I have done some calculations and I reckon I need 9kw at my design outdoor temperature (DOT) of -2C, so initially I though the 7kw Vaillant Arotherm Plus would be 2kw too low and so the 10kw Grant Aerona would be more appropriate, but on further investigation it seems the nominal kw given for heatpumps is not that meaningful. I found some figures for both heatpumps which I plotted on the graph below:
So in the UK, the DOT is -5C to -1C and in this range the Arotherm Plus outputs 8.2 to about 9.2 (for 35C flow) and about 9kw (shown by cross on graph) at my DOT of -2. Below are the figures from Vaillant for their 7kw Arotherm Plus with the range highlighted for DOT of -5C to 0C for 35C flow:
So why do Vaillant specify a nominal output of 7kw – they don’t even provide the DOT for 7kw which I think would be something like -9C .
For the 10kw Grant Aerona, Grant only provide a figure of 11.1kw output at 7C:
If I plot a line through the 7kw Arotherm Plus figures then at 7C, it would be the same as the 10kw Grant Aerona, however that does not mean the 10kw Grant Aerona would output 9kw at -2C as I only have one data point from Grant. So possibly the 10kw Grant Aerona gives the same 9kw output at -2C as the 7kw Arotherm Plus, but how I would I confirm this when Grant only seem to give kw output at 7C.
Am I misunderstanding outputs of the heatpumps or is my analysis above correct ?