The whole principle is based on the laws of nature. Heat pumps take heat from around the building – from the air, ground or water. The temperature changes during the change of state – evaporation, where the change from liquid to gas takes place, and condensation – the change from gas to liquid. This whole process takes place in a compressor where the pressure change takes place and where the higher the pressure, the faster the liquid becomes a vapour and vice versa. By compressing the cold vapour, it is heated. To change a liquid substance into a gas, we need energy from outside.
In an air-to-water heat pump, the outdoor unit takes air and leads it to the heat exchanger. At the same time, there is a liquid refrigerant that is colder than the incoming air, which leads to a temperature change and begins to evaporate. The compressor takes the refrigerant in its gaseous state, and uses pressure to raise its temperature. The warm steam goes into a condenser, which passes this produced energy to the home’s heating system. The refrigerant is converted back to a liquid state and the whole process is repeated again.
It’s easier to imagine this happening with a refrigerator, which takes heat from food and uses an evaporator to transfer it out – when you touch the outside of the refrigerator, it gets hot. A heat pump works on a similar principle.