On 15/06/2020 11:09, Harry Bloomfield wrote:
Dave Liquorice pretended :
They'll have less thermal output for a given water temperature. Old
systems with conventional boilers were designed such that the
radiators gave enough heat with water temps of 70+ C and a 10 C drop.

New systems with condensing boilers require the return to be about 55
C for maximmum boiler efficency. This means the flow is only 65 C and
if you have old radiators they won't push out enough heat.

Which will not matter, if the thermal efficiency of the home has been
improved - as seems likely, since the original rads were specced.

My house (1976) had single panel rads upstairs. In the two largest
bedrooms, the one where the class 2 flue blocks were (for the
baxi bermuda non-pumped back boiler) had a rad 2/3rds the size of
the room without this benefit.

The wall where the flue blocks were got so hot the rad was only
needed during extreme weather.