Delta T and Heat Output explained

As explained in a previous post, when selecting your radiators you will need to work with your plumber to determine the heat requirements of your rooms and you need to be aware of the different standards used in quoting the Heat Output (Watts or BTU) of  radiators, so that you get the right size radiators for your project.

The heat output of radiators varies with the size and design of the radiators.  The heat output of a given radiator varies with its temperature or more precisely the difference between the temperature of the radiator and the surrounding air temperature. This temperature differential is referred to as Delta T, written  ΔT. ΔT is defined as the mean water temperature of the water in the radiator minus the ambient air temperature. For example, if the water flowing into a Radiator is 85oC and the water flowing out of it is 75oC then the Mean Water Temperature (MWT) is 80oC, if the Air Temperature (AT)  in the room is 20oC then ΔT=MWT-AT = 80oC-20oC= 60oC or 60 K (Kelvin).

The heat output increases as ΔT increases but the relationship is not linear, halving ΔT more than halves the heat output, so for example the heat output at ΔT=30 is less than half the heat output at ΔT=60.

Heat Output can be measured in Watts, or BTU/h. Throughout our website we give heat outputs in BTU/h – British Thermal Units per hour. To convert Watts to BTU/h just multiply by 3.412 , to convert BTU/h to Watts divide by 3.412 or use the handy conversion tools on

Unless otherwise stated the heat outputs on our website are given for the traditional industry standard of ΔT=60, to find the heat output for other values of ΔT the following conversion table will need to be used:

ΔT Conversion Factor
10 0.097
15 0.165
20 0.240
25 0.320
30 0.406
35 0.496
40 0.590
45 0.688
50 0.789
55 0.893
60 1
65 1.110
70 1.223




So for example if a Radiator has a Heat output of 3600 BTU at ΔT=60, to find the heat output at ΔT=50 multiply by 0.789.
Heat output (ΔT=50)= Heat Output (ΔT=60 )* 0.789 = 3600BTU * 0.789 = 2840 BTU

Similarly, to calculate the heat output at ΔT=55 multiply the heat output at ΔT=60 by 0.893.

Heat output (ΔT=55)= Heat Output (ΔT=60 )* 0.893 = 3600BTU * 0.893 = 3215 BTU

Plumbing systems using modern Condensing Boilers typically work at a value of ΔT=50. “Low” temperature systems operate at still  lower values of ΔT and are becoming more popular as people consider the environmental impact of traditional heating systems. Our radiators work with ‘low’ temperature systems, however more of them are required to achieve the same result as a traditional high temperature system and they need to be balance correctly.