Turns a room's design heat load into the radiator output a catalogue must be SPECIFIED at, using the BS EN 442 power law F = (ΔT/50)n, with a quantified oversizing advisory for low-temperature / heat-pump systems.
Chains straight off Room-by-Room Heat Loss's room total, or enter a load directly.
For design guidance only. Always verify with a qualified engineer and manufacturer catalogue data.
About this radiator selection calculator
This free radiator selection calculator turns a room’s design heat load into the nominal catalogue output a radiator must be specified at, correcting for the flow, return and room temperatures the system actually runs at. It is aimed at heating engineers and M&E designers choosing panel radiators from manufacturer data, and is especially useful for low-temperature and heat-pump systems where the standard catalogue rating badly overstates real output. It chains straight off the Room-by-Room Heat Loss calculator’s room total, or you can enter a load directly. Everything runs in your browser. Nothing is uploaded.
How radiator selection is calculated
Radiators are catalogued at the BS EN 442 reference condition of 75/65/20 °C, giving a mean water-to-room difference of ΔT = 50 K. The calculator finds the actual mean water temperature (flow + return) ÷ 2 and the actual ΔT = mean water − room, then applies the BS EN 442 characteristic power law F = (ΔT ÷ 50)n, with the emitter exponent n ≈ 1.3 for a panel radiator. Inverting it gives the required nominal output = room load ÷ F and the oversize factor = 1 ÷ F. An advisory flags whether the radiator is at rating (F ≥ 1), needs up to 2× oversizing (F ≥ 0.5) or more than 2× at heat-pump flow temperatures.
Frequently asked questions
What exponent (n) should I use?
The exponent describes how a radiator’s output changes with ΔT. For a standard steel panel radiator it is about 1.3, and the calculator defaults to that; the practical range is roughly 1.2 to 1.35. Column and towel radiators differ slightly, so use the manufacturer’s quoted exponent where it is published rather than the default.
Why does a heat pump need such a large radiator?
Because output falls with the square-ish power of ΔT. At a 45/40 °C heat-pump flow into a 20 °C room the mean water-to-room difference is only about 22.5 K, giving F well below 0.5, so the radiator must be specified at more than twice its catalogue ΔT50 output to deliver the same heat. That is why the tool reports the oversize factor explicitly.
Is the required output the size I buy?
It is the nominal ΔT50 output to specify against a catalogue, since that is how manufacturers publish figures. Pick the next radiator whose catalogue output meets or exceeds it. This is design guidance only under BS EN 442. Verify against the manufacturer’s own output data and your project specification.