Heat loss calculator
Room heat loss calculator to BS EN 12831, with radiator and heat pump sizing
AeroCalc Pro works out the design heat loss of each room to BS EN 12831-1:2017 — the method behind the CIBSE Domestic Heating Design Guide and the MCS heat loss calculation — and tells you the radiator output to buy at ΔT50 for whatever flow temperature the boiler or heat pump will run at.
How the heat loss is calculated
Heat loss has two parts: heat conducted out through the room's fabric, and heat carried away by air leaving the room. Both are proportional to the difference between the room temperature and the outside design temperature.
A = area (m²), U = U-value (W/m²K), b = temperature factor for unheated spaces (0.5), θint = room temperature, θe = external design temperature, n = air changes per hour, V = room volume (m³). Ground floors use BS EN 12831's ground factors (fg1 = 1.45, against a UK annual mean of 10°C). A thermal-bridging allowance of 0.1 W/m²K is added to walls, roof and floor.
Room temperatures (CIBSE)
| Room | Design temperature |
|---|---|
| Living room | 21°C |
| Dining room | 21°C |
| Home office | 21°C |
| Bathroom | 22°C |
| Bedroom | 18°C |
| Kitchen | 18°C |
| Hall / landing | 18°C |
| WC / cloakroom | 18°C |
U-values by build era
Pick when the property was built and AeroCalc uses default U-values from RdSAP 10 and the Part L of the time. You can type in measured U-values instead.
| Built | Wall | Roof | Floor |
|---|---|---|---|
| Pre-1930 — solid brick/stone | 2.1 | 0.4 | 0.7 |
| 1930–1975 — unfilled cavity | 1.5 | 0.4 | 0.7 |
| 1976–1995 — partial-fill cavity | 0.6 | 0.35 | 0.6 |
| 1996–2006 — insulated cavity | 0.45 | 0.25 | 0.25 |
| 2007–2021 — Part L 2006/2013 | 0.28 | 0.16 | 0.22 |
| 2022 on — Part L 2021 | 0.18 | 0.11 | 0.13 |
| Glazing | U-value |
|---|---|
| Single glazed | 4.8 |
| Double, pre-2002 | 2.8 |
| Double, modern low-E | 1.4 |
| Triple glazed | 0.8 |
UK external design temperatures
Use the 99.6% winter design temperature for the site. For MCS work, use the figure MCS 031 gives for the property's postcode. AeroCalc includes these approximate CIBSE figures as presets:
| Location | Design temperature |
|---|---|
| London | -1.8°C |
| Southampton | -1.9°C |
| Bristol | -2.3°C |
| Plymouth | -0.2°C |
| Cardiff | -1.6°C |
| Birmingham | -3.4°C |
| Manchester | -2.2°C |
| Leeds | -2.6°C |
| Newcastle | -2.4°C |
| Belfast | -1.2°C |
| Glasgow | -3.9°C |
| Edinburgh | -3.4°C |
| Aberdeen | -4.4°C |
Radiator sizing for boilers and heat pumps
A room loses the same heat whatever heats it — so AeroCalc never inflates the heat loss for a heat pump. What changes is how much a radiator gives out. Radiators are rated at ΔT50 (BS EN 442): a mean water temperature 50°C above the room. At a lower mean water temperature the output falls:
Worked example: a 5 × 4 × 2.4 m living room built 1976–1995, one 5 m outside wall with 2.5 m² of modern double glazing, a solid ground floor and a heated room above, at an external design temperature of −1.8°C, loses 996 W. With a condensing boiler at 70/50°C (mean water 60°C) the radiators give 72% of their rating, so you need about 1,376 W at ΔT50. With a heat pump at 45/40°C (mean 42.5°C) they give about a third, so you need roughly 2,980 W at ΔT50.
What you get in AeroCalc
- Room-by-room heat loss with a breakdown by walls, windows, roof, floor and ventilation.
- Whole-house total per location, for sizing the boiler or heat pump.
- Radiator output at ΔT50 for 75/65, 70/50, 55/45 and heat pump flow temperatures down to 35/30°C.
- LiDAR room scanning on iPhone Pro to measure rooms for you.
- Rooms saved by property, shared with your team, and sent to LilaCRM as a survey.
AeroCalc gives design estimates. For MCS certification, check your inputs against MCS 031 and the manufacturer's data.
Try it free
No account needed for the demo. Sign up free to save up to 5 rooms.