Heat pump COP
The higher the outlet temperature of the heat pump, the more work the compressor has to do to achieve this temperature. Therefore the more power the heat pump requires and the lower COP.
The temperature leaving the heat pump will actually be less than this ‘compressor off’ temperature due to loses across the heat exchanger within the heat pump.
This graph gives an indication how for different kW rating the COP changes with a change in the temperature of the refrigerant coming off the compressor.
The graph above has been generated using the methodology laid down in EN14511 and is as a guide only.
The main outlet temperature test condition that all heat pumps should be tested to determine the COP is 35oC as per EN14511-2. This (with a set inlet of 0oC) is to try and provide a common point so one heat pumps performance can be compared to another.
It can be seen that the outlet temperature from the heat pump can have a significant effect on the efficiency and hence COP of the heat pump. This is why heat pumps are ideally suited for underfloor mounted in screed which only requires a low inlet temperature of 35oC and how using radiators may result in a lower COP due to the higher flow temperatures they require.