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  1. #1
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    Default Exhaust duct shared by bathroom fans

    I've always reported conventional (positive pressure, non-"inline") bathroom exhaust fans sharing a common duct as a defect under IRC M1506.1 / M1507.1 - plus common sense, plus the fact that the manufacturer's installation instructions usually say something like "run ductwork to a roof cap or wall cap".

    However, none of that is an explicit prohibition.

    Is anyone aware of a code commentary or other source where this of exhaust arrangement is explicitly prohibited?

    Thanks


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    Rod Butler's Avatar
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    Default Re: Exhaust duct shared by bathroom fans

    Michael,

    Not exactly sure how your specific arrangement is but as long as the inlet is on the negative pressure side of the fan and there are means to balance the air flow at each register or branch duct, there is absolutely no problem with exhausting multiple batrooms on one fan.

    Of course good engineering practices need to apply, i.e. proper fan sizing, proper duct sizing and routing, fan placement and air discharge location.

    One nice thing about multiple branchs leading to one central fan is the possibility of heat recovery at a later date.

    HTH


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    Default Re: Exhaust duct shared by bathroom fans

    I'm referring to the situation where you have two or more conventional (positive pressure) bathroom exhaust fans connected to the same exhaust duct somewhere ahead of the termination.

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    Vern Heiler's Avatar
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    Default Re: Exhaust duct shared by bathroom fans

    Quote Originally Posted by Michael Thomas View Post
    I'm referring to the situation where you have two or more conventional (positive pressure) bathroom exhaust fans connected to the same exhaust duct somewhere ahead of the termination.
    I know that back in the 1995 NC Mechanical Codes, kitchen, laundry, and bathroom exhaust could not be recirculated to any other room. This meant they could not be connected together because there could be recirculation of the air, even if it was not intentional.

    Not only has the code changed from 308 Exhaust Systems, but I can't find any reference in the new codes either.

    The beatings will continue until morale has improved. mgt.

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    Default Re: Exhaust duct shared by bathroom fans

    Are both bathrooms on the same floor, that is both bathroom fans installed in the same floor/ceiling assembly? What is the path of the exhaust ducting? That is, does it provide for "communicating" with more than one assembly?

    Chicago or a burb? Vintage? Residential? Single family, duplex, true "townhome" or multifamily such as a condo, apartment or co-op? Fusible link or Fire dampers? Are the vent ducts encased/separated from the walls/ceilings/floors and protecting the attic space and roof assembly in gyp soffits or boxes or similar separation assembly? rigid metal of specified thickness or wire coil supported flex ducting of some sort (foil, plastic, etc.)?

    We lost a few things with the adoption of the I-codes vs. the Uniform Codes, for example the I-codes don't require periodic testing of the listening assist device systems like the Uniform codes originally did.


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    Default Re: Exhaust duct shared by bathroom fans

    The UMC states that a Bathroom Fan
    "must not pass through ducts or plenums" 602.1
    Sharing a common duct would qualify as passing through a duct.

    Remember - any system that is UL tested and listed can only be used
    in the manner it was tested and listed. If the manufacturers installation
    manual doesn't provide for "sharing a duct" don't do it.

    Good call in my opinion -

    mf.

    Matt Faust
    Real Estate Inspector

  7. #7
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    Default Re: Exhaust duct shared by bathroom fans

    The bathroom fans that I have seen are provided with a damper that prevents backdrafting. Therefore, when one fan is on, air should be going out, not into the other bathroom. I cannot see that it would be a huge problem. Have you looked at the installation instructions?

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    Default Re: Exhaust duct shared by bathroom fans

    Quote Originally Posted by Gunnar Alquist View Post
    The bathroom fans that I have seen are provided with a damper that prevents backdrafting. Therefore, when one fan is on, air should be going out, not into the other bathroom. I cannot see that it would be a huge problem. Have you looked at the installation instructions?
    I would not state that those dampers prevent backdrafting. At best they may reduce it, they certainly do not seal well at all when closed, nor are they intended to seal back the positive pressure of another bathroom fan.

    The IRC is pretty clear on this: (underlining and bold are mine)
    - M1501.1 Outdoor discharge. The air removed by every mechanical exhaust system shall be discharged to the outdoors. Air shall not be exhausted into an attic, soffit, ridge vent or crawl space.

    That does not leave any room for any air to be pushed back into the living space by any other mechanical exhaust system.

    Then the IRC adds this:
    - M1507.1 General. Where toilet rooms and bathrooms are mechanically ventilated, the ventilation equipment shall be installed in accordance with this section.
    - 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.

    That clearly states that exhaust air from the bathroom/toilet room fan shall be exhausted ... directly ... to the outdoors.

    Based on the above two code sections, unless the installation instructions specifically state that two exhaust fans may be connected to a single duct, then two exhaust fans may not be connected to a single duct.


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    Default Re: Exhaust duct shared by bathroom fans

    An interesting argument, but the language and fact-set presented do not support your conclusion. A leap has been made which is unidentifed and as presented, is unsupported.


  10. #10
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    Default Re: Exhaust duct shared by bathroom fans

    The connection where the 2 fans tie together may determine whether this install is correct. They should be connected to a true Y connector with the branches at 4" and the discharge at 6". This design makes backdrafting less likely.

    Common sense seems to be less common.

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    Default Re: Exhaust duct shared by bathroom fans

    Quote Originally Posted by H.G. Watson, Sr. View Post
    Are both bathrooms on the same floor, that is both bathroom fans installed in the same floor/ceiling assembly? What is the path of the exhaust ducting? That is, does it provide for "communicating" with more than one assembly?

