During the heating cycle, the ground water, which has circulated through the
underground piping system and picked up heat from the soil,
is brought back to the heat pump unit inside the house.
It then passes through the refrigerant-filled primary heat exchanger.
The heat is transferred to the refrigerant,
which boils to become a low-temperature vapour.
The reversing valve directs the refrigerant vapour to the compressor.
The vapour is then compressed, which reduces its volume and causes it to heat up.
Finally, the reversing valve directs the now-hot gas to the condenser coil,
where it gives up its heat to the air that is blowing across the coil
and through the duct system to heat the home.
Having given up its heat, the refrigerant passes through the expansion device,
where its temperature and pressure are dropped further
before it returns to the first heat exchanger to begin the cycle again.
In some systems, a second heat exchanger called "desuperheater"
takes heat from the hot refrigerant after it leaves the compressor.
Domestic water is pumped through a coil ahead of the condenser coil,
in order that some of the heat that would have been
dissipated at the condenser is used to heat water.
The cooling cycle is basically the reverse of the heating cycle.
The direction of the refrigerant flow is changed by the reversing valve.
The refrigerant picks up heat from the house air and transfers it to the
ground water or antifreeze mixture.
The heat is then pumped outside into the underground piping.
Once again, some of this excess heat can be used to preheat domestic hot water.