How does a heat pump work?

History of the Heat Pump

The basic principle behind how a heat pump works was laid out in 1852 by Lord Kelvin in his second law of thermodynamics. This law is made up of several parts. The part that is most important when it comes to the principle behind the operation of a heat pump is the fact that heat always travels from the warmer area into the colder area.

The First Heat Pump

The first heat pump was built by the American inventor Robert C. Webber. It all basically happened by accident when, in the late 1940's, Webber was experimenting with deep freezing equipment and he unintentionally touched the outlet pipes of the cooling system, burning his hand.

This led him to the idea behind the basic mechanics of a heat pump. He connected the outlet piping from a freezer to a hot water heater and, since the freezer was producing constant excess heat, he hooked up the heated water to a piping loop and, using a small fan, he started to force the warm air into the building. He continued to experiment by successfully harvesting heat from the ground using below-grade heat collectors. In fact, he was so satisfied with his discoveries that only a year later, he decided to sell his old coal furnace.

Principle Behind the Heat Pump

(for the not-technically inclined)

A heat pump basically works on the same principle as a refrigerator, which removes heat from the food that is being stored in it (i.e. cooling) while generating heat to be exhausted out the back of the device (i.e. heating). A heat pump works in the same way but in a reverse fashion and with greater power. The pump is removing heat from water, air or from the ground and redistributes this heat into the desired location through radiators or an underfloor heating system.

(for the technically inclined)

The easiest way in which to understand how a heat pump works is to look at the following diagram (click to enlarge):

Step 1 - Evaporation: The coolant circulating in the heat pump system collects heat from the air, water or the ground and this process causes the coolant to evaporate and change to a gaseous state.

Step 2 - Compression: The heat pump's compressor rapidly compresses the coolant, which is now in a gaseous state and several degrees warmer. Thanks to the laws of physics behind the compression process, which cause the temperature of the compression medium with increases in pressure, the compression elevates the otherwise low potential heat of the coolant to a higher level – approximately 80°C.

Step 3 - Condensation: Using a second heat exchanger, the heat from the heated coolant is transferred to the water circulating through the radiators. This causes the coolant to drop in temperature and condense back into a liquid state. The radiators distribute the heat provided into the heated areas. The chilled water in the heat loop then travels back into the second heat exchanger, where it is heated again.

Step 4 - Expansion: By passing through the expansion valve, the coolant travels back to the first heat exchanger, where it is once again heated.

This cycle keeps repeating itself over and over again.