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

A boiler can go in the loft, flued through the roof.