How Does an Air Source Heat Pump Actually Work?

An Air Source Heat Pump (ASHP) works by harnessing the natural heat energy present in the outside air, even when it feels cold to us, and transferring it indoors to heat your home and/or hot water. It essentially operates like a refrigerator, but in reverse.

Here's a step-by-step breakdown of how it works, often referred to as the "refrigeration cycle":

Absorption (Evaporation):

The ASHP has an outdoor unit with a large fan. This fan draws in outside air (from the back) and blows it over a heat exchanger coil containing a special liquid called refrigerant.

Even at low temperatures (as low as -20°C), there's still heat energy in the air. The refrigerant used in an ASHP has a very low boiling point, meaning it can absorb this low-grade heat from the outside air and evaporate, turning from a cold liquid into a warm gas.

Compression:

The now-gaseous refrigerant then moves into a compressor. The compressor, powered by electricity, squeezes the gas, increasing its pressure and, crucially, its temperature significantly. This is where the "boosting" of heat happens.

Heat Transfer (Condensation):

The hot, high-pressure gas then flows into a second heat exchanger, located within the outdoor unit (for air-to-water systems) or directly into your living space (for air-to-air systems).

Here, the heat from the compressed gas is transferred to the cooler air or water circulating in your home's heating system (e.g., radiators, underfloor heating, or a hot water cylinder). As the refrigerant gives up its heat, it cools down and condenses back into a liquid.

Expansion:

Finally, the liquid refrigerant passes through an expansion valve. This valve reduces the pressure of the refrigerant, causing its temperature to drop significantly, bringing it back to its original cold, low-pressure liquid state.

Repeat:

The cold liquid refrigerant then flows back to the outdoor unit to repeat the cycle, continuously absorbing heat from the outside air and transferring it into your home.

Key Components of an ASHP:

Outdoor Unit: Houses the fan, primary heat exchanger (evaporator), compressor, and reversing valve. Secondary heat exchanger (condenser).

Indoor air-to-air Unit: Contains the indoor heat exchanger and a fan.

Refrigerant: The fluid that circulates through the system, absorbing and releasing heat.

Compressor: Increases the pressure and temperature of the refrigerant gas.

Expansion Valve: Reduces the pressure and temperature of the refrigerant liquid.

Types of ASHPs:

Air-to-Water Heat Pumps: The most common type in the UK. They transfer heat from the outside air to water, which then circulates through your wet central heating system (radiators or underfloor heating) and can also heat your domestic hot water.

Air-to-Air Heat Pumps: These systems blow heated air directly into your rooms through fans, similar to an air conditioning unit (but providing heat). They typically do not provide hot water.

Efficiency:

ASHP's are highly efficient because they transfer heat rather than generate it through burning fuel.

For every unit of electricity consumed to power the heat pump, they can typically produce 2 to 5 units of heat energy, making them significantly more efficient than traditional boilers.