My understanding is that boiler does not condense until return water temperature is less than 55C. So for a boiler which serves hot water cylinder and radiators, should radiators be sized on 80-55 delta T?
80C to ensure DHW tank gets up tp 65C reasonably quickly, or could this be reduced to say 70C?
55C so that mixed return watre temperature (from rads and DHW tank) is low enough to ensure condensing mode?
I understand that reducing the mean water temperature in the radiator reduces their published output so and I have to check selection.
Also do all condensing boilers have to operate on a closed system?
Any advice appreciated.