Friday, 7 August 2015

“Delta T” – What is it? What does it mean? How does it affect you?

‘Delta T’ is a term we heating & radiator manufacturers and plumbers use to describe a calculation gleamed from 3 variables that affect the heat output of radiators, they are:
The mean temperatures of the water found in central heating systems, both coming into and out of the boiler, and that of the localised ambient temperature( i.e. room temperature.)

The reason why it is calculated is to determine the number of radiators required in order to heat a room effectively or how many BTU’s (British Thermal Units) or Watts are needed.

In order to calculate the ‘delta t’, the mean temperature of the water in the central heat
ing system must be attained first.
This is worked out by taking the inlet (into the radiator) temperature and adding the outlet (leaving the radiator) temperature together, then dividing by two.
So if the water measures 90°C when it leaves the boiler and then is 70°C when it returns, having completed a circuit, the mean temperature of the water is 80°C.

Calculation 1. (90 + 70 = 160) / 2 = 80)).

So if the ambient room temperature is 20°C and the mean water temperature in the system is 80°C the ‘delta t’ is: 60Δt.

Calculation 2. 80 – 20 = 60

All our radiators are extensively independently tested, so we know the BTU and Watt output with the resulting variables to give us the 60Δt. In order to measure the actual output of any radiator across a range of differing delta t values a conversion chart is required which calculates correction factors.


A lower delta t value is the result of lower water temperatures. It is important to note that this means more radiators are required in order to heat the room to a suitable level.

Correction Chart
Delta t                    Correction Factor
60Δt                                        1.00
55Δt                                        0.89
50Δt                                        0.79
45Δt                                        0.69
40Δt                                        0.59
35Δt                                        0.50
30Δt                                        0.41
25Δt                                        0.32
20Δt                                        0.24

So if a 5 section radiator had a BTU output of 1500 at 60Δt it would reduce to 615 at 30Δt.
Calc 3. 1500 * 0.41 = 615
More radiator sections would be required to match the output of the original 60Δt.

In essence we use the above measurements and calculations to determine how many radiators or what size radiators are required to heat up a room effectively.

Basically, we use science and mathematics to keep you nice and cosy when it gets a little chilly!

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