Following. All except having the fan pulling from both legs... so it's vacating air from the bathroom
whenever it's running? That's not desirable in my case at all.
So I still follow... and I do have a 6" Vortex fan... matching 6" carbon filter... Already bought & used
previously as an "air scrubber" in the room...
So I think I could do this... Come from my exterior opening, put a Y in place. Put a back-draft damper
on each leg of the Y. On one leg, after the damper, run to the existing bathroom exhaust.
On the other leg of the Y, after the damper, I could put my 6" fan so it's pulling - not pushing - and then
run ducting to the grow room. In the grow room, hang the 6" carbon filter and attach ducting.
So the biggest variable left in my mind, is what size the new exhaust should be to accomodate the combined
volume of air flow from both sources. Would going 8" be enough? The bathroom fan obviously won't be
running that often... When it does, it won't push very hard especially against a competing 6" inline fan...
So if the grow-room fan is blasting away and pushing air hard, would that force the bathroom back-damper to close?
I've just talked myself into keeping them seperate I think. The exhausts up to the ridge-line vents sounds better & better.