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Thread: Bath fan exhaust venting

  1. #1

    Default Bath fan exhaust venting

    I would like verification that the following can be done -
    I have a master bathroom with separate toilet / shower / open areas. The toilet area has a bath fan that vents to the side of my house (stucco). The run is @15'-20'. The tiled shower area only has a light - no exhaust venting, therefore we get frequent mildew buildup in the shower. (we do have a ceiling fan in the master bath, but that does not help much).

    I'd like to install a bath light/fan kit in the shower, replacing the light - but I'd prefer not to cut another vent into my siding. I was hoping that I could use some kind of 'T' into the existing toilet fan ducting that is currently vented to the side of my house. This way, each could operate independently and I would not have to cut an additional vent into my siding.

    Can this be done with no concerns? Do I need to be concerned about duct sizes (the shower fan would likely need to be sized much bigger than the toilet fan). What about backdrafting? Any special parts I may need?

    thanks

  2. #2
    Plumber jimbo's Avatar
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    Default

    Use a wye instead of a tee to reduce backdraft. Also, most fans have an integral backdraft damper. If you have a 4" duct on the toilet fan, that is adquate for even a very large CFM bath fan.

    Fans and lights directly over a tub or shower are required to be UL listed for that specific application. This may limit your choices.

    In any event, such a fan must be connected to a GFCI protected circuit.
    Last edited by jimbo; 11-16-2004 at 07:57 AM. Reason: add gfci

  3. #3
    Plumber, Contractor, Attorney LonnythePlumber's Avatar
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    Default Larger exhaust fan

    If you have mildew then it sounds like you need a larger fan to move more CFM or let it run longer. I have not seen a ceiling fan in a bathroom. You must have a very large room.

  4. #4

    Default possible problem...3" and 4" ducting

    It is a large master bathroom, the only venting to outside is in the 'enclosed' toilet area (bath fan and window). This is the reason I need to vent the shower area.

    I just found that the existing ducting (from the toilet room to outside) is 3". The intended bath fan I'd like to install in the shower requires 4" ducting. Does this mean that I am out of luck in trying to tie the 2 bath fans into the same outside vent? I assume the 4" ducting above the shower would lose efficiency if I tap into the existing 3" ducting.
    If so, my choices appear to be:
    1) Run a separate 4" duct for the shower and cut a new vent in my siding for it
    2) Upgrade my toilet fan to a 4" duct, enlarge the existing vent, and tie the 2 together.
    3) Let the 4" duct share the 3" duct/vent and hope efficiency is OK

    correct?

  5. #5
    Retired Defense Industry Engineer jadnashua's Avatar
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    Default

    I'd consider a remote fan that connects ducts to both places. That way it is quiet, and efficient. You can probably replace the existing 3" vent cap with a 4" or larger without messing up the siding, but I've not done it (I'm not a pro).

    Does this have an attic above, or does it have living space above? I put in a Solatube ceiling fan/light/circular skylight in my bathroom. Very quiet, and free light. Check out their website as an option. My guess is that you could use this in the shower as well, and when the sun is up, you wouldn't need the light.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  6. #6

    Default

    There is an attic above the master bath.

    Thanks for the suggestion, but I'm looking for a quick and easy fix rather than the Solatube solution.

  7. #7
    Plumber jimbo's Avatar
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    Default

    I suspect you could up the ductwork to 4" and possibly keep the same outlet fitting at the exterior wall.

  8. #8

    Default Thanks

    Thanks to all for your responses.

  9. #9

    Default

    I have coupled two together with no problem and I have coupled two together with a problem.

    Same fan and duct size with minimal duct runs and it seems to work fine.

    Different fan size and longer / different ducts you will have a problem. When both are in use at the same time, the stronger fan can reduce the output of the weaker fan to almost no discharge.

    Try and keep it as balanced as you can.

  10. #10
    DIY Junior Member stephnej's Avatar
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    Default

    since we are on this subject of bathroom fans, i am replacing an exhust fan and not sure how to take out the old? it's in a finished ceiling, so i don't know if it is nailed in or what. i took out all visible screws and something is holding it in. any tips?

  11. #11
    Retired Defense Industry Engineer jadnashua's Avatar
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    Some of them use hangers - sliding bars that go between the studs. Depending on how concentious the installer was, it could be a pain getting it out. Most of the time, they are just tacked in. If it is up against a joist, look for a nail or a screw through the side into the joist. WIthout access, how do you plan to get the new one in and stable? There often isn't enough play in the exhaust vent to do the wiring and make an airtight connection, then put it in place.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  12. #12
    DIY Junior Member stephnej's Avatar
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    Question

    Not sure what you mean. so, if it is on a hanger then what is the best way to get it out? can i just pull it down by force? also, i can see the wires and it appears that i can get my hands it there and wire it together. is a possible change?

  13. #13
    Moderator & Master Plumber hj's Avatar
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    Default fan

    IF you pull it by force you will pull down a portion of the ceiling also. Most fans are larger than the opening in the ceiling so you will probably have to cut a section out to remove the old and install the new one.

  14. #14
    DIY Senior Member Hube's Avatar
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    Default

    As that piece of foolhardy advice suggests, yup, your a plumber

  15. #15
    Retired Defense Industry Engineer jadnashua's Avatar
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    Default

    You indicated in an earlier post that there was an attic above. Time to crawl up there. Once there, it should be obvious how it is attached. You will be able to then make a much better connection to the vent pipe and do the electical connections as well. Probably need to be up there to get it attached properly, too. You don't want your connections on the vent pipe to be leaking. You're exhausting moist air - it will condense and potentially make a mess up there. Depending on the length of the run, it will probably get some condensation. It is much better to use the metal ductwork for this rather than the corregated stuff, unless run vertically through the roof. That way, you can pitch the run to the outside so any condensation will run outside rather than drip back into the room. Seal the joints, too. Some of the cheaper fans do not have a damper in them to block cold air from coming back into the room. This may be a useful thing to add, if necessary. Much easier in a solid vent. Most vent caps have one, but not all (they don't seal that well usually, and I personally like to have one closer to the unit. If it gets really cold there, insulate the pipe, too. Helps keep the moisture as a gas rather than condensing.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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