Heat loss calculation: the basics of TS EN 12831
Sizing a heating system correctly starts with knowing how much heat each room loses on the design day.
A heat loss calculation finds how much heat each room needs to hold its indoor temperature on the coldest design day. The result is the basis for choosing radiators, fan coils or underfloor heating, and for boiler capacity.
Three components
- Transmission loss: heat conducted out through walls, windows, roof and floor.
- Ventilation loss: heat needed to warm incoming cold outdoor air.
- Reheat allowance: extra capacity to warm the room back up after intermittent heating.
Q_T = Σ A · (U + ΔU_TB) · (T_in − T_out) · bQ_V = 0.33 · V · (T_in − T_out)Terms
- U: thermal transmittance of the element (W/m²K). Falls as insulation increases.
- ΔU_TB: simplified allowance for thermal bridges.
- b: temperature correction factor. Below 1 for elements next to unheated spaces.
- V: ventilation or infiltration air flow (m³/h). The 0.33 factor is the volumetric heat capacity of air (W·h/m³K).
From calculation to equipment
Once room losses are summed, heaters are selected with a safety factor. Radiator output is found by correcting the nominal rating for actual water temperatures. Boiler capacity comes from room losses, the air handling unit heater and domestic hot water demand taken together.
Note: this article explains the core logic of the method. In a project calculation, element values, climate data and correction factors must be set for the specific project.
Want to see these calculations on your own plant?
Request a demo