Heat pumps are classified based on the source fluid and the discharge fluid. The full designations consist of two parts, for example, air-to-water heat pump:

  • First fluid: heat source
    • A (Air): outdoor air
    • W (Water): groundwater
    • B (Brine): heat transfer fluid in geothermal heat pumps

Pumps that use groundwater are rarely permitted for residential use; therefore, air and heat transfer fluid are the most common options.

  • Second fluid: heat emitter

The heat is transferred to the heating water or domestic hot water. In passive houses, the heat can be distributed directly into the air via a ventilation-
system.