Radiator Output Calculator (Preliminary)

Why does radiator output depend on temperature?

The output printed in a catalogue applies to one condition. In EN 442 that condition is 75 °C flow, 65 °C return and 20 °C room temperature, a difference of 50 K between mean water temperature and the room. If your system runs at lower water temperatures the same radiator gives less heat.

The correction formula

Actual output is Q = Q_n × (ΔT / ΔT_n)^n, where ΔT is the actual mean temperature difference, ΔT_n the rated difference in the catalogue and n the radiator exponent. The exponent is around 1.3 for panel radiators and is listed for each model.

Example

A radiator rated at 1000 W runs on 55/45 °C water in a 20 °C room. The mean difference is (55 + 45) / 2 − 20 = 30 K and the correction factor (30 / 50)^1.3 = 0.515. The radiator gives 515 W, about half its catalogue value.

Looked at the other way: if the room needs 1000 W, a radiator with a catalogue output of 1943 W is required at this condition.

Radiators with condensing boilers and heat pumps

Condensing boilers are efficient at low return temperatures and heat pumps at low flow temperatures. When water temperature drops, so does the output of existing radiators. This tool gives a rough view of what your radiators will deliver at the new temperature and how much larger they need to be.

Limits

The tool does not calculate room heat loss, it only corrects radiator output for temperature. The heat required must come from a heat loss calculation such as EN 12831. Position, obstructions and the connection arrangement affect output as well.