Each bathroom does have its own exhaust fan that vents to the roof separately. While two fans with one vent aren’t possible, one fan and one vent can service two bathrooms. This design necessitates the installation of an in-line centrifugal fan in the attic that draws air from both bathrooms at the same time.

Bathroom exhaust fans must vent to the exterior. Venting this fan into the attic is a recipe for disaster. Condensation will form on the roof members and insulation, eventually leading to mold growth. Even if the attic is ventilated, it is never a good idea to vent straight into it.

If all of the following are true, you could use 2 bath fans flowing into the main exhaust line:

1. There must be a damper for each fan housing.

2. The diameter of the combined exhaust vent pipe should be one size greater than the diameter of the two separate exhaust lines that feed it. That is if each fan’s individual exhaust lines are 3 inches in diameter, the common line where they are joined together with a wye must be at least 4 inches in diameter to allow both fans to run at the same time. 6′′ would be preferable.

3. The fans must have identical capacities. That is, you should not connect a 200 CFM fan to an 80 CFM fan on the same line.

4. There must be no “kinks” in the common exhaust duct with a turning radius of less than 2x the duct’s diameter. As a result, the minimum turn radius for a 4″ duct is 8.”

5. The number of turns in the general exhaust duct should be limited to four, even at the smallest radius.