We just had our old gas water heater replaced with a tankless unit, so the vent for the old tank is now unused. I'd like to install exhaust fans in our 2 small bathrooms, and ideally use the old vent instead of cutting a new hole.
My plan is to use 4" insulated flexible duct to run from the first fan, have the second fan wye into that same duct (with a back draft damper if needed), and then run the rest of the way to the 4" exhaust from the old water heater. Like so:
Will this setup work, or will I need different/bigger ducts? An alternative could be to use the 8" vent that used to connect to the furnace (which is now in the crawlspace) that's in between Fan 1 and Fan 2, but that seems like it may require more powerful fans.