gas grid and currently rely on oil, LPG or electric heating, where the running-cost comparison is often more favourable.
- Can accommodate a hot water cylinder, since heat pumps do not heat water instantly the way a combi boiler does.
Older, solid-walled properties — common across the South Downs villages and historic towns — are not ruled out, but they usually need more attention to insulation and larger or additional radiators to run efficiently at low flow temperatures. In a conservation area or on a listed building, planning permission may be needed for the external unit, so it is worth checking with the local authority early.
Coastal exposure is a practical factor too. Salt-laden air near the seafront can accelerate corrosion, so units intended for those locations should be specified with suitable protective coatings.
What an installation involves
The process usually starts with a survey and a heat loss calculation. An installer assesses how much heat each room loses on a cold day, which determines system sizing — the capacity the heat pump needs and the size of each radiator. Correct sizing matters: an oversized unit cycles inefficiently, while an undersized one struggles in cold weather.
A typical fit-out includes the outdoor unit, a hot water cylinder, controls and any new or larger radiators. Existing pipework is checked, and some sections may be upgraded to carry the higher flow rates a heat pump needs. Underfloor heating, where present, pairs particularly well because it operates at low temperatures across a large surface.
Installation generally takes a few days, depending on how much radiator and pipework change is required. After commissioning, the system is balanced so each room reaches its target temperature, and the controls are set to run the pump steadily rather than in short bursts.
Funding may be available. In England, the Boiler Upgrade Scheme offers a grant towards installing an air source heat pump in eligible homes, administered through approved installers. Eligibility rules and grant levels change over time, so the current details should be confirmed through the official government scheme. Installations that follow the Microgeneration Certification Scheme (MCS) standard are usually required to qualify for grants and consumer protections.
It helps to ask any installer how the flow temperature has been set, whether existing radiators were assessed, and what the expected running costs are based on your home's heat loss. Those questions reveal whether the design has been tailored to the property rather than fitted to a standard template.