How Does an Air Source Heat Pump Work?

The reason heat pumps are so efficient is that they move existing heat rather than generating it from scratch. Even on a frosty January morning in Leeds, the air outside contains a large amount of thermal energy. A heat pump captures that energy and concentrates it to warm the water flowing through your radiators and to heat your hot water cylinder.

Think of it this way: an electric heater converts 1kWh of electricity into 1kWh of heat, and that is as good as direct conversion gets. A gas boiler burns 1kWh of gas and delivers slightly less than 1kWh of heat after flue losses. A heat pump uses 1kWh of electricity to move 3-4kWh or more of heat from the outside air into your home. It is not creating energy from nothing; it is using a small amount of electricity to harvest a much larger amount of ambient heat.

This is the same technology behind your fridge, run in reverse. What has changed in recent years is the efficiency of the hardware, the installer base, and the £7,500 Boiler Upgrade Scheme, which together have made the heat pump a mainstream replacement for the gas boiler in UK homes.

An air source heat pump works through four stages in a continuous cycle:

1. Evaporation: A liquid refrigerant flows through the evaporator coil in the outdoor unit. As the fan draws air across the coil, the refrigerant absorbs heat and evaporates into a gas. This works because the refrigerant has an extremely low boiling point, so even cold winter air carries enough energy to boil it.

2. Compression: The compressor squeezes the low-pressure gas into a high-pressure, high-temperature gas. This is where the heat gets concentrated. The compressor is the main electricity user, but because it only compresses gas rather than generating heat directly, it uses a fraction of what a resistive heater would.

3. Condensation: The hot gas passes through a heat exchanger, transfers its heat to the water in your heating circuit, and condenses back into a liquid. That hot water then flows to your radiators, underfloor heating and hot water cylinder.

4. Expansion: The liquid refrigerant passes through an expansion valve, dropping its pressure and temperature so it can absorb heat again. The cycle repeats.

The whole system modulates continuously: rather than blasting on and off like a boiler, it adjusts its output to match what the house is losing, which is part of why it is so efficient.

COP (Coefficient of Performance) is the ratio of heat output to electrical input at a single test condition. A COP of 4 means 4kWh of heat per 1kWh of electricity.

SCOP (Seasonal COP) is the number that matters for your bills: the same ratio averaged across a whole heating season of real weather, including cold snaps and defrost cycles. Good modern systems achieve SCOPs of 3.8-4.5, with premium units at the top of that band.

Why SCOP decides the gas comparison: under the Ofgem cap, electricity costs roughly 3.6-4.3 times as much per kWh as gas (24.67-26.11p versus 5.74-7.33p across the April-September 2026 cap periods). If your heat pump multiplies each kWh of electricity into 3.8-4.5kWh of heat, you are producing heat at or below the cost of a gas boiler, and the margin improves on heat pump time-of-use tariffs. The arithmetic is in our running costs guide.

An important detail: efficiency varies with the outdoor temperature and with your flow temperature. The colder the air and the hotter you ask the radiators to run, the harder the compressor works. That is why system design, not just the unit on the wall, determines the SCOP you actually get.

This is the single biggest behavioural difference from a boiler. A gas boiler typically sends very hot water to the radiators in short bursts. A heat pump is most efficient sending cooler water for longer: the lower the flow temperature, the higher the efficiency.

To deliver the same warmth with cooler water, you need more radiator surface area. That is why a heat pump installation often includes upsizing a few radiators, and why underfloor heating (a huge emitter surface) pairs so well with heat pumps.

Practical implications:

• Run it steadily. A heat pump home is kept at temperature with long, gentle running rather than morning blasts. Set it and leave it; do not operate it like a boiler.
• Weather compensation: most systems automatically adjust the flow temperature to the outdoor temperature, keeping efficiency high without you touching anything.
• Design matters: the MCS heat loss survey that precedes every Boiler Upgrade Scheme installation calculates each room's heat loss and specifies the radiators to suit. It is the difference between a SCOP of 4+ and a disappointing one.

Two different products share the "air source" name, and the grant treats them very differently:

Air-to-water (the subject of this site) connects to your wet central heating: radiators, underfloor heating and a hot water cylinder. It replaces the gas boiler outright. This is the system the £7,500 Boiler Upgrade Scheme grant supports in England and Wales.

Air-to-air blows warm (or cool) air directly into rooms, like the units common in shops and offices. It does not heat water, so it cannot replace your boiler for hot water. Air-to-air systems were added to the Boiler Upgrade Scheme in 2026 at a lower £2,500 grant level.

For a whole-home boiler replacement, air-to-water is the standard answer, and it is what installers mean by "air source heat pump" in a heating quote. If a quote seems surprisingly cheap, check which type it is for.

A heat pump heats your tap water by warming a hot water cylinder, typically overnight or in scheduled windows, and the cylinder stores it for showers and taps.

If you currently have a system or regular boiler, you already have a cylinder (a heat pump cylinder is usually a swap-in with a larger heat exchange coil). If you have a combi boiler, you have no cylinder, and the installation adds one: a real cost and an airing-cupboard conversation, covered in our boiler replacement guide.

Cylinder behaviour to expect: the heat pump heats water to a comfortable storage temperature very efficiently, and the system runs a periodic higher-temperature sterilisation cycle as a hygiene safeguard. All automatic, all configured by the installer.

Yes. The most common doubt about heat pumps is winter performance, and it is the most thoroughly answered: air source heat pumps are standard equipment in climates far colder than anywhere in the UK.

The physics: as the air temperature drops, there is less heat to harvest per cubic metre of air, so the compressor works harder and efficiency falls. Below about zero, moisture in the air can freeze on the evaporator coil, and the unit periodically runs a defrost cycle to clear it, borrowing a little heat back. Defrost cycles are normal, automatic, and already baked into SCOP figures.

What that means in practice across the UK: in the milder south, units run near their rated efficiency most of the year. In Scotland, the North East and the Pennines, winter efficiency dips further and the heat loss survey will size the system accordingly. Even at its mid-winter worst, a properly sized heat pump keeps the house warm; it just uses more electricity in January than in May, exactly as a boiler burns more gas.

Efficiency is not constant through the year, and knowing the pattern helps you read your bills.

Summer: the system mostly just heats the hot water cylinder, at high efficiency. Heating is off.

Spring and autumn: mild-weather heating is where heat pumps shine: low flow temperatures, gentle running, excellent efficiency.

Winter: longest run times and lowest spot efficiency, with defrost cycles in frosty spells. Heat demand also peaks exactly when efficiency dips, so winter dominates your annual cost, just as it does with gas.

This is why the seasonal figure (SCOP) is the one to plan with: 3.8-4.5 for a well-designed modern system. Most controllers and apps show live and historical performance, so you can see your actual numbers against the design figure. If they diverge badly, something is set up wrong: see our common problems guide.