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How They Work
A heat pump's refrigeration system consists of a compressor and two coils made of copper tubing (one indoors and one outside), which are surrounded by aluminum fins to aid heat transfer. In the heating mode, liquid refrigerant in the outside coils extracts heat from the air and evaporates into a gas. The indoor coils release heat from the refrigerant as it condenses back into a liquid. A reversing valve near the compressor can change the direction of the refrigerant flow for cooling, as well as for defrosting the outdoor coils in winter.
When outdoor temperatures fall below 40° F, a less efficient panel of electric resistance coils, similar to those in your toaster, kicks in to provide indoor heating. This is why air-source heat pumps aren't always very efficient for heating in areas with cold winters. Some units now have gas-fired backup furnaces instead of electric resistance coils, allowing them to operate more efficiently
The efficiency and performance of today's air-source heat pumps is one-and-a-half to two times greater than those available 30 years ago. This improvement in efficiency has resulted from technical advances and options such as these:
- thermostatic expansion valves for more precise control of the refrigerant flow to the indoor coil;
- variable-speed blowers, which are more efficient and can compensate for some of the adverse effects of restricted ducts, dirty filters and dirty coils;
- improved coil design;
- improved electric motor and two-speed compressor designs; and
- copper tubing, grooved inside to increase surface area.
Some heat pumps are packaged systems. These usually have both coils and the fan located outdoors. Heated or cooled air is delivered to the interior from ductwork that protrudes through a wall or roof.
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