Pros : The most efficient type of heat pump, particularly during the colder months of the year, when heat is drawn from the ground, dam or river. Suitable for heating swimming pools, hydronic floor slabs, domestic hot water systems, and room heating using radiators.

Cons : More expensive than other units.

Efficiency : Between 4.2:1 and 5.4:1; ie. 4.2kW of heat output requires 1.0kW of electricity input, depending on ambient air temperatures.