Don’t let the name fool you; a heat pump is a home heating and cooling system device that works by “moving” heat from one place to another. Heat is just another word for thermal energy. Different kinds of energy can be converted into thermal energy. Most commonly, our homes are heated by turning either electric energy (in the case of baseboard heaters) or chemical energy (natural gas) into thermal energy. A heat pump works by simply moving existing thermal energy from one place to another.
Heat naturally moves to cooler places—the easiest demonstration of this principle is what happens when you open a window in winter. The warm air moves outside to where the air is cold, making the room cool down rapidly. Heat pumps reverse this process by taking warm air from a cold area (like outside) and moving it to a warmer area (like your home).
Most of the cost of home heating comes from warming up cold air. For example, if you turn on a space heater, the fan that blows warm air out doesn’t use nearly as much energy as the heating element that warms the air. Any electrical heating system relies entirely on electricity to produce and distribute heat. Heat pumps only rely on electricity to distribute existing heat.
The diagram demonstrates how a heat pump system works by pushing a fluid (in this case, refrigerant) through a pair of coils. One coil is indoors, the other outdoors. In cooler weather, when you want to heat your home, the outdoor coil acts as an evaporator of refrigerant (a fluid used in many different systems that are meant to change the temperature of a given area), and the indoor coil acts as a condenser. When the refrigerant is turned to gas in the outdoor evaporator coil, it absorbs thermal energy from the surrounding air and then moves to the cooler indoor coil. That coil condenses the refrigerant vapor back into a liquid using compression, and the thermal energy (heat) is released into the indoor air.