Radiator size calculator
To calculate the radiator size, enter the heat demand and operating temperatures: you see the rated output the radiator must have, plus values for common flow temperatures.
Source: Wikipedia – Heizkörper: standard output according to DIN EN 442 at 75/65/20 °C, radiator exponent n. Updated: .
How it is calculated
The lower the flow temperature, the less output a radiator gives. At 55/45 °C and 20 °C room temperature it delivers only about 51 % of its rated output; for 1500 W of heat demand you then need a radiator rated around 2940 W.
How it is calculated
The rated output applies at 75 °C flow, 65 °C return and 20 °C room temperature, which is a logarithmic excess temperature of 49.83 K. At other temperatures Q = QN · (ΔT / 49.83 K)n, with ΔT = (flow − return) / ln((flow − room) / (return − room)). The exponent n is around 1.3 for most radiators.
Good to know
- Heat pumps work most efficiently at low flow temperature; radiators must then be larger than in high-temperature systems.
- The room's heat demand comes from a heat load calculation; as a rough guide 50–100 W/m² is often quoted, depending on insulation.
- The result is the minimum output; some reserve for warm-up phases is usual.
Frequently asked questions
How do you calculate the size of a radiator?
Divide the heat demand of the room by the output factor at your flow temperature. The result is the rated output (75/65/20) the radiator must have in the catalogue.
How much output does a radiator give at 55/45 °C?
About half the rated output at 75/65. A radiator rated 1000 W gives roughly 510 W at 55/45 °C.
What is the radiator exponent?
The exponent n describes how strongly output rises with excess temperature. For panel radiators 1.3 is the usual value.
Is this calculator enough instead of a heat load calculation?
No. It only converts output. The heat demand of the rooms has to come from a heat load calculation.
Sources and legal basis
- Wikipedia – Heizkörper: standard output according to DIN EN 442 at 75/65/20 °C, radiator exponent n
- Wikipedia – Logarithmic mean temperature difference (mittlere logarithmische Übertemperatur)
As of: · Editorial policy
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