    Chicago or a burb? Vintage? Residential? Single family, duplex, true "townhome" or multifamily such as a condo, apartment or co-op? Fusible link or Fire dampers? Are the vent ducts encased/separated from the walls/ceilings/floors and protecting the attic space and roof assembly in gyp soffits or boxes or similar separation assembly? rigid metal of specified thickness or wire coil supported flex ducting of some sort (foil, plastic, etc.)?

    We lost a few things with the adoption of the I-codes vs. the Uniform Codes, for example the I-codes don't require periodic testing of the listening assist device systems like the Uniform codes originally did.
    I was was thinking of the generic residential situation frequently encountered by home inspectors:

    1) Usually an ad hoc rather than an "engineered" system, with the exact fan location, duct material and routing decisions made by the installer, and often the result of at least one of the fans being a retrofit.

    2) "Residential" grade ceiling mount fans, with the standard integral back-draft damper at the fan housing and (perhaps) a second back-draft damper at the termination.

    3) Duct materials commonly employed in such situations, might be rigid or metal or plastic flex, likely not insulated at passages through unconditioned spaces.

    4) Length usually 5-20 feet, with one to two 45 or 90 degree bends.

    5) Exhaust route might or might not included a (typically one floor) change of height, typically to a basement sidewall or attic roof, soffit or gable.

    In other words: a typical residential installation as see the the Chicago area, and with the exhaust ducts combining somewhere between the fans housings and a common termination.

    Michael Thomas
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  12. #12
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    Default Re: Exhaust duct shared by bathroom fans

    For the amount of good the bath fans do I don't see how it really matters how they are connected as long as they vent outside. Most of the fans do more noise making than exhausting. IMO


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    Default Re: Exhaust duct shared by bathroom fans

    Quote Originally Posted by James Duffin View Post
    For the amount of good the bath fans do I don't see how it really matters how they are connected as long as they vent outside. Most of the fans do more noise making than exhausting. IMO
    Well... there is that...

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  14. #14
    James Duffin's Avatar
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    Default Re: Exhaust duct shared by bathroom fans

    I didn't say I was right.....only expressing my opinion.


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    Default Re: Exhaust duct shared by bathroom fans

    Quote Originally Posted by James Duffin View Post
    For the amount of good the bath fans do I don't see how it really matters how they are connected as long as they vent outside. Most of the fans do more noise making than exhausting. IMO
    Yep, this is not on the top of my look for list! I'm giddy with excitement when I find them venting to the exterior...

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  16. #16
    David Bell's Avatar
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    Default Re: Exhaust duct shared by bathroom fans

    Quote Originally Posted by Scott Patterson View Post
    Yep, this is not on the top of my look for list! I'm giddy with excitement when I find them venting to the exterior...
    Giddy!? I'd have to see that in person.

    Common sense seems to be less common.

  17. #17
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    Default Re: Exhaust duct shared by bathroom fans

    Quote Originally Posted by David Bell View Post
    ...They should be connected to a true Y connector on the mangaflow exhaust systems with the branches at 4" and the discharge at 6". This design makes backdrafting less likely.
    It's sounds all the more complicated to understand but will try it and get back to you for further advise and oh! I'm new here.

    Last edited by alelogman; 12-03-2010 at 03:13 PM.

  18. #18
    Rick Hill's Avatar
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    Default Re: Exhaust duct shared by bathroom fans

    If there is a wye conection and an increase in duct size I have found it ok to vent two fans from the same general area into one exhaust duct to the outside. With an increase in duct size the backdraft damper at the fan seems ok to prevent recirculation of air.


  19. #19
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    Default Re: Exhaust duct shared by bathroom fans

    Whether the bathroom exhaust fans seem okay when two are connected to the same exhaust duct does not matter. What does matter is whether or not they are listed for that installation.

    I checked a couple of installation instructions from Broan and Nutone and (like this one: http://www.nutone.com/PDF/InstallGui...tion_Guide.pdf ), none of them say anything about connecting multiple fans to a common exhaust duct. In fact, what they do say implies just the opposite: (underlining is mine)
    - PLANNING DUCTWORK AND WIRING
    - - Ductwork
    - - - 1. Use 4Ē round duct.
    - - - 2. Plan to run duct from discharge opening of fan to the outside. For best fan performance, make the duct run as short as possible and use a minimum number of elbows.
    - - - 3. Use optional ducting accessories as required.

    and

    - INSTALLING DUCTWORK
    - - 1. Refer to Figure 2. Place duct collar over flanges at discharge opening of fan. Secure collar by snapping tabs into slots in flanges.
    - - 2. Run 4Ē round duct from outside to fanís discharge opening.
    - - 3. Connect duct to fanís duct collar.

    Which comes back to my original post above

    Quote Originally Posted by Jerry Peck View Post
    ... unless the installation instructions specifically state that two exhaust fans may be connected to a single duct, then two exhaust fans may not be connected to a single duct.


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  20. #20
    Rod Butler's Avatar
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    Default Re: Exhaust duct shared by bathroom fans

    Quote Originally Posted by Jerry Peck View Post
    - - - 2. Plan to run duct from discharge opening of fan to the outside. .
    I don't know Jerry, I don't think that's quite concrete enough. (all due respect)

    Since my first post where I was trying to better understand the OP's question I have been lurking and researching this one a bit.

    Whether the backdraft damper is effective or not, it is called a "backdraft damper". It's intent is to stop infiltration from wind and building negative pressure. (My interpretation.) With that in mind I can't find anything more specific not allowing this type of installation.

    Not sure I would flag this in a report but one's thing for sure I would never recommend it.


  21. #21
    Denny Waters's Avatar
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    Default Re: Exhaust duct shared by bathroom fans

    My friend up the street installed the bath fan ducting from two baths to one exit. Not uncommon at all to receive the odors from one bath into the other.


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