Bathroom exhaust duct length

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EthylOH

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Hi all,

I'm in the middle of my bath renovation, and I'm having some difficulty deciding where to run the vent.

I have 3 choices:
  • through the roof: (not my favorite idea here in New England. We definitely get enough snow that the vent would get covered during the winter. And I'd rather not put another hole in the roof if other options are available)
  • through the rim joist: I could run the duct DOWN in the adjacent closet and out the rim joist. However, I may put a deck on the back this summer, and the vent would likely need to be moved at that point. ( I got the idea from here: http://www.uexpress.com/first-aid-for-the-ailing-house/2014/6/4/down-not-up-is-the-best )
  • through the gable: this would be fairly straightforward. The only problem is that the duct length would be 20 - 25 feet. The bath is 770 cubic feet (8*12*8) and the fan is a NuTone rated at 130CFM if that makes a difference.
As an aside, my roof has gable vents, but does not have soffit vents. Could I vent through the soffit in this case? (This is not ideal as I'll be replacing the flashing and the soffits this summer. I'll probably add soffit vents at that point).

Any thoughts as to my best option in this case?

Thanks in advance.
 

Jadnashua

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Trying to push hot humid air down is not an ideal path. Mine is routed through the roof. I don't use the fan that much in the winter, so snow has not been an issue for me. The spec sheet on the fan will usually list the max duct length which gets shorter with each elbow, and may offer more than one size. The Panasonic I last added to my mother's house ran about 20' out to the gable end. That fan has a stepped outlet that would take either a 4 or 6" duct. I used a 6" solid duct with added sleeve insulation on it and it has been working fine for about 5-years now. Venting through soffits is not a great idea as you tend to get some condensation there, and the grill can add a fair amount of restriction.
 

EthylOH

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I called NuTone tech support.

They said my fan would work fine with a 22 foot run (w/ 6" galvanized ridgid pipe).

Do I need to be concerned that the wall vent would be 5 feet away from the gable vent? I don't have vented soffits.
 

EthylOH

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So I'm in the process of installing the duct in the attic.

My length is exactly 20 feet. If I slope the duct to the outside (1/4"/ft), I end up with approximately a 6" height differential from fan to wall cap. Now, adding the diameter of the duct (6") and the height of the ceiling joist that the duct sits on (6"), the total height of the duct from the ceiling panel below is 18" or so.

I have approximately 24" of blown in fiberglass insulation. Since this duct will be near the top of the insulation (and thus close to the ambient attic air), do I need to insulate the duct with separate insulation?



Out of curiosity, what is the point of insulating a duct? For example, say the duct is run over the top of the joists in an attic with batted insulation in the joist cavities. The duct in this case would be the same temperature as the ambient air, since there is no heat source present to insulate. If we were to insulate the duct in this case, it would still be the same temperature as the uninsulated duct since there is no heat source to insulate. \

In the case bathroom exhaust, there will be warm, humid air. Is the point of insulation on the duct to retain the heat provided by this warm, humid air (as opposed to the heat from the ceiling below)?
 

Jadnashua

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It's a good idea to prevent the moisture from condensing. Keeping it warm until it hits the outside can help with that.
 

EthylOH

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Just thought I'd give an update...

What an ENORMOUS pain in the ass this is.

This might be the single most tedious part of the entire renovation. It literally has taken me all day to run the rigid duct from the fan to the end of the house. And I haven't even insulated it yet.

I can't think of a better way to spend my weekend than to be lying across ceiling joists in a hot attic in 2 feet of itchy insulation with no more than 10" of headroom.


It's a good thing I'm not claustrophobic.
 

Jadnashua

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FWIW, I had the same issue when I installed one at my mother's house except, I don't think I had 10" of headroom (a nearly flat roof). I ended up putting insulating sleeves on my ductwork, and pushing most of it to where I'd made the hole in the wall for the exhaust hood, so I didn't have to crawl all the way there, but it still was a major pain.
 

dosby

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I retrofitted a new exhaust fan (http://www.homedepot.com/p/NuTone-L...LED-Night-Light-ENERGY-STAR-QTNLEDB/203449905) recently and I did it through the 12" x 12" hole in a ceiling. The lower floor bathroom has 2 other bathrooms right on top of it, so there were a lot of drainage pipes between the floors, too. HVAC as well. I could find a clear path along the HVAC pipe and barely fit my 4" flexible alum duct there, all the way (6') to the soffit vents area. It took some pulling and shoving.
I also wired a 14-4 romex to feed all 3 loads of the exhaust fan independently.
 

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