Best Bathroom Exhaust Fans for 3-Inch Ducts Fans built for 3-inch bathroom ... If it’s working well to remove moisture and odors, there’s no need to replace it—though you certainly can ...

A common use for inline exhaust fans is to vent two bathrooms. A plastic exhaust grille in each bathroom (usually in the ceiling) attaches to ducts, which then fasten to a "Y" connector at the fan.