Sizing radiators for condensing boilers

[Pritchard]

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.

David

[wishiwascorgi]

not too sure what your asking as the condensate on the condensing boilers is for the actual combustion of the gases as they leave the flue and and condensate returns through the boiler and drains away via the correct pipe work. I have installed a few Keston boilers in pubs that were just connected to the open vented system for cost reasons. This was within regulations and the systems worked fine but the manufacturer recommended sealing the system with an expansion vessel.

[Pritchard]

Thanks for reply. If condensing boilers only condense when return water is 55C or less then I thought that rads should be selected with flow=70C and return=50C. This downrates radiator output, mean water temperature is lower, the required radiators are larger, but hopefully the condensing boiler can operate more efficiently for more of the year? Any thoughts?