Heat loss calculator: room by room
The calculator needs JavaScript. The worked example, formula and figures below show exactly how it works, so you can do the sum yourself. A guide, not advice. Check your supplier's or maker's figures before ordering.
Short answer
Worked example
| You enter | Value |
|---|---|
| Rooms | Bedroom: room temperature 21 °C, floor area 20 m², ceiling height 2.4 m, external wall 10 m², external wall 0.3 W/m²K, windows area 2 m², windows u-value 1.4 W/m²K, external doors area 0 m², floor area 0 m², roof or ceiling area 0 m² |
| Region | I'll enter my own |
| Design outdoor temperature | -1 °C |
| Air changes per hour | 1 /h |
Heat loss: 476 W (0.48 kW) at the design temperature
| Result | Value |
|---|---|
| House heat loss | 476 W (1,624 BTU/h) |
| House heat loss | 0.48 kW (1,624 BTU/h) |
| Fabric loss (walls, windows, doors, floor, roof) | 128 W (435 BTU/h) |
| Ventilation loss | 348 W (1,189 BTU/h) |
| Heat loss per m² of floor | 24 W/m² |
| Design outdoor temperature used | -1 °C |
| Room | ΔT | Fabric | Ventilation | Total | Per m² |
|---|---|---|---|---|---|
| Bedroom | 22 K | 128 W | 348 W | 476 W | 24 W/m² |
Show the working
- Design outdoor temperature: -1 °C (your figure)
- Bedroom: ΔT = 21 − (-1) = 22 K
- Wall: 10 m² × 0.3 W/m²K × 22 K = 66 W
- Windows: 2 m² × 1.4 W/m²K × 22 K = 61.6 W
- Fabric: 127.6 W
- Ventilation: 0.33 × 1 /h × 48 m³ × 22 K = 348.5 W
- Room total: 127.6 + 348.5 = 476.1 W
- House: 127.6 W fabric + 348.5 W ventilation = 476.1 W (0.48 kW)
How it works
- Temperature difference ΔT (K) = room temperature − design outdoor temperature
- Fabric loss (W) = Σ (area A × U-value U × ΔT) for each wall, window, door, floor and roof, plus any thermal bridging allowance
- Ventilation loss (W) = 0.33 × air changes per hour n × room volume V × ΔT (V = floor area × ceiling height)
- Room heat loss = fabric loss + ventilation loss; house heat loss = the sum of the rooms
Not an MCS heat loss survey. An installer sizing a heat pump for the Boiler Upgrade Scheme must do their own room-by-room calculation to MCS MIS 3005-D.
Simplified BS EN 12831-1: no allowance for heat-up, intermittent heating or heat flow to neighbours. Floors on the ground use the floor U-value you enter.
Our regional design temperatures and air change rates are still being checked: enter your own until they are filled in.
Our default (adjustable)
These are typical planning figures and our own allowances, not facts about your product. Change any of them in the calculator to match your bag, pack or tin.
| Figure | Our default (adjustable) | Kind | Source |
|---|---|---|---|
| Air changes per hour | Enter from the bag, pack or tin | Our assumption Not yet checked against the source | Our choice: change it to suit your job Brief v2 H1: air change rate. No figure stated in the brief; Researcher to fill from CIBSE Domestic Heating Design Guide / BS EN 12831-1 by room type and age. Until then the user enters it. |
| Thermal bridging allowance on fabric loss | Enter from the bag, pack or tin | Our assumption Not yet checked against the source | Our choice: change it to suit your job Brief v2 H1: thermal bridging allowance is an assumption. No figure stated; Researcher to fill. Blank means no allowance. |
Regulations and standards
- BS EN 12831-1 (simplified room-by-room method)
- CIBSE Domestic Heating Design Guide
Explained: BS EN 12831 and CIBSE DHDG (heat loss), MCS MIS 3005-D (heat pump design).
The method is the same across the UK. Design outdoor temperatures vary by region.
| Table | Clause | Nations | Status |
|---|---|---|---|
| Design outdoor temperature by region | CIBSE Guide A / MCS MIS 3005-D design outdoor temperatures (Researcher to cite the exact table) | UK-wide | 0/11 values entered Not yet checked against the source |