How do Air Source Heat Pumps (ASHPs) work?

An air source heat pump (ASHP) heats water for space heating and domestic hot water needs by extracting heat from the outside air. This system is particularly suited for underfloor heating, radiators, and hot water tanks, making it popular in residential and commercial heating applications.

In heating mode, an ASHP extracts heat from outdoor air through an outdoor unit that includes a fan and an evaporator coil. The process starts when the fan blows outdoor air over the evaporator coil, which contains a refrigerant. This refrigerant absorbs heat from the air, even at low temperatures, causing it to evaporate into a gas. The refrigerant, now in a low-temperature gaseous state, flows into a compressor, where it is compressed, raising its temperature and pressure.

The high-temperature gas then travels to a heat exchanger, typically located inside the building, where it transfers its heat to water stored in a tank or circulated through a closed-loop system in radiators or underfloor pipes. As the refrigerant releases its heat, it cools down and returns to its liquid state, then flows back to the outdoor unit to restart the cycle. The heated water produced by the heat exchanger is then circulated throughout the building, delivering warmth to rooms via radiators or underfloor heating systems.

ASHPs can also provide domestic hot water. Some models feature an additional tank or coil specifically designed to heat water for taps and showers, ensuring a steady supply of hot water while efficiently managing space heating.

ASHPs are energy-efficient because they transfer heat rather than generate it directly. Their efficiency decreases as outdoor temperatures drop, which means that in extremely cold climates, they may require supplemental heating to maintain comfort. Despite this, modern ASHPs operate effectively in a range of temperatures due to advancements like variable-speed compressors and enhanced refrigerants.

The initial installation cost of an air-to-water ASHP is typically higher than traditional heating systems, but the long-term energy savings, reduced greenhouse gas emissions, and potential subsidies often make them a sustainable choice.

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