Heat pumps are a mystery to most people, even those who own them. After all, how can these devices heat a home in winter and then cool it in summer?

According to experts at the Trane Home Comfort Institute, a consumer information service on heating and cooling, the answer lies in the fact that heat is present in all air, even air that's below freezing.

Think of the way your refrigerator removes unwanted heat that accumulates when you open the door and place warm food inside. You can feel that heat coming back into the kitchen from the refrigerator's exhaust fan.

In a similar way, a heat pump simply extracts the heat that's present in outdoor air in winter and delivers it inside your home to keep you warm and comfortable.

In summer, the process reverses. The heat pump pulls the heat out of indoor air and releases it outside to keep your home cool and dry. A heat pump's ability to both heat and cool makes it a very economical and efficient home comfort system.

The Trane Home Comfort Institute explains that a typical heat pump installation consists of two parts: an indoor unit and an outdoor unit. The indoor unit is called an air handler and looks similar to a gas furnace. The outdoor unit looks exactly like a central air conditioner in both size and appearance.

A special liquid called a refrigerant circulates between the indoor and outdoor units, absorbing and releasing heat as it travels through the loop.

Regardless of whether the heat pump is heating or cooling, the compressor is considered the "heart of the system" because it is the pump that circulates the refrigerant through the loop.

Because of its importance, experts recommend that you look closely at the compressor's reputation and warranty when selecting a heat pump.