An air-source heat pump is an energy-efficient heating and cooling system that uses outside air as its heat source. With energy-efficiency ratings of well over 100 percent, such heat pumps provide year-round comfort in millions of U.S. homes. But how do heat pumps use cold air for heating, and how do you get energy efficiency of more than 100 percent out of any device?

How they work

Heat pumps provide both heating and cooling. During the heating season, heat pumps move heat from the cold outdoors to the warmer indoor space. How does cold air provide heat? Hot and cold are relative terms. All outside air down to absolute zero (-460°F) contains some heat. An air-source heat pump extracts some of this heat and moves it indoors to provide space heating.

A heat pump system consists of a compressor and two coils made of copper tubing; one coil is located indoors and the other one is located outdoors. In the winter, liquid refrigerant in the outdoor coil converts into a gas after absorbing heat from the air. In the indoor coil, heat is released from the gas refrigerant as it condenses back into liquid. In the summer, this process is reversed; the heat pump provides cooling by moving heat from warm inside air outdoors.

Source: http://www.energystar.gov/

How do heat pumps have efficiencies greater than 100 percent? Because heat pumps move or pump heat, rather than create it. A conventional furnace creates its own heat by burning fuel. Combustion losses result in an energy efficiency of less than 100 percent. While a furnace can achieve a high efficiency level, the amount of heat produced is ultimately dependent on the amount of fuel used.

Since heat pumps are merely moving heat from one place to another, they are not bound by such restrictions. Heat pumps still use energy, just not nearly as much as the heat energy they move. When properly installed, an air-source heat pump delivers one-and-a-half to three times more heat energy to a building than the electrical energy it consumes, according to the U.S. Department of Energy. This translates into energy efficiencies of 150 to 300 percent. The measure of heating efficiency is the Heating Seasonal Performance Factor (HSPF) in units of Btu heat output divided by watt-hours of electricity input (6.8 HSPF = 200% efficiency).

Air-source heat pumps lose both efficiency and heating capacity as the outdoor air temperature decreases. When outdoor temperatures drop below 32°F, a less efficient, electric resistance or gas-fired backup system kicks in to provide extra heating capacity. For this reason, standard air-source heat pumps are not always the most cost-effective heating option in areas with cold winters. Though cold-climate heat pump models have been developed by some manufacturers, they are not yet widely available. In moderate climates, however, heat pumps can provide highly efficient and economical space conditioning all year long.