I am running a SAP calculation on a proposed building which will be utilising a 8.5 kW ASHP for space and water heating. I am also running a comparative analysis against a mains gas boiler to show the reduction in dwelling emission rate (DER).
I have noticed that when specing the heat pump in my model the TER shoots up from 14.49 kgCO2/yr/m2 (in the gas powered model) to 21.5 kgCO2/yr/m2. This is having a pretty massive impact on the percentage improvement of DER over TER between models.
Can any one explain why this happens? I cannot find an explanation elsewhere and I would hate to overstate the potential improvement the heat pump brings.