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Old 12-30-2009, 02:30 PM   #1
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Bathroom Fan Duct question


I am replacing my bathroom fan due to the low CFM for the size of room. I am getting mixed answers on how to run the duct in the attic from the fan. I watch "This Old House" all the time and they are saying you should send it either through the soffit (which is not applicable), or through the siding ( I can, but since we added a room the closest area to run is about 12 to 15 ft. The show says its better to run it pretty close to the fan like 6 ft. Would that be too far or is that ok? Will any pools of condensation form in the duct since it is ran so far? I talked to a contractor and he was telling me the show that I am watching takes place further north and being in Chattanooga TN you really do not need to exhaust out of the house(no hose is needed). Is this correct in the south, or if you have any other recommendation I would appreciate it. Average temp in winter is 37 to 45, summer 80 to 92. Thanks for your help

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Old 12-30-2009, 04:14 PM   #2
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The bath exhaust should always be vented outside. Many contractors think it is okay since most do it wrong and learn from each other. Venting out the roof is best. galvanized 4" round pipe is best but should be insulated to avoid condensation build up.

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Old 12-30-2009, 08:12 PM   #3
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Originally Posted by bemgolf View Post
I talked to a contractor and he was telling me the show that I am watching takes place further north and being in Chattanooga TN you really do not need to exhaust out of the house(no hose is needed).

For starters: (Underline mine)

IRC
M1506.2 Recirculation of air. Exhaust air from bathrooms and toilet rooms shall not be recirculated within a residence or to another dwelling unit and shall be exhausted directly to the outdoors. Exhaust air from bathrooms and toilet rooms shall not discharge into an attic, crawl space or other areas inside the building.
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Old 12-30-2009, 09:52 PM   #4
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"bemgolf"--You are in Chattanooga, I've been there many time, you have humidity problems much like we do down here in S. GA. You don't need any more humidity in your attic space than you have now. When I replace a bath fan, or fan/light fixture that is not vented to the outside, I try to convince the homeowner it needs to be done this way. I also prefer "out the roof" also. Another item I have gotten into promoting is a delay-timer on the fan or fan/light switch. People just don't let the bath exhaust fan run long enough to adequately remove moisture. It's not that much of a job to do. Thanks, David
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Old 12-30-2009, 10:21 PM   #5
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Further info on timers: http://www.cbs.state.or.us/bcd/progr...commentary.pdf

Sizing, which you may not need: http://www.efi.org/factoids/bathroom_sizing.html

The soffits exhaust, as mentioned, will return back to condense in the attic. Through the roof: http://www.rd.com/how-to-install-a-q...icle18312.html

Be safe, Gary
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Old 12-30-2009, 10:29 PM   #6
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Can you believe someone using what was said during a TV for entertainment only show as a true how to do it?
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Old 12-30-2009, 10:54 PM   #7
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The bath exhaust should always be vented outside. Many contractors think it is okay since most do it wrong and learn from each other. Venting out the roof is best. galvanized 4" round pipe is best but should be insulated to avoid condensation build up.

Thanks for all the answers. How about out the siding and how far is the max run? I will probably look at the roof. Any recommendations of vent caps and type of caulk around the vent and in the nail holes? Can you get all that at lowes or HD?
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Old 12-30-2009, 11:15 PM   #8
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Can you believe someone using what was said during a TV for entertainment only show as a true how to do it?
Bob,
Is that any more ridiculous than getting it off a forum from "some guy"?
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Old 12-30-2009, 11:29 PM   #9
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Bob,
Is that any more ridiculous than getting it off a forum from "some guy"?
Some of us here actually do this for a living. we are not actors. And here one can check other posts and determine if we know what we are doing. Often we refer to code requirements and even point out that these are a minimum and should be exceeded not ignored. Actors only care if the wind is messing their hair not if it is blowing moisture into the insulation making it less effective. So I will continue to learn from scientific facts and other professionals not TV actors.
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Old 12-30-2009, 11:33 PM   #10
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most all supplies can be bought from box stores. A roofing supplier will be better. Just try to keep the distance as short as possible. I use under 12 foot as a rule. Keep in mind every 90 ell will greatly reduce the distance allowed. The roof penetration needs a roof boot. This gets nailed only at the top, which gets covered by the shingles above. So no exposed nail holes will take play. The boot flange will be sealed all around with roofing caulking.
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Old 12-31-2009, 07:14 AM   #11
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This may help visualize how much humidity a bathroom fan pumps into an attic:

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Old 01-01-2010, 01:28 AM   #12
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most all supplies can be bought from box stores. A roofing supplier will be better. Just try to keep the distance as short as possible. I use under 12 foot as a rule. Keep in mind every 90 ell will greatly reduce the distance allowed. The roof penetration needs a roof boot. This gets nailed only at the top, which gets covered by the shingles above. So no exposed nail holes will take play. The boot flange will be sealed all around with roofing caulking.

Something like this






and foil tape would that be all that I need?
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Old 01-01-2010, 06:56 AM   #13
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limit the use of the flexible duct and use the 4" galvanized round duct. Less friction.

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