Exhaust Fan and H2O..

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Beachfront71

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All,

Thanks in advance.
When installing the duct work for an exhaust fan .. is condensation buildup/backflow an issue ? Do I need to slope the pan 1/4 per foot on the long horizantal runs?

Did not know.

Thanks again.
 

Jadnashua

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Yes, it can be...probably not in the summer, but as the attic cools, certainly. It depends on how long the duct is, whether it is insulated, and (obviously) the current dew point.
 

Dana

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Condensation issues on bathroom or kitchen exhaust would be all but unheard of in Newport Beach CA, but in colder climates it's common. Sealing the duct seams & joints with duct-mastic, then insulating them to R6 where the runs are outside the home's insulation boundary reduces the problem. (In Newport Beach just sealing them would be good enough.) It's the dew point of the exhaust stream relative to the duct temperature that's relevant, not the outdoor dew point, but at SoCal temps unless you're taking a 1 hour shower at 4AM on the coldest day of the decade you won't condense enough to cause damage, and even then it would be a pretty minimal thing. It's only chronic condensation that becomes a real issue.

In MN in January the attic & outdoor temps are ALWAYS below the dew point of the interior air, let alone the even moister vent exhaust, and drips & dribbles leaking out of uninsulated unsealed ducts can cause all sorts of problems with accumulating water/frost/ice in the insulation & attic, etc.

Backflow preventers are important to limit outdoor air infiltration from adding to the cooling & heating loads in any climate.
 

CarlH

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What do you folks up north use as far as terminating the duct? Just wall caps or do you also use roof caps? I would expect the vertical rise needed for a roof cap would allow moisture build up and leak from the duct.
 

Dana

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In my neighborhood they're usually side-vented. For roof venting it would have to stick up above the snow line (which can be pretty deep some years.) I've seen frost buildup on heavily used vents during colder weather, but not icicles. I've also seen paint-failure from humidity on the exterior at the outflow too (primarily on kitchen exhausts, not bathrooms.)
 

Jadnashua

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Mine is roof vented, but it's not elevated. Then, I rarely use it in the winter as the added humidity normally isn't a big deal. I do think that if I used it then, it would likely clear its own path as snow typically isn't all that dense. It gets used lots more in the non-winter seasons. In my condo, there really isn't a side wall where it could go, and before I remodeled, they fools just ran it into the attic (not good!).
 

CarlH

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Jim, I'm in the same boat as you were. For my top floor bathrooms, the ducts for the bath fans were run to the soffit. I need to redo those ducts and get them vented to the outside and was debating on whether or not to go with a roof cap or wall cap. In one case, using a roof cap would be a much shorter duct run. I think it would be best if I just went with the wall cap.
 

Jadnashua

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Running out a sidewall is likely a better solution, but in my case, I don't have a sidewall to vent to (townhouses on either side of me)! It's certainly better than just dumping it into the attic like they had it originally...
 
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