I've got a 100mm inline extractor fan in the ensuite; there's a vent in the
ceiling and it ducts the air up into the roofspace (in corrugated flexible
ducting) and as this is a dormer-style house with a very low roofline, it
passes down under the tiles in 100mm rigid flat ducting to a grille just
below the guttering.

It's really very ineffective though, and while I'm scrabbling around in the
roof space sorting out my downlighter wiring I thought I might try fixing
this too... I was considering replacing the 100mm fan with a 150mm model;
however, realistically there's no easy way to upgrade the length of fixed
100mm ducting (about 1-1.5m worth) which runs under the roof tiles. I
would replace the flexible stuff in the roof space with 150mm ducting,
probably rigid rather than the flexible hose.

Question is, would that be worthwhile at all? Would the old 100mm section,
which I can't upgrade, just act as a bottleneck to eliminate any benefit?

Thanks

--
David