Radiator Size Calculator
At heat pump temperatures, output more than halves.
Work out Radiator Size. At heat pump temperatures, output more than halves. States the assumption instead of hiding it.
Actual output at this flow temperature
668 W
33% of the catalogue figure at Δ21.5
Radiators are catalogued at Δ50 — a mean water temperature 50 °C above the room. Run the same radiator from a heat pump at 45 °C flow and the working ΔT is closer to 22, which cuts its output by well over half. That single fact drives most of the cost of a heat pump retrofit, and it is the thing quotes routinely leave out. A house whose radiators are correctly sized for a 70 °C boiler will be cold on a heat pump unless they are changed or the flow temperature is raised — and raising it wrecks the efficiency. The derating follows ΔT to the power 1.3, the standard exponent from EN 442. The exponent exceeds 1 because radiators shed heat by convection as well as radiation, and convection strengthens faster than the temperature difference itself.
How the Radiator Size Calculator works
Actual radiator output at a real flow temperature, against the Δ50 figure catalogues quote. Run a radiator at a heat pump's 45 °C and it delivers well under half its rated output — the fact that drives most of a retrofit's cost.
Also known as: radiator output at 45 degrees · do i need bigger radiators for a heat pump · delta t 50 explained · radiator watts for a room
Frequently asked questions
What is delta T 50?
The rating condition for radiators: mean water temperature 50 °C above the room. Catalogue outputs assume it, which means they assume a boiler running at roughly 75 °C flow.
Why do heat pumps need bigger radiators?
Because output collapses at lower flow temperatures. At 45 °C flow the working ΔT is around 22 rather than 50, which cuts output by well over half — so the same heat needs roughly twice the radiator.
What is the exponent 1.3 for?
Radiator output scales with ΔT to the power 1.3, the standard from EN 442. It exceeds 1 because radiators shed heat by convection as well as radiation, and convection strengthens faster than the temperature difference.
Can I just raise the flow temperature instead?
You can, and it wrecks the efficiency. Every degree of extra flow temperature costs COP, so running a heat pump at 60 °C to suit existing radiators throws away most of the reason for fitting one.
What about underfloor heating?
It works at much lower flow temperatures — often 35 °C or less — because the emitting area is enormous. That is why it pairs so naturally with a heat pump and why retrofits often combine the two.
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