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Old 11-30-2006, 12:08 PM   #1
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bathroom vent fan - condensation problem


I recently installed a vent fan in the bathroom ceiling. Now that colder weather is here- colder weather = cold attic. Condensation inside the duct is running down the duct and dripping from the ceiling in the bathroom. Has anyone ever had a similar problem?

I thought that a possible solution would be to make some sort of trap in the ducting. It is a flexible vinyl duct, and if I bought a longer one I could make a U-shaped section right above the point where it connects to the fan housing- same concept as a drain trap. I think that would work, but I thought I would ask here first. Any ideas or suggestions would be appreciated.

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Old 11-30-2006, 12:31 PM   #2
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bathroom vent fan - condensation problem


I've been fighting that since moving in two years ago. I over-insulated the ceiling of the bathroom and on top of the fan unit, then wrapped insulation around the vent pipe in the attic. It has still happened once since then, but only once. We get it dripping from our stove vent as well, but it usually only happens during the transition from warm to cold, once it's cold for good, it seems to stop. Let me know if you find something that works!

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Old 11-30-2006, 01:41 PM   #3
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bathroom vent fan - condensation problem


Don't make a trap in the ductwork. This will fill with water and defeat the purpose of the vent fan. Insulation is the answer. If you are using the duct with ridges in it, that is part of the problem. The ridges catch condensation and fall back down as droplets. Smooth metal or plastic duct is better. It may be that you are not leaving the fan on long enough after using the shower or tub. Recommended time is 20 to 30 minutes after shower use. This will also dry up most of the condensate in the duct.
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Old 01-20-2008, 02:40 PM   #4
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bathroom vent fan - condensation problem


While we were painting the ceiling of our bathroom, we also found condensation coming from the ceiling fan. We have metal, ridged tubing. Can you tell us if you found a way to fix this problem. We had to cut out some of the ceiling drywall because of the damage. Any help would be appreciated.
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Old 01-20-2008, 02:56 PM   #5
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bathroom vent fan - condensation problem


I have installed way to many of these fans to contemplate. Most were ducted with 26 ga round metal pipe and insulated if running through an attic in the open (not under insulation), some were run in pvc sch 40. All were run to vent terminations that had dampers to stop back drafts. If the piping is insulated and some of the outside air is stopped via a damper, you shouldn't have issues with condensation.
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Old 01-20-2008, 06:19 PM   #6
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bathroom vent fan - condensation problem


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I have installed way to many of these fans to contemplate. Most were ducted with 26 ga round metal pipe and insulated if running through an attic in the open (not under insulation), some were run in pvc sch 40. All were run to vent terminations that had dampers to stop back drafts. If the piping is insulated and some of the outside air is stopped via a damper, you shouldn't have issues with condensation.
Agreed, a decent termination cap with a damper and insulating the pipe should solve it. If the vent is ducted with some type of flexible garbage, get rid of it and go to ridgid metal. Whenever I duct through an attic space, I always add a couple of inches of vertical riser right at the vent fan, so that I can have a positive slope towards the vent outlet, just in case of any condensation.
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Old 01-20-2008, 07:02 PM   #7
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bathroom vent fan - condensation problem


use insulated flexible duct work , same as used for heat runs on furnace. When warm moist air from shower hits cold attic air it is condensing before exiting home , if insulated it should stop dripping.
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Old 01-26-2008, 11:50 PM   #8
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bathroom vent fan - condensation problem


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use insulated flexible duct work , same as used for heat runs on furnace. When warm moist air from shower hits cold attic air it is condensing before exiting home , if insulated it should stop dripping.
Pumping hot moist air through this stuff will over time result in condensation pooling in every dip in it, and it will eventually look and smell exactly like the sewer main from your house. Spoken from experience of years of renovating. . Metal pipe with a positive fall to the outside, insulate it if it runs in a cold attic, spend all the time you want enjoying a steaming shower.
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Old 01-27-2008, 06:27 AM   #9
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bathroom vent fan - condensation problem


Flex of any type is not what you want for this install. Use hard pipe, avoid problems.
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Old 01-27-2008, 07:46 AM   #10
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bathroom vent fan - condensation problem


Areas that may be difficult to reach on the duct work can also be spray foamed over (layered)to insulate...
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Old 08-11-2010, 11:34 AM   #11
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Pumping hot moist air through this stuff will over time result in condensation pooling in every dip in it, and it will eventually look and smell exactly like the sewer main from your house. Spoken from experience of years of renovating. . Metal pipe with a positive fall to the outside,....
Same issue here. Installing a bathroom exhaust.
My duct is running horizontal for 7 feet and elbowing up 2 1/2 feet through the roof? The entire run is insulated.
If I slope away from the fan I'm afraid I'll have water/condensate sitting at the elbow. If I slope towards the fan I'm afraid I'll get dripping out of the fan.
Which direction should I pitch my horizontal duct?

Last edited by buildjack; 08-11-2010 at 11:38 AM.
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Old 08-11-2010, 11:38 AM   #12
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bathroom vent fan - condensation problem


This is an old thread that you bumped, you may want to start a new one.

I had condensation issues even with insulation. I fixed it by doing two things:

First I got a much higher CFM fan, a Panasonic Whisper Quiet. Second I started using a timer so that the fan runs for 10-15 minutes after my shower. These two things blow the moist air out and dry up the duct work.
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Old 08-11-2010, 11:59 AM   #13
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Yup, a 2006 post is old. Any exhaust fan should be run while a shower is in use and 30 minutes afterwards. The moisture is still in the air after the shower is shut off.
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Old 08-11-2010, 12:01 PM   #14
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Yup, a 2006 post is old. Any exhaust fan should be run while a shower is in use and 30 minutes afterwards. The moisture is still in the air after the shower is shut off.
30 minutes is a bit much, IMO. After 10 or so I think the job is done. During the Winter and Summer the fan is removing the air that you paid to heat or cool, so you don't want to go too far with it.
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Old 08-11-2010, 12:14 PM   #15
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Thanks for the replies. I found this thread by googling my problem. I'll start a current one.

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