Heat loss & emitters
Radiator output at operating temperatures
- where
- operating mean-water / air difference (K)
- flow temperature (°C)
- return temperature (°C)
- room temperature (°C)
- radiator output (W)
- published rated output (W)
- published rated mean-water / air difference (K)
- manufacturer exponent
Explain this equation
Estimates what a radiator will actually give out at your flow, return and room temperatures, compared with its catalogue rating. Use it when running lower flow temperatures, for example with a heat pump.
T_f and T_r are the flow and return temperatures; their average is the mean water temperature. Take away the room temperature T_a and you get ΔT, the mean-water to air difference. P_rated is the catalogue output at ΔT_rated, often 50 K. n is the manufacturer's exponent for that radiator.
A radiator's output does not fall in straight proportion with ΔT. Convection weakens as the surface gets cooler, so output follows a power law with n usually around 1.3.
Sense check: a 2000 W radiator rated at ΔT 50, run at 55/45 °C in a 20 °C room (ΔT 30), gives about 1030 W, roughly half.
Watch out: use the actual radiator's exponent or correction factors. The arithmetic mean is an approximation.
Enter your values to see the result.
Good to know
Use the specific radiator exponent or manufacturer correction data. Arithmetic mean-temperature power law is an estimate; unusual flow conditions can require a different method.
Sources
Results are arithmetic only. Worked examples are not recommended design settings. This library does not provide automatic gas pipe selection, live gas tightness tests, purging procedures, combustion acceptance limits, installation approval or commissioning certification. Manufacturer data and the applicable current method remain essential.