The best way to describe a heat pump to somebody who is unfamiliar with them is to explain that a heat pump is similar to an air conditioner—with the important difference that a heat pump can switch the direction it works so that it will also heat the interior of a house. An air conditioner operates by evaporating refrigerant indoors to absorb heat and then condensing the refrigerant outdoors to release heat. A heat pump can do that, but it can also reverse the process so it releases heat inside.
That’s the difference in how the two operate. But what is the difference in the actual systems? What allows a heat pump to be able to change the direction it runs with only a single adjustment on the thermostat? We’ll go into those details below.