Whilst end energy use is ultimately the most important (kWh/year) there are lots of variables that influence this which can be difficult to measure e.g. occupancy factors and weather. Instead a HTC is a very pure indicator of the fabric performance and its effectiveness in retaining heat. By extracting a HTC value from PHPP you can then compare this design performance value against a measured HTC using a measurement solution such as SmartHTC.
Deriving HTC from PHPP
PHPP does not total the losses in W/K but calculates the losses directly into kWh/year using a value G that accounts for the temperature difference (internal minus external). It is however very easy to extract the areas and U-values, thermal bridging and ventilation losses from the ‘Annual Heating’ or ‘Heating’ tabs in PHPP to derive the HTC based on the PHPP input data using the linked template.
A HTC is expressed in W/K and is the sum of all fabric losses arising from the U-values of the building components, thermal bridging, infiltration and ventilation. A HTC extracted from PHPP, SAP (field 39) and as measured using SmartHTC are all directly comparable.