The following appendix gives details on how to size a radiator for a room with an example heat loss of 500W. This example details how to size the radiators for heat generators with varying flow and return temperatures.

Radiator selection

Radiator outputs are typically based on a water-to-air temperature difference ΔT 50°C in manufacturer's product catalogues.

The higher the ΔT value, the higher the radiator output will be. Traditional gas or oil non-condensing boilers operate with higher flow and return temperatures 82/71°C, while condensing boilers operate at lower flow and return temperatures 65/55°C, to increase their efficiency. Heat pumps normal operating temperatures are 45/40°C, to increase their efficiency – see examples below on how to size radiators for various heat generators.

Traditional gas or oil non-condensing boilers, flow and return temperatures 82/71 and room temperature 20:
Condensing boilers, flow and return temperatures 65/55°C and room temperature 20°C:
Heat pump technology, flow and return temperatures 45/40°C and room temperature 20°C:

The reduced temperature difference reduces the output of the radiator. Many radiator manufacturers supply information for radiator output based on a ΔT value of 50°C. Table B1 is an example of typical information detailed in a radiator manufactures radiator catalogue:

Table B1: Radiator sizes and outputs

Where radiators are to be installed for different ΔT values, the stated radiator outputs must be multiplied by a conversion factor to account for the different ΔT value. Manufacturers should be asked to provide conversion factors for different ΔT values.

The following table is an example of conversion factors to be applied to outputs quoted at ΔT 50°C. Radiator manufacturers supply specific conversion factors for their specific products and these should be requested by the designer:

Table B2: Example of Radiator conversion factors

The conversion factor allows the calculation of the radiator output where the operating temperatures result in a ΔT value different to that of the stated ΔT 50°C. As the ΔT value reduces, larger radiator dimensions will be required to achieve the same heat output. Check with radiator manufacturer for specific conversion factors.

If a room had a calculated heat loss of 500W the following are examples detail how to select a suitably sized radiator for the space.

As detailed above:

Traditional gas or oil non-condensing boilers, 82/71°C and room temperature 20°C has a 56.5°C ΔT

Using Table B2 you need to calculate the factor for 56.5°C ΔT and as the table is only in increments of 5°C ΔT , then take factor of 60 ΔT°C and multiply output of 1.268 from Table B2.

Therefore, to achieve the target of 500 Watts output for the room, we need to select a radiator from Table 1 with an output of at least:

Selecting a radiator size of 424 Watts (400mm L x 450mm H) from Table B1 and multiply the calculated multiply output of 1.186

The radiator selected will give an output of 502 Watts for a room with a heat loss of 500 Watts.

Condensing boilers flow and return temperatures 65/55°C and room temperature 20°C, has a 40°C ΔT

Using Table B2 you need to use the factor 40°C ΔT of 0.748

Selecting a radiator size of 758 Watts (700mm L x 450mm H) from Table B1 and multiply the calculated multiply output of 0.748

The radiator selected will give an output of 566 Watts for a room with a heat loss of 500 Watts.

Heat pump technology, 45/40°C and room temperature 20°C, has a 22.5°C ΔT

Using Table B2 you need to calculate the factor for 22.5°C ΔT and as the table is only in increments of 5°C ΔT, then take factor of 20°C ΔT and multiply output of 0.304 from Table B2.

Selecting a radiator size of 1536 Watts (1400mm L x 450mm H) from Table B1 and multiply the calculated multiply output of 0.342

The radiator selected will give an output of 525 Watts for a room with a heat loss of 500 Watts.

Table B3 below illustrates the changes in and how this effect the radiator sizes for varying flow and return temperatures

Table B3: Changes in radiator sizes with varying temperature differences

Appendix B: Guide to the radiator sizing for various heat generators