Heat loss calculation
The heat loss calculation indicated below is a simple example of how the heat requirement is calculated for a room. Our chart to how hot you should heat each room in your home will help you calculate the required radiator size and output [here]
First the air volume must be heated Measure the room length height and width in metres and multiply by 0.33 and then by the number of air changes an hour you expect that room to have. Generally this will vary from 1 to 2 depending on its location and use.
The next step is to calculate the heat loss through the windows. Measure the area of the windows in square metres and then multiply by the thermal properties of the window i.e. The U value. For a single glazed window the U value will be 5.8 w/m2/degC and 2.9 for double glazed windows. If you have very special glazing system the U value may be different so check with the manufactures.
A quick ready recknoner is provided here to help you work out the output required from your radiator to heat your room [guide]
The heat loss through the external walls is calculated similar to the windows. Measure the area of the external wall and multiply by the U value which could range from 1.5 for a conventional cavity brick wall to 0.3 if highly insulated. Again you will need to check with the manufactures. Heat losses through the ground floor and roof are also calculated in a similar way. When all the surfaces which are lose heat have been calculated add up the total and also include the air volume. This will give you a value fore the room expressed in watts/degC. Multiply this by the maximum temperature you wish to heat the room usually 21 degrees C. with an outside temperature of 1degreeC. This represent a temperature rise of 22 deg C. This value will now be the heat required in Watts to heat the room. If you want this value in kW the just divide by 1000.
Example calculation for heat loss