One limitation of utilizing heat pumps in lieu of boilers is that the maximum possible temperature of the majority of commercially available heat pumps is limited to 50 – 55 deg C with 45 deg C being ideal for efficient operation. Domestic Hot water and various other heating applications require heating temperatures of 65 deg C or better, this is very difficult to achieve using standard heat pumps only.

One possibility is to use a booster heat pump to take water from a medium temperature from the first stage heat pump to high temperature (via a second stage heat pump). Such heat pumps use specialist refrigerants that can operate as elevated discharge temperatures to provide the higher water set points. While this requires additional energy allowing both of the heat pumps to operate at the preferred temperature ranges means that even the combined efficiency is still superior to other conventional methods