Home’s Heat Demand (kWh Year) for Heat Pumps.

Original Article
February 17th, 2018


The Space Heating kWh per year figure in the “your home’s heat demand” section of the EPC gives different results for SAP default heat pumps and heat pumps selected from the product characteristics data file (PCDF).  On the face of it this seems counter-intuitive because in all other cases the ‘heat demand’ is independent of the heating system (and efficiency) used to deliver that heat.  Changing from a SAP boiler to a boiler from the product database has no impact, and the same is true for other heating types. 

Why is this different for heat pumps, and why does it have such a significant impact (33% in some scenarios)?

When a heat pump is selected from the product database, SAP is calculated in a different way for the calculation from section 7 ‘Mean Internal Temperature’ onwards, according to Appendix N.

The main purpose of Appendix N is to provide more precise energy performance according to the plant size ratio.  This is a measure of how well suited the particular heat pump is to the specific dwelling and energy use.  If a heat pump is not ideally ‘sized’ for the dwelling, then it will experience a reduced part-load efficiency, impacting the secondary heating fraction, water heating, and internal gains.

PCDF heat pumps use a 24/hour day heating model, whereas SAP default heat pumps have a standardised heating pattern (9 hrs/ day weekdays, 16 hours weekends).  Appendix N also accounts for the responsiveness of the heating controls.  Taken together, these factors have an impact on the pattern of daily temperature fluctuation and changing the total amount of heat required.