In the heating mode, a circulating fluid flowing through the coils absorbs energy from the outside and releases it to the interior of the building from the interior coils.
The fluid is cold and at low pressure when it is in the external coils, where it absorbs energy by heat from either the air or the ground.
The resulting warm fluid is then compressed and enters the interior coils as a hot, high-pressure fluid, where it releases its stored energy to the interior air.