Advice - updating a CH system

And in this case a heating engineer will design the system and fit it. Or do you think it would be likely to be a direct swap new for old? I don't.

In boiler repair, modification is a no-no, they can only swap the faulty part (and any other bits they swap along the way until they get it right)

As for designing a system, don't make me laugh - educated guesswork is the best that most can manage, it's mainly a matter of turning a well worn handle

The boiler they fit tends to be the one they can get the best deal on

Of course, I don't include all fitters in this category, but it does account for a fair proportion of those I encounter

No idea what you're prattling on about there, sorry

The OP is getting a new system not a repaired boiler

Which has what, exactly, to do with engineering ?

You took the bite !

Of course, you could work out the size of rads req'd dependant on the room parameters etc, but how many fitters do ?

They just go for a guestimate of what they think will do

Well yes. The only crime is not to install a large enough unit. The TRVs will do the rest and the condensing boiler won't mind about the cool return water.

This is a worry as I'm in the same position and going to replace the boiler when the weather gets warmer.

Had your system had inhibitor from new? I know for certain mine has, and it's been changed about every 5 years. The water on draining isn't a nasty colour either.

Don't know but quite likely not. This would have happened about 6 years ago when we'd been living in the place 3 or 4 years; although I'd put inhibitor in myself the CH system was a lot older than that, and there was (is) plenty of evidence of neglected maintenance overall in the house.

David