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How Air-Source Heat Pumps Work

Heating with cold air? Cooling off with hot air? Heat pumps performing these feats (especially mini-splits and VRF systems) have taken off, but how do they work?

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Air-source heat pumps have been growing rapidly in popularity over the past decade. In many regions, simple point-source mini-split systems are now the leading choice for designers and builders of high-performance green homes, and more sophisticated variable-refrigerant-flow (VRF) multi-split systems are popular in multifamily and commercial buildings.

All heat pumps perform the same basic function: they move heat from one place to another, using electricity. In heating mode, air-source heat pumps move heat from the outside air into a building; in cooling mode, they take heat from the inside air of the building and dump it outdoors, cooling the indoor space in the process.

Keeping an indoor space at 70°F using 0°F outdoor air is a bit counterintuitive; we’re used to heat flowing from warmer to colder, as the second law of thermodynamics says it should. The key to heat pumps—used for decades by our refrigerators—is the refrigerant cycle or vapor-compression cycle, driven by electricity.

With the vapor-compression cycle, a specialized refrigerant fluid alternately evaporates and condenses—absorbing and releasing heat in the process. All this takes place in a closed loop that is typically split, with part of it outside the building and part of it inside.

The operation of a heat pump varies depending on whether it is used in heating or cooling mode. In the heating mode, evaporation takes place in the outdoor coils and condensation in the indoor coils. In the cooling mode, the opposite occurs, with the outdoor coils becoming the condenser and the indoor coils becoming the evaporator.

In heating mode, very cold liquid refrigerant is pumped through heat-exchange coils exposed to the outside air. This outside air—even at 0°F in winter—is warmer than the sub-zero refrigerant, and it warms the refrigerant enough to evaporate it (change its phase from liquid to gas).

Still outside the building, that refrigerant gas is then mechanically compressed using a compressor (the loudest part of the heat pump), which raises its temperature. As a hot gas, this refrigerant then flows into the house through insulated piping to the interior heat-exchange coils. Indoor air is blown across these coils, warming that air. As this happens, the refrigerant cools and condenses back into liquid.

Finally, to complete this loop, the refrigerant (mostly liquid at this point) flows through an expansion valve, where the pressure is suddenly reduced, causing the refrigerant to cool off further.

In cooling season, the process is reversed. What had been a heat source (the outside air) 

becomes a heat sink, and what had been a heat sink (the inside air) becomes a heat source. The same indoor and outdoor heat-exchange coils are used, but their function reverses.

The technical wizardry inside these heat pumps allows for impressive coefficient of performance (COP) numbers, often 3 or more. That means that for every one unit of energy used by the system (as electricity), three units of heating or cooling energy are provided.
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