Room-by-room heat loss: the step-by-step method
A heat loss calculation works out how many watts each room loses on the coldest design day. Add the rooms together and you have the figure that sizes the heat pump. Keep them separate and you can size each radiator.
Here is the method, the two formulas behind it, and a worked example.
The two parts of the heat loss
Every room loses heat through its fabric (walls, windows, doors, floor and ceiling) and through ventilation (air leaking or being extracted and replaced with cold outside air).
Fabric loss (W) = U-value × area (m²) × temperature difference (K)
Ventilation loss (W) = 0.33 × air changes per hour × room volume (m³) × temperature difference (K)
The temperature difference is the room's design temperature minus the outside design temperature, or minus the temperature of the space on the other side for internal walls and floors.
Design temperatures
| Room | Typical design temperature |
|---|---|
| Living room, dining room | 21 °C |
| Kitchen | 18 °C to 20 °C |
| Bedroom | 18 °C |
| Bathroom | 22 °C |
| Hall and landing | 18 °C |
The outside design temperature depends on where the house is. MCS uses values for weather stations around the UK, corrected for altitude. Colder sites and higher altitudes mean a bigger temperature difference and a bigger heat loss.
Worked example: one external wall
- Wall area after taking off the window: 9.5 m².
- Cavity wall with insulation, U-value 0.55 W/m²K.
- Living room at 21 °C, outside design temperature -2 °C, so ΔT = 23 K.
- Fabric loss = 0.55 × 9.5 × 23 = 120 W.
Repeat for every element of every room, add the ventilation loss, and you have each room's heat loss. Add the rooms for the whole house.
Where calculations go wrong
- Guessing U-values instead of identifying the construction.
- Using the wrong outside design temperature for the site.
- Forgetting heat lost to unheated spaces such as garages and lofts.
- Missing ventilation, which can be a large share of the loss in draughty older houses.
Do it with the free tool
Measure each room and pick the constructions, and the free tool does the U × A × ΔT and ventilation sums for every room as you type.
Frequently asked questions
What is the formula for heat loss?
For each part of the building fabric, heat loss = U-value × area × temperature difference. Ventilation heat loss = 0.33 × air changes per hour × room volume × temperature difference. Add them for each room.
What is a U-value?
A U-value measures how much heat passes through a square metre of a wall, window, roof or floor for each degree of temperature difference, in W/m²K. Lower is better insulated.
Why do a heat loss calculation room by room?
Because radiators are sized room by room. A whole-house figure can size the heat pump, but not tell you whether each room's radiator is big enough at a low flow temperature.
How much does a heat loss survey cost?
Prices vary widely by company and house size. Installers often include it in the heat pump quote, and you can do the calculation yourself with free tools if you know the method.
Is there a free heat loss calculator?
Yes. Free Heat Pump Design is a free room-by-room heat loss calculator for UK installers. It follows the MCS method, but isn't an MCS product, so check results against the official MCS calculator.
Related guides
Sources: MCS MIS 3005-D; CIBSE Guide A and the CIBSE Domestic Heating Design Guide; BS EN 12831. Free Heat Pump Design follows the MCS method but is not an MCS product and is not endorsed by MCS. Check designs against MCS MIS 3005-D and the official MCS Heat Pump Calculator.