What radiator inlet/boiler flow temperature is used in the condensing boiler design calcs?
Depends on the boiler manufacturer. Some quote outputs for a 20°C differential, while others still quote the output for an 11°C differential. The important thing to remember is that you need the return temperature below 55°C if the boiler is to condense.
How does this impact on performance when installing a condensing boiler in a 20 year old house where the radiators would have been sized for 85C?
Is that 85°C flow or the mean temperature, i.e 90°C flow and 80°C return?
Radiator output 20 years ago used to be measured with 90°C flow and 70°C return, i.e mean of 80°C - BS3528. The current standard, introduced in 1997, is BS EN442 which uses 75°C flow and 65°C return.
If you want to see the effects of different temperatures on radiator output, try the calculators at
http://www.engineeringtoolbox.com/heat-emission-radiators-d_272.html
Twenty years ago, heating systems were not designed so carefully as nowadays, so boilers and radiators were probably considerably oversized - to make sure they could reach the "guaranteed temperature". The reduced output due to the increased differential of the boiler will usually be compensated by the oversized radiator. If the house has been well insulated, the oversize will be even more noticeable.
My house is about 20 years old. The living room radiator is about 2kW. Now that the house has been insulated and double glazed, the required radiator is only about 1kW, so the existing one is 200% oversized. On average my radiators are 180% oversized, the worst case is 375% oversized. The size of the radiators seem have been selected to fit the width of the window they are under, not the heat loss in the room. Only one radiator is possibly undersized - the one in the kitchen. This is because there is only limited space to put a radiator.
