In article ,
David writes:
,nip
Will the fan be powerfull enough to suck open the louvres if installed
at bathroom end, or blow them open if installed outside under soffit ?
Which end is preferred ?

When I installed ours, as part of the kit I fitted on the outside a
plastic 'flap valve'. A disk which filled the aperture pivoted off
centre, such that it would close under its own weight or if the wind
blew against it, yet the draft from the fan when it was turned on was
enough to open it fully.

Was it one of the B&Q/Greenwood/Marley fans ?

I fitted a Vent-Axia Silhouette 100mm fan a few years ago,
and that has just such a back draft stop built into the rear
of the fan. (Screwfix item 47989 or 92561.)

--
Andrew Gabriel