A radiator's heat output is not one fixed number. It depends on how much hotter the radiator is than the room, which is called ΔT (delta T). Output figures quoted as ΔT50 and ΔT30 are the same radiator rated under different conditions, and comparing them directly is a common source of undersized radiators.
What ΔT means
ΔT is the difference between the average water temperature in the radiator and the room air temperature. In the common European rating method, ΔT50 corresponds to a flow temperature of 75°C, a return of 65°C and a room at 20°C. The mean water temperature is 70°C, which is 50°C above the room, hence ΔT50.
Common ΔT values
| Rating | Typical example (flow / return / room) | Typical use |
|---|---|---|
| ΔT50 | 75°C / 65°C / 20°C | Standard catalogue rating |
| ΔT30 | about 55°C / 45°C / 20°C | Lower-temperature systems, including many heat pumps |
Other ΔT ratings, such as ΔT60, may also appear on specification sheets. The exact flow and return temperatures behind each rating are defined by the manufacturer, so check the datasheet.
Why output falls at a lower ΔT
A radiator gives off heat in proportion to how hot it is compared with the room, so a cooler radiator releases less. The reduction is not the same for every radiator type, but as a rough guide a ΔT30 output is often around half of the ΔT50 output. Treat that only as a rule of thumb and use the manufacturer's published ΔT30 figure wherever it is available.
How to compare outputs correctly
- Find your system's flow and return temperatures from your installer or heat-loss report.
- Work out which ΔT matches them.
- Compare each radiator's output at that ΔT, not at the headline ΔT50 number.
- Check the unit: watts or BTU/h. One watt is about 3.412 BTU/h.
What this means when you are shopping
If a product lists only ΔT50, ask whether a ΔT30 figure is available before choosing it for a low-temperature system. Our BTU calculator helps estimate the heat a room needs; then compare that with the output at your ΔT. Browse designer radiators and check each product page for its specifications. For a full introduction to sizing, read the radiator size guide.
Common mistakes
- Comparing a ΔT50 radiator with a ΔT30 radiator as if the numbers were equivalent.
- Choosing a radiator for a heat pump using ΔT50 outputs.
- Forgetting that older boilers and newer condensing or heat pump systems run at different temperatures.
In short
ΔT tells you the conditions behind an output figure. Always compare radiators at the ΔT your system actually runs at, and use the manufacturer's data for the final decision.