Although heat pumps have been around since the 1950s, how they operate remains a mystery to most people. Essentially, a heat pump is a reversible air conditioner. Think of a heat pump this way. When a typical room air conditioner is running, it is cool on the inside where a fan blows air over one set of coils (evaporator), but hot to the touch at another set of coils on the outside (condenser). While this works fine during the summer months, how could this arrangement provide heat during the winter? Well, you simply spin the window unit around so that the evaporator and condenser coils are on opposite sides. Now with the condenser on the inside of the house, hot air is provided to the living space. This is basically what a heat pump does, only without physically rotating the unit!
With a heat pump, rather than reverse the position of the evaporator and condenser coils, the flow of refrigerant between them is reversed. Refrigerant is the chemical medium used to absorb heat. This change in refrigerant direction effectively switches the roles of the evaporator and condenser coils. During summer months, the heat pump operates like a conventional air conditioner, with the evaporator extracting heat from inside the living space. During the winter, the reversing valve causes the condenser to operate as an evaporator, extracting heat from the outside air and delivering it inside the building. Although this system is very effective, it does have limitations. This is why a heat pump is equipped with an auxiliary electric heater. Once the temperature outside drops below freezing, the ability to extract heat from the cold air is significantly reduced. At this point, the electric backup heater turns on to compensate. During this time, the heat pump operates as a pure electric furnace, which in frigid climates can become cost-prohibitive.
