Air Source Heat Pumps (ASHP)

How Air-Source
Heat Pumps Work

How air-source heat pumps work

Air-source heat pumps are heating and cooling systems that move heat into a home in the winter and draw heat out of the home in the summer. Instead of burning fossil fuels, they operate on the same principle as your refrigerator: powered by electricity, they use a refrigerant cycle to move heat and to keep your home at a comfortable temperature year-round. They are much more efficient than electric resistance (electric baseboard) heating and also provide highly efficient air conditioning.

Air-source heat pump systems feature an outdoor unit (containing a compressor, reversing valve, heat exchanger and expansion device) connected to one or more indoor units by small refrigerant piping. The refrigerant is a substance with properties that enable it to easily absorb and release heat.

Winter

Winter Mode

In the winter, very cold, low-pressure refrigerant absorbs heat from the outside air at the outdoor unit’s heat exchanger (1). (Yes, even sub-zero outdoor air has heat in it!) The refrigerant then flows to the air-source heat pump’s compressor (2), which mechanically pressurizes the refrigerant, causing it to heat up. The reversing valve (3) directs the hot refrigerant to flow to an indoor heat-exchanger where the refrigerant transfers its heat to the indoor air (4). No longer hot, the refrigerant then passes through an expansion device (5), which makes it very cold. Because it is now colder than the outdoor temperature, the refrigerant can again absorb heat from the outdoor air to begin the cycle again (1).

Summer

Summer Mode

In the summer, the process is reversed. The refrigerant passes through the expansion device, which makes it very cold (1). The cold refrigerant absorbs heat from air inside the home at the indoor heat exchanger (2), cooling down the interior. Once outside, the warmed refrigerant goes through the compressor (3), which pressurizes and heats it up further. This time, the reversing valve (4) directs the hot refrigerant to the outdoor heat exchanger. Because the refrigerant is now hotter than the outside temperature, it gives up its heat to the outdoor air (5), much the way a conventional air-conditioning system works.