Heat loss
Indicative heat loss from EPC heating demand (MCS 031 method)
- where
- specific heat loss (W/K)
- EPC space heating demand (kWh/yr)
- degree days for the postcode (K·day/yr)
- indicative design heat loss (W)
- design inside temperature (°C)
- outdoor low temperature (°C)
- heat loss per m² of floor (W/m²)
- total floor area (m²)
Explain this equation
Gives an early indication of a home's design heat loss from the space heating figure on its EPC. Use it for a first conversation with a customer, before a full room-by-room survey.
The EPC demand Q is a yearly total in kWh. Degree days D add up how many degrees colder than the base it is, day by day, over a year. Converting kWh to Wh with 1000 and days to hours with 24 gives the watts lost per degree, P_s. Multiplying by the design temperature difference gives the peak loss.
Annual energy equals heat loss per degree times degree-hours, so the equation runs that backwards to find the heat loss per degree.
Sense check: 15,500 kWh with 1835 degree days gives about 352 W/K, and at a 24 K difference about 8.5 kW, roughly 58 W/m² over 145 m².
Watch out: EPC figures are modelled, not measured, and can be well out. This is for an early estimate only; MCS requires a full room-by-room calculation for the design.
Turns the space heating demand on an EPC into an indicative design heat loss, the way MCS 031 does for pre-sale estimates. Useful before a survey, or to sense-check a room-by-room calculation.
Enter your values to see the result.
Good to know
MCS 031 uses 21 °C inside and the postcode degree days and outdoor low temperature from its Table 1 (for example SW/E London 2,033 degree days and −3.0 °C; Bristol and Cardiff 1,835 and −3.1 °C; Glasgow 2,494 and −5.6 °C; Aberdeen 2,668 and −5.4 °C). The result is indicative only: the system must still be designed room by room.
Common mistake
Treating the EPC figure as a measured demand. EPC demand is modelled with standard occupancy, so the heat loss derived from it can be well off for the real house.
Limits
Assumes demand is proportional to degree days, with no allowance for internal or solar gains beyond what the EPC model already includes. MIS 3005-D requires a full room-by-room heat loss for the design.
Sources
- MCS 031 Issue 4.0 Heat pump pre-sale information and calculations (Pspecific = 1000 × Q / (24 × D); Pt = Pspecific × (21 − do); W/m²; Appendix A Table 1 degree days and outdoor low temperatures by postcode; Appendix B worked example)
- MCS MIS 3005-D Heat pump design standard (heat pump to meet the design heat loss at the design outdoor temperature; room temperatures and outdoor design temperatures; design to BS EN 12831-1)
Results are calculations from the published methods and figures shown, and depend on the data you enter. Follow MCS standards, manufacturer data, BS 7671, the ENA connection rules and Building Regulations. System design and DNO applications need a competent, certified installer.