Radiators are rated at ΔT50 (mean water temp 70°C, room 20°C — the EN 442 standard). A heat pump runs far cooler, so each radiator delivers much less than its catalogue figure. This checks, room by room, whether the existing emitters still meet the heat loss at your design flow temperature — and how much bigger they'd need to be if not.
1. Set your design flow temperature (the heat pump's target, e.g. 45°C or 50°C) and the system ΔT (flow minus return — typically 5°C on a heat pump).
2. For each room, enter the heat loss (W) from your BS EN 12831 calc, the existing radiator's rated output at ΔT50 (W, from the manufacturer datasheet or the CIPHE radiator guide), and the room design temperature.
3. The tool computes the radiator's actual output at your conditions using the EN 442 relationship: output = rated × (ΔT ÷ 50)1.3, where ΔT = mean water temp − room temp.
A radiator passes if its actual output meets the room heat loss. If not, the tool shows the rated (ΔT50) output you'd need and the upsize factor.
Most steel panel radiators use n ≈ 1.3. Column and some designer radiators can differ — check the manufacturer's data if you need exact figures.
| Room | Heat loss (W) |
Existing rad @ΔT50 (W) |
Room °C |
ΔT | Output now (W) |
Status | Need @ΔT50 (W) |
Upsize |
|---|
Factor applied to a radiator's ΔT50 output to give its output at a given ΔT, using (ΔT ÷ 50)1.3. At a 45/40°C flow/return into a 20°C room (ΔT 22.5), a radiator delivers about 35% of its catalogue figure — so it needs roughly 2.8× the output to do the same job.
Indicative tool. Results depend on accurate room heat loss figures (BS EN 12831) and correct ΔT50 radiator data. The n = 1.3 exponent is a standard panel-radiator assumption. This check supports system design but is not a substitute for a full MCS heat pump design, and does not produce an MCS-compliant certificate.