Heat pumps … a not-so-modern marvel of technology.
It is my understanding that household heat pumps work by utilizing an outdoor condenser unit to pressurize and depressurize a liquid refrigerant so that one side of the system will absorb heat from the surrounding air, while the other side releases it. The two ends of the system being: 1) the outdoor condenser unit that many people associate with having ‘central air’, and 2) the coils inside of the air-handler through which the air your home is circulated.
By selectively reversing the pressurization/depressurization process via a reversing valve, the heat pump is able to either heat or cool the house, depending on what the thermostat determines is needed at the time. In many circumstances, heat pumps will also have an auxiliary heating system (be it natural gas, propane, electric coils, etc) to augment the heat produced by the condenser, or to provide emergency back-up heat in the event that the condenser fails.
Recently.. my condenser failed.