A simple stylized diagram of a heat pump's vapor-compression refrigeration cycle: 1) condenser, 2) expansion valve, 3) evaporator, 4) compressor.

A heat pump is a machine or device that moves heat from one location (the 'source') to another location (the 'sink' or 'heat sink'), using energy.

Common examples would be the refrigerator and split A/C units, both are heat pumps. Heat pumps can be used in either direction to warm or cool air.

The term coefficient of performance (COP) is used to describe the ratio of useful heat movement to work input. A simple electrical heater consuming 1 watt of electricity would release 1 watt of heat into the air. However a heat pump with a COP of 4 would release 4 watts of heat for the same amount of electricity.

The COP of a heat pump is effected by the differential temperature, for example ; when there is a wide temperature differential, e.g., when an air-source heat pump is used to heat a house on a very cold winter day, it takes more work to move the same amount of heat indoors than on a mild day. We intend to reduce energy consumption when the heat pump is used by using the geothermal mass below the house as a heat sink. This is achieved by running the pipes of the heat sink side below the floor and into the ground.

The table below shows the temperature gradients for a hot summers day and a cold winters day. The columns to the right show how the temperature gradient is vastly reduced using the ground as a heat sink, resulting in a higher COP and reduced energy consumption.