You can download the document for “An Introduction to Renewable Heating” here. Or view all documents available for downloads here.

If we were to repeat the process of calculating the heating requirement for a single room for the rest of the building, obtaining heat loss values for every single room of the house, we would get:


Here we get a total heating requirement of 14,164 Watts and thus for this house we would need to choose the next largest boiler size up from this for the space heating requirements, i.e. 15kWp.

However, for a property we tend to use a more simple process that looks at the bigger picture. In the heating world, radiators are sized per room whilst boilers tend to be sized to the house!

One such method is the “The Whole House Boiler Sizing Method” from the Energy Saving Trust. This is a simple and easy way to correctly size a boiler for a house. This method should not be used for combi-boilers, and the U values used are generalised for the UK, meaning that older houses will need to use a more detailed calculation, as expressed above.

The assumptions used are:

  • 19.2 degrees Celsius is required for the internal temperature
  • External temperatures depend on the property location and factored to reflect as such
  • 10% allowance for intermittent heating (included in the location factor)
  • 5% allowance for pipe losses (included in the location factor)
  • Ventilation rate of 0.76 air changes per hour

In this method we split a property into rectangles and measure the external wall lengths. For a simple property such as the one below this can be very straight forward:


For more complicated structures we must separate the building into rectangles and create a calculation for each (similarly for extensions and conservatories we separate them and do a separate calculation):


Once we have the measurements we then determine what type of property we are dealing with (detached house, semi-detached, mid terrace or flat); the properties location in the UK; the Window type; the floor type; the wall insulation levels and finally the roof insulation levels. Given this data we can then get a very accurate measurement of a properties space heating requirement.

Luckily, Cernunnos Homes has a spreadsheet calculator that will do this for you!


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