"Ed Sirett" wrote
I'd be interested to know what web sites are suggesting a larger output
unit. This existing unit will be sized for the space heating requirements
which for a modern house will be relative modest.

So it comes down to what 15kW condensing plain heating boiler you need.
Some thermal stores have a coil for indirect primary heating and so a
sealed boiler can be used. Otherwise you are needing a plain heating
boiler.


Try
http://www.vaillant.co.uk/homeowners...e-right-system

They recommend the ecoTEC plus 428 which is a 28.2Kw condensing boiler for
a 3-4 bedroom house. I have seen another website but can't find it now