F wrote:
Relative lives in an inner, three story town house and needs a new
boiler. The current ~20 year old boiler is located on the ground floor
with the flue running up through the other two (concrete) floors.
There's a garage to the front and a kitchen to the rear and the boiler
is in a walk-in cupboard between the two.

She's been told that the boiler must go on an outside wall with the flue
through the wall, or on the top floor with the flue through the flat
roof.

That sounds like ******** to me.

Question is, why can't it go where the existing boiler is? Is
there a limit on the length of flues for modern boilers?


May be an issue with condensing types. Dunno.

My guess is that otherwise water drips down the flue pipe or summat.