Water dripping out of exhaust fan

[Nathansdad]

This is the second year in a row I've got water dripping out of the exhaust fan in the bathroom. There is flexible insulated duct running from the fan and out the roof. Am I getting frost build up in the duct then it melts and runs down into the fan? Any other thoughts? How can I prevent this from happening. Thanks for your help.

[reddog]

Insulate the duct where it goes out of the attic insulation to the roof, or let the fan run continuous.

The damper probably is not working real well if theres one in there at all.

The duct in not insulated or under insulated going thru the cold attic space. When the fan is not running, stack effect carries warm moist air up thru the duct like a chimney. When the moist air meets the cold part of the duct, the moisture vapor condensates in the inside of the pipe. During long periods of cold weather, this moisture turns into ice inside the pipe and builds up.

When it finally warms up (like today) the ice starts melting and running back down, eventually exiting back out thru the fan

Its possible that the frost buildup is on the termination at the roof also.

[PurpleFloyd]

Agreed.

[RoosterR]

Sounds like no drip loop. Should always be one in the duct for that reason.

[soldoncass]

That is a common problem with going through the roof.....we quit doing that and started using flexible, insulated pipe, and taking it out the nearest gable end. A matching siding color outlet(with the floppers), and some blown attic insulation over the pipe, eliminated all the problems we had before. If you did that you would have to fix the roof hole. If you don't have a gable end, that probably isn't possible though. There are many things that CAN happen to get your problem . Snow gets blown into vents, they get covered with snow, melting can turn to water and run in, or the frost thing that was mentioned. I would reroute it out the house gable.

[CAMAN]

Sounds like no drip loop. Should always be one in the duct for that reason.

You never want a drip loop. It should be the straightest possible insulated line with no areas for water to settle. If enough water builds up in that line the exhaust fan is completely useless.

[cavalierowner]

I always run to a gable end when there is one. My second choice is thru a wall and my absolute last choice is thru the roof. In my current house, the gable for the master bath was about 30' away (far too long for a conventional run), so from the fan, I immediately transitioned up to 6" round and ran toward the gable. Then just before the outlet I installed an inline booster fan and wired it with the exhaust fan. That really works well.

[dillon]

I had the same problem until I installed insulated flex ducting. Keep the run as short and straight as possible. I had uninsulated ducting and a drip loop...that just resulted in condensation settling in the low point and freezing. At that point my fan was basically deadheaded and useless. As mentioned above, through a gable end is the preferred way to run the duct.

[Nathansdad]

That was my thought, run the duct under the blown insulation and out the gable end. Sounds pretty basic. Aparently the HVAC contractor I hired didn't know any better. I've got 16' of insulated flex duct running through the atic and out the roof on the opposite side of the house as the bathroom. I guess my second bathroom will have the same problem as soon as my son starts to shower. I've got 18' of duct on that one. When I asked the contractor why they just didn't go strait up and out the roof I was told how unatractive the vent would be on the front of the house. I could care less about that if the system actually worked properly. I wish I'd have thought about the gable end as I am only 6' away. Why didn't the contractor know this. I guess I've got a summer project now. Maybe I'll foreward this thread to my contractor and educate him a little.