In our terraced house, we have a Ferroli Modena 102 combi in the
kitchen, with a horizontal 100mm concentric flue of 0.5m length, ending
on the outside wall which at that point is part of a single-storey
extension to our house. Unfortunately, it's rather close to next door's
kitchen door and patio, and the smell of the initial exhaust when the
boiler starts up is quite noticeable.

According to the manufacturer's manual, the maximum flue length is 4m,
but any bend counts as 1m, so a horizontal flue like mine, attached to
the top of the boiler by a 90deg bend, has a maximum length of 3m, after
the bend.

Fine, I thought, I can add a 90deg bend where the flue emerges from the
wall, and then a 1.5m concentric flue going vertically upwards to end
about half a metre above the top of the wall. That would be an
effective length of 0.5 (existing) + 1 (new bend) + 1.5 (new flue) = 3m.
But my local Corgi-registered plumber says that you can't mix horizontal
and vertical flues in this way, and he says Ferroli confirmed this to
him. The only answer would be a new vertical flue system which would
have to pass through the existing flat roof, which I am reluctant to do.

If the boiler will accept either horizontal or vertical flues, why can't
you mix them?
--
Richard J.
(to e-mail me, swap uk and yon in address)