Steady-state design heat loss room by room: fabric Σ(U×A×ΔT) per element plus ventilation, rolled up to a whole-dwelling total, with a per-room emitter-sizing (ΔT50) handoff.
Tender-stage only. Verify final design to project spec and CIBSE Guide A / BS EN 12831-1.
For design guidance only. Always verify with a qualified engineer.
About this room heat loss calculator
This free room-by-room heat loss calculator works out steady-state design heat loss for each room and rolls it up to a whole-dwelling total, so you can size heat sources and emitters. It is aimed at heating engineers, M&E designers and installers doing a room-by-room load assessment for boilers, heat pumps and radiator or underfloor systems, following the CIBSE Guide A / BS EN 12831-1 elemental method. Add rooms and their fabric elements, set the ventilation, and each room also carries a per-room emitter-sizing handoff. Everything runs in your browser. Nothing is uploaded.
How room heat loss is calculated
Fabric loss is the sum over each element of U × A × ΔT (BS EN 12831-1 / CIBSE Guide A §5.2), with ΔT taken to each element’s own adjacent-space temperature, defaulting to the external design temperature. Ventilation loss is 0.33 Wh/(m³·K) × flow × ΔT × (1 − heat recovery%), where the flow is either air-changes × volume or a direct l/s rate converted at 1 l/s = 3.6 m³/h. The room total is fabric + ventilation. The emitter handoff then applies the BS EN 442 power law F = (ΔT ÷ 50)n at the system mean water temperature to give the required ΔT50 output = room load ÷ F.
Frequently asked questions
What external design temperature should I use?
Use the local winter design temperature from CIBSE Guide A Table 2.5 for your area, the calculator defaults to −3 °C, a common London figure. It sets the ΔT for both fabric and ventilation losses, so a colder external design temperature raises the calculated heat loss.
Can I model an internal wall to a cooler space?
Yes. Each element takes an optional adjacent-space temperature; leave it blank and the wall loses heat to the external design temperature, or enter a value for a party wall, unheated garage or corridor so the ΔT reflects the real temperature difference across that element rather than to outside.
What is the required ΔT50 output for?
It is the nominal catalogue output an emitter must be specified at to meet the room load at your system flow and return temperatures, using the BS EN 442 correction. It is most useful for low-temperature and heat-pump systems, where the corrected figure is well above the raw room load. This is a tender-stage estimate. Verify to project specification and BS EN 12831-1.