The heat pumps delivers comfort by circulating refrigerant. In the winter, the refrigerant absorbs heat from outside (even in subfreezing temperatures), causing it to boil into a gas. A compressor then squeezes the refrigerant gas, making it hotter, because gases heat up under pressure. This really hot compressed gas flows into a spring-shaped coil. Then a fan blows over this coil, pushing warmed air into a building. The refrigerant cools to a liquid, and after it pours through an expansion valve to chill it even further, it’s ready to absorb more heat from the outdoors once again.
In summer, a heat pump operates in the same way, just in reverse; instead of absorbing heat from outside, the refrigerant collects heat from indoors and dumps it outdoors.
Heat-pump types are divided by their thermal-energy source. If they absorb heat from the ambient air, they’re called “air-source heat pumps.” If they suck thermal energy from a pond or stream, they’re “water-source” heat pump. If they draw heat from the ground or groundwater, you guessed it, they’re “ground-source” or “geothermal heat pump.”
There are also “absorption” heat pumps that can run on fossil gas, but they’re not effective decarbonization tools.
Heat pumps are categorized not only by what they gather thermal energy from, but also by what they transfer that energy to. A heat pump can gather thermal energy from the outdoor air and then release it into indoor air, making it an air-to-air heat pump. Or it could gather warmth from the air and use it to heat water that circulates through radiators, making it an air-to-water system.
