Two bathroom exhaust fans are joined into one duct with no backdraft dampers. The consequence to report is:

Joining two exhaust fans into one duct creates a shared path, and without backdraft dampers at each branch that path runs both directions. When one fan runs, the easiest route for some of its air is not always the long run to the exterior termination — it is the short branch to the other, idle fan, whose grille offers little resistance. So moisture-laden air from one bathroom is delivered into the other, where it condenses on cool surfaces and does exactly what the exhaust system exists to prevent. The remedy is a backdraft damper on each branch, which permits flow outward and blocks it inward. The inspector reports the missing dampers and the cross-flow they permit; the shared duct itself is not necessarily a defect if properly sized and damped.

With no dampers on the branches, air from the running fan takes the low-resistance path into the idle fan's duct and out its grille. Moist air is delivered into the other bathroom, defeating the purpose of the exhaust.

Exhaust systems require backdraft dampers to prevent reverse flow. Where multiple fans share a duct, each branch needs a damper and the common duct must be sized for the combined flow, with a single properly terminated discharge to the exterior.

No Damper, Two-Way Street. The running fan finds the idle fan's grille easier than the roof — and delivers the steam next door.

Confirm the common duct is sized for both fans and terminates outdoors. Undersizing and terminating into the attic are frequent companion defects.

• •Reporting the shared duct as the defect rather than the missing dampers
• •Failing to test by running one fan and checking the other grille
• •Overlooking whether the common duct is sized and terminated correctly