Boiler Selection

The correct sizing of a boiler is important, whether it is required as a replacement, or for a new installation. An oversized boiler costs more to buy and generally operates less efficientlywhich means higher running costs and increased emissions to the atmosphere.

Boiler size is often established on the basis of the number and size of existing radiators. This nearly always leads to oversized boilers and is not a recommended practice.

The determination of boiler output, other than for a combination boiler, is related directly to the ‘heat loss’ of the property and pipework, plus additions for domestic hot water requirements and an allowance for intermittent heating.

For most domestic dwellings, the heating output of combination boilers far exceeds the needs of the property, because such boilers also have to provide hot water. Other than for large homes with lots of people living there, boiler size is normally arrived at in the following way:

Heat loss of the whole dwelling including extensions.

  • 10% for intermittent heating
  • up to 5% for pipework losses
  • 2 kW for hot water

This, of course assumes that a cylinder is used to provide hot water and that the cylinder is heated by the boiler.

You can go to Selection Wizard in our site for a quick calculation of the capacity you need, however you should contact your dealer for the most accurate calculation of the capacity needed in your home.