Heat pumps are energy-efficient central air conditioners that provide cooling in the summer and heating in the winter, as an alternative to furnaces and air conditioners. According to the U.S. Energy Information Administration, roughly 12.1 million households use heat pump systems to heat and cool their homes. Most of them are located in hot-humid or mixed-humid areas with mild winters, where heat pumps work best.

Although only about 10 percent of U.S. households use heat pumps, the number is growing fast as new builders opt for a more energy-efficient system. It’s important for HVAC contractors to learn and understand how they work, especially as they become more popular.

Here’s a basic overview that will help you better understand how a heat pump works:

Now that you know how heat pumps work, get to know the individual components, from suction accumulators to heat exchangers. Familiarize yourself with the maintenance and upkeep of heat pump systems, and you’ll be better prepared to service a growing number of homes in the U.S.

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