Heat loss
Room design heat loss (fabric U·A·ΔT plus ventilation 0.33·n·V·ΔT)
- where
- inside to outside design temperature difference (K)
- room design temperature (°C)
- design outdoor temperature (°C)
- fabric heat loss coefficient (W/K)
- U-values of wall, glazing, roof and floor (W/m²K)
- areas of wall, glazing, roof and floor (m²)
- fabric heat loss (W)
- ventilation heat loss (W)
- air change rate (per hour)
- room volume (m³)
- room design heat loss (W)
Explain this equation
Works out how much heat a room loses on the coldest design day, through its walls, windows, roof and floor and through air leakage. This is the figure each radiator or emitter has to meet in a heat pump design.
ΔT is how much warmer the room is kept than the design outdoor temperature. Each element loses U × A watts for every degree of difference; adding them gives H, then multiplying by ΔT gives the fabric loss. Ventilation loss is the heat needed to warm incoming outside air: n air changes per hour of a room of volume V. The 0.33 is the heat to warm one cubic metre of air by one degree, in watt-hours (about 1.2 kg/m³ × 1.005 kJ/kgK ÷ 3.6).
Heat flow through a layer is proportional to its area, its U-value and the temperature difference; air loss is the heat capacity of the air swapped each hour.
Sense check: a typical room with about 22 W/K of fabric and 1.5 air changes of 36 m³ at a 24 K difference loses roughly 1 kW. Double ΔT and the loss doubles.
Watch out: use net wall area with windows taken out, and the MCS design outdoor temperature for the postcode. Rooms next to unheated spaces lose heat to those too.
Works out the heat a room loses on the design cold day: conduction through walls, windows, roof and floor, plus the heat carried away by air changes. Use one card per room, then add them up.
Enter your values to see the result.
Good to know
MIS 3005-D uses room temperatures of 21 °C for living rooms, 22 °C for bathrooms and 18 °C for bedrooms and most other rooms. Take the outdoor design temperature for the postcode from the MCS table (MCS 031 Table 1 gives, for example, −3.0 °C for SW and E London, −3.1 °C for Bristol and Cardiff, −4.5 °C for Manchester, −5.6 °C for Glasgow). Walls to a heated neighbour lose little; walls to unheated spaces can be entered with a reduced U-value or area. CIBSE DHDG lists air change rates by room type and age: about 1.5 ach for an older living room and 1 ach for bedrooms are common starting points; a blower-door test is better.
Common mistake
Using the gross wall area and then adding the windows on top, so the glazed area is counted twice. Enter the wall area net of windows and doors.
Limits
Simplified steady-state method with one ΔT for every element. BS EN 12831-1 treats ground floors, unheated spaces, thermal bridges and ventilation in more detail; the MCS heat loss calculator follows that method. Use this card for checks and quick room sizing, not as the formal MCS design record.
Sources
- 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)
- 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)
- CIBSE Domestic Heating Design Guide (2026 edition; design to BS EN 12831-1, room temperatures, air change rates, pipe and emitter sizing)
- The Open University, Energy in buildings, section 3.3: ventilation heat loss Qv = 0.33 × n × V × ΔT (0.33 Wh/m³K volumetric heat capacity of air)
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.