Radiator Output Calculator
The real heat output of a radiator at your actual flow and return temperatures, against its catalogue rating.
Results
What this tool does
Radiator catalogues quote output at delta T 50, meaning a mean water temperature fifty degrees above the room. Very few systems run that hot, and a heat pump certainly does not. Drop the mean water temperature from 70 to 45 °C and a radiator gives back barely half its rated output — which is precisely why replacing a boiler with a heat pump so often means replacing radiators too.
Formula
Q = Qn (dT / 50)^1.3 ; dT = (one way + return)/2 - temperature ambient
Variables
| Symbol | Meaning | Unit |
|---|---|---|
qn | Catalogue output at delta T 50 | W |
tf | Flow temperature | °C |
tr | Return temperature | °C |
ta | Room temperature | °C |
ex | Radiator exponent | — |
QQ | Actual output | W |
DT | Temperature difference | K |
PC | Share of rated output | % |
LO | Output lost | W |
Worked example
- Catalogue output at delta T 501500 W
- Flow temperature65 °C
- Return temperature55 °C
- Room temperature20 °C
- Radiator exponent1.3
- Actual output1122.3 W
- Temperature difference40.00 K
- Share of rated output74.82 %
- Output lost377.7 W
Limitations
- Coverage and consumption figures vary between products. Take the values from the manufacturer's datasheet for the product you are actually using.
- The result is an estimate based only on the values you type. Real situations often include factors this calculator does not know about.
- For work that must comply with a standard or be signed off, check the result against the applicable code and have it reviewed by a qualified engineer.
Frequently asked questions
Why is the exponent 1.3?
Because a radiator does not transfer heat in proportion to its temperature difference: part of the output is natural convection, which strengthens more than linearly as the surface gets hotter, and part is radiation, which follows a fourth-power law. The combined behaviour is fitted with a single exponent, and manufacturers publish it with the rated output. Panel radiators are usually around 1.3, while fan-assisted emitters are closer to 1.0.