We've extended onto a wall that contained the outlet for an internal
toilet extractor fan. So temporarily, this fan ducts into a cavity.

The original plan was to re-route the ducting to the other side of the
house. But this seems tricker than expected - we'd have to destroy a
lot of ceilings, fix a long run of ducting just to get to the nearest
outside wall. Even then - that wall contains the outlet for the CH
condensor boiler so it may not be a good idea to come out too near
that.

We've had the suggestion that ducting into a cavity from a toilet only
room (not a bathroom or shower with moist air) can be acceptable if you
use a fan with a carbon filter for odour control.

But I can't seem to find any such fans - does anyone know if they
exist?

Thanks
Paul