Radiator & Heat Emitter Sizing
Size radiators from a heat load: ΔT from flow, return and room temperatures, the correction Φ = Φ₅₀·(ΔT/50)ⁿ with your emitter exponent, and the count.
The mean ΔT (arithmetic or logarithmic), the correction factor, the catalogue output the emitter must carry (with margin), and for a chosen emitter the count, installed output and margin.
Example: 1 500 W load, 55/45 °C in a 20 °C room: ΔT 30 K, factor (30/50)^1.3 = 0.515, so 2 914 W of catalogue rating is needed (3 205 W with 10 % margin) — three 1 200 W radiators.
Rating at 50 K,
reality at 30.
Why catalogue outputs shrink at low flow temperatures, how the correction is made, and what the exponent is.
The rating convention
Radiator catalogues (EN 442 in Europe) quote output at an excess temperature ΔT of 50 K — mean water 70 °C in a 20 °C room. A condensing boiler at 55/45 or a heat pump at 45/35 runs far below that, so the same radiator gives a fraction of its rating.
The correction
Output follows Φ = Φ_ref × (ΔT ÷ ΔT_ref)^n, where ΔT is the mean water-to-room difference — the arithmetic mean (flow + return)/2 − room, or the logarithmic mean for a wide drop — and n is the emitter’s exponent from its data sheet (panel radiators are typically listed around 1.3; convectors and underfloor differ). The catalogue rating the room needs is load ÷ factor, plus any margin; dividing by one emitter’s rating gives the count.
What you supply
The heat load (from a loss calculation), the design temperatures, the reference ΔT and exponent from the catalogue, and the margin. No emitter data or "typical" figures are embedded. Fan-assisted and underfloor emitters follow other laws. Nothing leaves the browser; the same four anonymous usage counts as the rest of the site apply.
SOURCES
- EN 442 rating convention (ΔT 50 K) and the power-law correction Φ = Φ_ref·(ΔT/ΔT_ref)^n with the emitter’s own exponent; arithmetic or logarithmic mean ΔT
Last reviewed 21 September 2026. How results are checked: How we verify.