Exhaust Fan Condensation Drip

Asked March 9, 2015, 9:29 PM EDT

I have a small bathroom (8.5 ft Long, 5.5 ft Wide and 12.5 ft Tall) and I'm having a problem in the winter with water dripping down from the bathroom exhaust fan mostly in the winter when it's cold. The room is about 600 cubic ft and duct is less than 10 feet long so I got a 110 cfm fan. I first I thought the duct was too small so I replaced with a 4 inch as recommended by the fan but still drips. I checked insulation in attic and not wet so condensation is just making it up to fan and dripping back down. What can I do? The ceiling is tall and slanted so not sure. The original fan was loud but didn't have this problem. The fan was installed by a professional.

Sacramento County California home ownership engineering home energy

3 Responses

- The exhaust air at some point is being cooled enough to cause condensation. Some possibilities to look into include:
- outside (attic) air leak into the fan unit unitself, from not being well sealed to the ceiling/attic floor

- exhaust damper not opening properly, so humid exhaust air doesn't exhaust and stays in the duct getting cold enough to condense and drip back down

-crimp or sharp angle in the duct, impeding air flow, allowing enough cooling in winter to cause condensation.

I suggest you check the packaging or online for the manufacturer's installation instructions, and make sure the ducting is installed to exhaust to the outdoors with a functioning damper, has no sharp turns or crimps, and the fan is caulked to the attic floor and duct is sealed to it to prevent air leaks into the fan housing.

Another simple test is the toilet paper test. If when turned on, the fan doesn't pull hard enough to pull a dangled strip of toilet paper tightly to the grille, it's not pulling anywhere the near the cfm it should



Thank you very much for the information. The duct is not kinked but there is a 1.5 inch gap between the drywall and the fan so attic air can get into contact with the warm air before it gets into the fan. Could that be the problem? The grate covers gap but there is a gap. Maybe I should seal it completely

It's possible that gap is the source of the problem. The cold attic air may be drawn into the fan, making the components cold enough to sweat. Plus the gap can cause other problems, air leaks that increase your energy bill or reduce comfort, and pulling dirty attic air into the fan, getting it dirty and potentially moldy.

So yes, seal the gap.