Watts and BTUs
Radiator output is quoted in watts (W) or British Thermal Units per hour (BTU/h). One watt is about 3.41 BTU/h. A higher figure means more heat. Bigger radiators, double panels and extra fins all increase output.
What Delta T means
Manufacturers state output at a particular temperature difference between the radiator water and the room, known as Delta T. The figure is usually given at a standard test condition. If your system runs at a lower water temperature – common with condensing boilers set efficiently and with heat pumps – the radiator gives out noticeably less heat than the headline figure.
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Sizing a radiator
The output a room needs depends on:
- Floor area and ceiling height
- Number and size of windows and type of glazing
- Outside walls and their insulation
- Which rooms are above, below and beside
- The temperature you want the room to be
