Bathroom ceiling fan with light with timer

no you can't do that

what's restricting you from using the full area ducting for the entire route ? - bear in mind that you can get all manner of flat channel and rectangular ducting that maintains the volume

http://www.screwfix.com/c/heating-plumbing/ducting/cat840506

 

Obviously difficult to fully understand on a forum but why cant you vent out through the wall without taking this convoluted route ?

You've said the shower is on ground floor, outside wall so sure, you may need to cause some destruction but use a ceiling fan, with or without light and duct out via ceiling void to outside wall

I realise in real life not always this easy.....joists running wrong way, lack of access, etc but still think with the proposed venting run you will be wasting your time with an average power fan

100mm soil pipe for the ducting would improve air flow as offers less resistance than the flexi pipe stuff and a descent spec fan

 

can you swap that for this - compare the amount of air they shift - add the Manrose humidistat and you have fully automated operation - adjustable timer - only comes on for extraction when it needs to, goes off when it needs to - and can be over ridden to extract on demand