You are making the assumption that the existing radiators were correctly sized when installed and no changes have been made since then, e.g. improved insulation.
Rad sizing may have been done properly it was a one-off contract for an individual customer, but large housing estate contracts??
To give you a simple example, my house is 24 years old and on a small development of about 40 houses. Most of the radiators are under the windows and, without exception, they are all 600mm high and the same length as the window. The size was obviously chosen because it "looked nice", not because it was the size needed to meet the calculated heat loss. In fact some of them were double the required size. Now that the house has had uPVC DG windows, cavity and loft insulation fitted the total required rad size is 6.5kW, but I have 13kW of rads.
You should always do a
Whole House Boiler Size Calculation first. (The [DLMURL="http://www.energysavingtrust.org.uk/england/Publications2/Housing-professionals/Heating-systems/Domestic-heating-sizing-method-2010-edition"]Latest Version[/DLMURL] has not been automated yet.)
It doesn't matter if the rads are oversized compared to the heat loss as it means the boiler will be able to run at lower temperatures with increased condensing, so more efficiently.
