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Air-Source Heat Pump Basics

August 19, 2013 - 11:03am

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Air-source heat pumps transfer heat between the inside of a building and the outside air.

How Air-Source Heat Pumps Work

This diagram of a split-system heat pump heating cycle shows refrigerant circulating through a closed loop that passes through the wall of a house. Inside the house the refrigerant winds through indoor coils, with a fan blowing across them, and outside the house is another fan and another set of coils, the outdoor coils. A compressor is between the coils on one half of the loop, and an expansion valve is between the coils on the other half. The diagram is explained in the caption.

In heating mode, an air-source heat pump evaporates a
refrigerant in the outdoor coil; as the liquid evaporates it pulls
heat from the outside air. After the gas is compressed, it
passes into the indoor coil and condenses, releasing heat to the
inside of the house. The pressure changes caused by the
compressor and the expansion valve allow the gas to
evaporate at a low temperature outside and condense at a
higher temperature indoors.

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 a 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

Most central heat pumps are split-systems—that is, they each have one coil indoors and one outdoors. Supply and return ducts connect to a central fan, which is located indoors.

Diagram of a split-system heat pump cooling cycle that shows refrigerant circulating through a closed loop, which passes through the wall of a house. Inside the house the refrigerant winds through indoor coils, with a fan blowing across them; outside the house is another fan and another set of coils, the outdoor coils. A compressor is between the coils on one half of the loop, and an expansion valve is between the coils on the other half. The diagram is explained in the caption.

In cooling mode, an air-source heat pump evaporates a
refrigerant in the indoor coil; as the liquid evaporates it pulls
heat from the air in the house. After the gas is compressed, it
passes into the outdoor coil and condenses, releasing heat to
the outside air. The pressure changes caused by the
compressor and the expansion valve allow the gas to
condense at a high temperature outside and evaporate at a
lower temperature indoors.

Some heat pumps are packaged systems. These usually have both coils and the fan outdoors. Heated or cooled air is delivered to the interior from ductwork that protrudes through a wall or roof.

More Information

Visit the Energy Saver website for more information about the selection and performance of air-source heat pumps in homes.

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