The flu kits with combi boilers now allow for this possibility in that the
whole thing can be fitted from inside.
The seal against the wall is from a rubber fringe which opens when you pull
the flu tube back a bit, very neat idea.
Maybe you can get something similar for your extractor flu?

I was thinking of something like that. I know you can get them
somewhere because I've used them before, but I can't actually find
them at the mo.


SJW
A.C.S. Ltd.