How to Use an Inline Exhaust Fan to Vent More Than One Bathroom First, install plastic exhaust grills in each bathroom ceiling. Run 4-in. flexible ductwork from the grills to the inline fan.

Bathroom exhaust fans are designed to fit a particular duct size. Generally, fans built for larger air ducts produce better airflow. Newer bathrooms tend to have vents with 4-inch ducts or larger ...