My brother-in-law has a traditional system boiler (not room sealed) with the flue fed into a chimney stack that also contains the unused flue from the living room. The building is a chalet bungalow (with loft conversion) with numerous windows and nowhere on an outside wall to hang a new boiler except high on the gable ends which would be (to say the least) extremely inconvenient (and even that might fall foul of the flue near window exclusion zone).
The boiler is getting old and he has been told by the firm that services it that any replacement *must* be a condensing boiler and that the replacement *must not* use the chimney. I haven't taken much notice of developments over recent years but ISTR recent discussion that said that lack of suitable position could make a non condensing boiler a legitimate replacement.
I am not sure why the chimney should be a no-no these days. Is it due to the difficulty of fitting a condensate drain or some other reason that as yet hasn't occurred to me? If it is just verboten for condensing boilers I presume that a replacement non condensing boiler could continue to use the existing flue subject to modern safeguards. (I am not sure whether or not it is actually lined but I presume it is as the house was built as late as 1968).