The collector liquid (cooling medium) is pumped up from the borehole in tubing and passed to the heat pump.

Another fluid circulates in the heat pump in a closed system with the most important characteristic of having a low boiling point. This fluid is called a refrigerant. When the refrigerant reaches the evaporator, which has received energy from the borehole, the refrigerant evaporates.

The vapour is fed to a compressor where it is compressed. This results in a high increase in temperature.

The warm refrigerant is fed to the condenser, which is positioned in the boiler water. Here the refrigerant gives off its energy to the boiler water, so that its temperature drops and the refrigerant changes state from gas to liquid. The refrigerant then goes via filters to an expansion valve, where the pressure and temperature are further reduced.