Investigating a Dripping Bath Fan
HVAC pro Noah Racette confirms one homeowner's assumption that connecting an exhaust-fan duct to a furnace vent is a dangerous idea.
My in-laws had an exhaust fan installed as part of a bathroom remodel, and they’ve noticed water dripping from the fan after showering. I poked around the attic and found the flex duct for the vent is attached to the furnace’s concentric vent, which exhausts via a pipe running through the middle, and takes in outdoor air through a larger pipe that surrounds it. It’s this outer intake that the bath vent is connected to. Am I right to think this is crazy? What are the potential problems here, and how would you fix this?
—SEAN via email
Noah Racette (@HVAC_artisan), an HVAC installer in Delavan, Wis., replies: You are definitely right to think this is crazy. I would say this is a classic example of why it’s important to hire a reputable contractor to do the work and make sure they are using qualified subcontractors. Based on your description of the issue, this installation would not meet manufacturers’ specs for either the bath fan or the furnace.
There are several things that could be causing the water to drip from the fan, but the root of the problem is condensation. If the flex duct isn’t insulated and is routed through a cold space, the warm, damp air venting out can condense on the inner walls of the duct and drain back through the vent. It’s also possible that the duct is constricted where it enters the concentric vent termination, which might also cause water in the air to condense and dribble back through the vent.
I can see no reason why, under any circumstance, it would be appropriate to terminate a bath fan into the furnace intake. If the furnace is running while the bathroom exhaust is operating, the furnace would be drawing extremely humid air for combustion, which could lead to the deterioration of the burners, and water dripping inside the furnace cabinet could potentially damage other furnace components. The correct fix would be to install a separate roof jack to terminate the bath-fan exhaust. If this work was done recently enough, it’s worth contacting the contractor and demanding that the issue be remedied and a separate exhaust termination be installed correctly.
From Fine Homebuilding #286
More about bathroom fans:
Venting a Bath Fan in a Cold Climate – Use a high-powered fan, a large-diameter duct with the shortest run possible, and wrap the pipe with insulation.