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Thread: Ventilation for 2 bathrooms/1 roof penetration?

  1. #1
    DIY Junior Member nola mike's Avatar
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    Default Ventilation for 2 bathrooms/1 roof penetration?

    I'm installing a bathroom directly adjacent to the existing one. The current vent exhausts air directly into my attic insulation. I'd like to remedy that, as well as install a proper vent in the new bathroom. I'd really like to only need 1 roof penetration. It looks like the way to do this is to have a central fan that exhausts both bathrooms. I would prefer to have a separate fan in each bathroom, and didn't know if there was some type of circular pipe with 2 separate chambers. So that I could have each bathroom have its own piping, but only 1 pipe going through the roof, if that makes any sense. Any other good solutions?
    Last edited by Terry; 01-05-2013 at 10:43 AM.

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    Test, Don't Guess! cacher_chick's Avatar
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    The fans can be piped together into one outlet. In-line back-draft dampers must be used to prevent each fan from blowing into the opposite bath.

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    DIY Junior Member nola mike's Avatar
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    Quote Originally Posted by cacher_chick View Post
    The fans can be piped together into one outlet. In-line back-draft dampers must be used to prevent each fan from blowing into the opposite bath.
    For some reason I thought that wasn't allowed? Do you have a link to the dampers that you're referring to?

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    Test, Don't Guess! cacher_chick's Avatar
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    What you might be thinking of is that a kitchen vent hood cannot be connected to other vent systems.
    You can get vent dampers at any HVAC supply house.

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    Electrician djlazar's Avatar
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    Commercial buildings do this all the time, having one big exhaust fan sucking air out of several bathrooms. The problem would be the extreme cost of this project and the fact that it would be a pain in the ass. If you have an exhaust fan in your attic, it will surely go bad after a few years and you'll be up there changing it out, plus the wiring to it would be a pain. If you put a big exhaust fan on your roof, even more expense and pain the the butt. Do yourself a favor and just put an exhaust fan in each bathroom and vent them separately through their own penetrations, be it through the roof or the soffit. You'll be better off.
    Just don't do like you are thinking and connect the ducts. No matter how good you think your backdraft damper is, I can guarantee that you'll smell whatever is going on in the other bathroom.

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    DIY Junior Member nola mike's Avatar
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    Quote Originally Posted by djlazar View Post
    Commercial buildings do this all the time, having one big exhaust fan sucking air out of several bathrooms. The problem would be the extreme cost of this project and the fact that it would be a pain in the ass. If you have an exhaust fan in your attic, it will surely go bad after a few years and you'll be up there changing it out, plus the wiring to it would be a pain. If you put a big exhaust fan on your roof, even more expense and pain the the butt. Do yourself a favor and just put an exhaust fan in each bathroom and vent them separately through their own penetrations, be it through the roof or the soffit. You'll be better off.
    Just don't do like you are thinking and connect the ducts. No matter how good you think your backdraft damper is, I can guarantee that you'll smell whatever is going on in the other bathroom.
    I wasn't planning on connecting the ducts; what I was hoping for was a circular piece that penetrated the roof. Within that piece would be 2 separate chambers, each of which would be exhausting 1 bath. That way there are 2 separate circuits, but only one roof penetration.

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    Moderator & Master Plumber hj's Avatar
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    what is often done is to install a rectangular "roof vent" above the rooms. Then the circular pipes are teminated under/inside it. The "backflow" preventers on exhaust fans are very rudimentary, and do not seal tightly. One fan with many inlets is an entirely differnt thing than many fans into one duct.
    Licensed residential and commercial plumber

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    DIY Junior Member nola mike's Avatar
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    Quote Originally Posted by hj View Post
    what is often done is to install a rectangular "roof vent" above the rooms. Then the circular pipes are teminated under/inside it. The "backflow" preventers on exhaust fans are very rudimentary, and do not seal tightly. One fan with many inlets is an entirely differnt thing than many fans into one duct.
    Right. What you're talking about is a fantech or similar setup with a central fan, correct? As another route, is it acceptable to terminate the run by going through the soffit? I know you can't just leave it lying next to it, but if I actually place a vent cap through the soffit? I'd much rather do that...

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    Moderator & Master Plumber hj's Avatar
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    That is the "one fan with many inlets" design. My initial suggestion was to install a "common" ventilation roof vent and then run your individual fan vents up to it and terminate them inside it without any real "connection" between them or the vent.
    Licensed residential and commercial plumber

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    DIY Junior Member nola mike's Avatar
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    Oh, I see. That was I guess my original question. I was looking for a special made pipe/vent to do that. So I could just buy an oversized roof vent and stuff both vent runs into it...
    What about the soffits?

  11. #11
    Retired Defense Industry Engineer jadnashua's Avatar
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    I would not run that hot/moist air to the soffits...first, it is pointing down, and the hot air wants to go up, carrying the moisture with it back into the attic and around the soffit area, and second, many soffits are not wide enough to run a decent sized vent to. Your best bet is out a gable endwall or through the roof. My bathroom in the condo does not have any windows, so to get some light in there as well as replace the cheapo vent fan that was there venting into the attic (like yours), I installed a tubular skylight with a vent fan kit from www.solatube.com . They've updated the design since mine was installed (mine sticks down into the room, the new one is essentially flush). During the day, the light from overhead can be the equivalent of a 300W lightbulb! It even lights the room on a moon-lit night. One of the best things I did during my remodeling. But, this now would require yet another roof penetration. Barring the SolaTube, I'd probably use one of the Fantec units and put it on a 3-way switch, maybe with a timer. It would ventilate both baths whenever on, but would be the cleanest solution.
    Last edited by jadnashua; 01-11-2013 at 02:21 PM.
    Jim DeBruycker
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    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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