Has anyone come across an extractor fan designed for a 3"
or 3.25" or 8cm duct (not sure which it's meant to be)?
The duct is already there, and since it goes through an
18" solid wall, I'm not inclined to widen it. (It used
to be a boiler flue.)

Ideally, a fan which went on the outside and sucked
through the duct would be perfect, but I've never seen
such a fan either.

--
Andrew Gabriel