If I am grasping some basics right, radiator outputs vary depending on the temperature difference between the water in the rad and the air in the room. The higher the difference the higher the heat output of the rad.
So, for example (using the info in
this pdf) a Stelrad Softline 600x800 K2 with a 50c delta will output 1386w. At 40c delta it will be 1037w and at 30c it will be 714w.
So, if you planned on running with a 80c flow and 69c return, and the room was say 20c, that radiator would be putting out a fair deal of heat. However with a condensing boiler you want a return of 55c or lower, assuming the same 20c room temp then that same rad is now only shifting 860w ish.
So, if the room in question lost 1000w your choices for that particular radiator would be to a) run at a 40ish delta or b) increase size of rad to that the delta can be kept lower but still heat the room.
Did I get that right?
If so, does that mean on a condensing boiler the way to keep it most efficient is to have rads sized to run at say 35C delta rather than 50?