This system uses outside air as heat source. The heating and DHW circuit contains a stratified storage. In addition, a circulation pump is installed after the storage. Underfloor heating and low inlet temperatures provide good preconditions for the heat pump system.   

Efficiency of 2.9 in 2010 reflects the average value of 2.9 of air source heat pumps. As the back-up heater was not used, less electrical energy was needed and thus efficiency was influenced positively. The fan's share is not considered separately. The DHW share in 2010 amounted to 38 % and is far above the average of 19 %. This influences efficiency negatively due to the higher temperature level needed. The reason for the high DHW share seems to result from an unintended recirculation inside the stratified storage. This leads to the supply of unnecessarily high temperatures for the underfloor heating .