Heat loss
Whole-house design heat loss and W/m²
- where
- whole-house design heat loss (W)
- room design heat losses (W)
- whole-house heat loss (kW)
- heat loss per m² of floor (W/m²)
- total heated floor area (m²)
Explain this equation
Adds up the room heat losses to give the whole-house design heat loss, which sets the size of the heat pump. It also gives the loss per square metre as a quick benchmark.
Each P is a room's design loss from the room calculator. Their sum is the house total in watts, and dividing by 1000 gives kW. Dividing by the heated floor area gives watts per square metre, a figure that lets you compare houses of different size.
Heat losses from separate rooms simply add, because they are all at the same design conditions.
Sense check: rooms totalling 5 kW in a 90 m² house is about 56 W/m². Modern homes are often 30 to 50, older solid-wall houses 80 to 120.
Watch out: do not count heat passing between two heated rooms, as it cancels out across the house. Over about 100 W/m², look at improving the fabric first.
Adds the room heat losses into the design heat loss for the whole dwelling and expresses it per square metre of heated floor, the figure MCS uses to band performance.
Enter your values to see the result.
Good to know
For more than eight rooms, add the extra rooms into one box. MCS 031 bands the specific heat loss (W/m²) to look up the expected seasonal performance factor; its worked example is 8,483 W over 145 m², about 59 W/m². A typical modern or well-insulated house is often 30–60 W/m²; older solid-wall houses often 80–120 W/m².
Common mistake
Adding a safety margin to every room and then again to the total. BS EN 12831-1 and the current CIBSE guide design without built-in margins; add one deliberately at the end only if you have a reason.
Limits
A straight sum of the room figures. Heat flows between heated rooms at different temperatures cancel out across the house, so the whole-house total from BS EN 12831-1 can be slightly lower than the sum of room loads.
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)
- BS EN 12831-1:2017 Energy performance of buildings. Method for calculation of the design heat load (transmission H = Σ U·A; ventilation loss from air volume flow × ρ·cp)
- 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.