Uses the standard radiator exponent n = 1.3. Catalogue outputs are normally quoted at ΔT50; low-temperature and heat-pump systems run a smaller ΔT and deliver less.
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This calculator converts a radiator's rated (catalogue) heat output to the output it actually delivers at your system's ΔT — the difference between the mean water temperature and the room temperature. Enter the rated output, the ΔT it was rated at (usually 50°C) and the ΔT your system runs at, and it returns the correction factor and the corrected output in watts and BTU/h.
It is built for UK heating engineers designing or checking systems that run cooler than the catalogue conditions — heat pumps, low-temperature and weather-compensated systems. Manufacturers quote radiator outputs at ΔT50, but a heat pump running 45°C flow into a 20°C room only sees a ΔT of around 25–30, at which the same radiator gives far less heat.
Getting this right is the difference between a warm room and a cold one. Size emitters on the catalogue ΔT50 figure for a low-temperature system and every radiator will be undersized — this tool makes the derating explicit so you can pick a radiator that actually meets the room's heat loss at your flow temperature.
Output at ΔT = rated output × (ΔT_actual / ΔT_rated) ^ 1.3
Radiator output does not scale linearly with temperature difference — it follows a power law with an exponent n of about 1.3 for panel radiators. Dividing your system's ΔT by the ΔT the radiator was rated at, raising the result to the power 1.3, and multiplying by the rated output gives the corrected output. Because the exponent is above 1, a small drop in ΔT causes a proportionally larger drop in output, which is why low-temperature systems need noticeably bigger radiators.
Type the radiator's catalogue output in watts. This is almost always quoted at ΔT50 — check the manufacturer's data to confirm the ΔT the figure is rated at.
Enter the ΔT the output was rated at. The default is 50°C, the standard EN 442 rating condition; some older or continental data is quoted at ΔT60 or ΔT70.
Enter the ΔT your system runs at — the mean water temperature minus the room temperature. A boiler at 75°C mean into a 21°C room is ΔT54; a heat pump at 45°C flow / 40°C return (42.5°C mean) into 21°C is about ΔT21.
The calculator shows the correction factor and the real output in watts and BTU/h. Compare that corrected figure — not the catalogue figure — against the room's heat loss when selecting the radiator.
The exponent n = 1.3 is the standard value for panel radiators and gives a good approximation across the normal working range. Individual products have a slightly different exponent published in their technical data (typically 1.24–1.34), so for critical designs use the manufacturer's own correction table where available.
Mean water temperature is the average of flow and return, and ΔT is that mean minus the room's design temperature. Work in mean water temperature consistently — mixing a flow temperature with a return-based ΔT gives the wrong factor. For weather-compensated systems, design against the flow temperature at the design outdoor condition, not the mild-weather setpoint.
This corrects a known radiator's output between conditions; it does not size the room. Establish the room's heat loss first (a full heat loss calculation for low-temperature systems), then use this factor to check a radiator delivers that requirement at your flow temperature. Guidance only — verify against the manufacturer's data and system design.
Uses the standard radiator exponent n = 1.3. Catalogue outputs are normally quoted at ΔT50; low-temperature and heat-pump systems run a smaller ΔT and deliver less.
ΔT50 means the radiator's output is quoted with a 50°C difference between the mean water temperature and the room air temperature — for example 70°C mean water into a 20°C room. It is the EN 442 standard rating condition, so most UK catalogue outputs are ΔT50 figures. If your system runs a smaller ΔT, the real output is lower.
Divide your system's ΔT by the rated ΔT, raise the result to the power 1.3, and multiply by the catalogue output. For example, a 1500 W ΔT50 radiator at ΔT30 gives 1500 × (30/50)^1.3 ≈ 1500 × 0.52 = 780 W — barely half its rated output.
Heat pumps run much lower flow temperatures than boilers, so the ΔT between the water and the room is small — often 20–30 rather than 50. Because output falls with ΔT to the power 1.3, a radiator at ΔT25 gives roughly 40% of its ΔT50 catalogue output, so you need a much larger radiator to deliver the same heat.
It depends on the flow temperature the system is designed to. A heat pump at 45°C flow and 40°C return has a mean water temperature of 42.5°C, so into a 21°C room the ΔT is about 21. Lower design flow temperatures (35–40°C) give even smaller ΔTs and require correspondingly larger emitters.