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Thread: Can vent go downward below the highest fixture?

  1. #1
    DIY Member keano016's Avatar
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    Default Can vent go downward below the highest fixture?

    Might seem like a silly question, but please bear with me.

    I know that venting must go vertically up 6 inches above the highest fixture, which is usually the sink prior to going horizontally.

    My question is, once I go vertical, let's say 20inches above the rim of the vanity, can I go horizontal for a foot or so, then go downwards vertically for 30 inches, and connect to a vent?

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    One who lurks Basement_Lurker's Avatar
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    No you can not.
    Broken promises don't upset me. I just think, why did they believe me? -Jack Handy


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    In the Trades Tom Sawyer's Avatar
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    It will trap condensation and over time will seal off the vent.

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    Moderator & Master Plumber hj's Avatar
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    It will work because condensation will drain both ways, but I doubt that you would ever find an inspector who would approve it. They usually only know what is "in the book" and cannot visualize how anything else would work properly.

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    DIY Member keano016's Avatar
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    Here is my situation. The bathroom is in a dormer in the attic that has one wall that is an outside wall but there is a 2 feet crawl space on the bottom. So I was thinking of wenting from the sink go up then down then around to connect to the 2 inch meta venting that goes to the roof.

    What would you do?


    Last edited by keano016; 03-12-2011 at 05:29 AM.

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    Test, Don't Guess! cacher_chick's Avatar
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    I cannot make sense of the drains in your drawing, but it appears that the lav can be vented into the 3" CI or go straight up through the roof.

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    DIY Member keano016's Avatar
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    Yes, the cast iron is there, but I am nor sure how I can tap into the cast iron, especially since there are two cast iron pipes connected there, one 3 inch that is the drain from the toilet plus a 2 inch that is a vent from the lower floors.

    I want to avoid going through the roof because of the fear that I may not do it properly and then there is a leak or I fall

    if I can go around the small wall, which requires going below the highest fixture, then I can connect to the 2 inch metal venting on the oposite wall.

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    Test, Don't Guess! cacher_chick's Avatar
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    Quote Originally Posted by keano016 View Post
    if I can go around the small wall, which requires going below the highest fixture, then I can connect to the 2 inch metal venting on the oposite wall.
    You cannot go down with the vent.

    Because you don't know how to do something right is never a good reason to do it wrong.

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    DIY Senior Member dlarrivee's Avatar
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    The "proposed vent" on your drawing doesn't make any sense at all...

    You're going to have a tough time tying into the 3" CI stack at the proper height, so your best bet is to go through the roof... It's not a big deal...

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    DIY Member keano016's Avatar
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    I agree. That's why I was asking, altough I kinda expected you guys will say no to going down below the fixture. I still do not see the problem thou.

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    DIY Member Cubey's Avatar
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    This (I think) would be the correct way of sharing a vent (the red line). So why is it you can't do it that way? It might need to be higher than I placed it but that's the general idea.
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    DIY Member keano016's Avatar
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    Quote Originally Posted by dlarrivee View Post
    The "proposed vent" on your drawing doesn't make any sense at all...

    You're going to have a tough time tying into the 3" CI stack at the proper height, so your best bet is to go through the roof... It's not a big deal...
    Seems the easiest way to go through the roof. One thing I have not drawn out on the picture is a plastic vent through the roof from a high efficiency furnace. Now, the guys who did it they connected the fresh air supply and the venting together to a wye and then one 3 inch pipe is going through the roof. Is that set up ok (mixing the supply and the condensation vent? If so, can I tap into that with the sink vent since it is plastic and easy to work with?
    Last edited by keano016; 03-13-2011 at 10:34 PM.

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    DIY Member keano016's Avatar
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    Quote Originally Posted by Cubey View Post
    This (I think) would be the correct way of sharing a vent (the red line). So why is it you can't do it that way? It might need to be higher than I placed it but that's the general idea.
    That is a Cast Iron 3 inch vent so it is challenging to connect to.

  14. #14
    DIY Member Cubey's Avatar
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    Does it have a tee currently where I am showing the red line tapping into it? If so, could you not remove the tee and plus a 4-way ( + ) one? And anyway, your proposed vent had it going over there anyway to that exact spot. I just cut out the extra wasted amount going up, across and back down.

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    DIY Member keano016's Avatar
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    sorry about the confusion in the drawing. A 2 inch cast iron ties with a 45 degree tee into the 3 inch cast iron. The propose venting was passing trough that area but it was not connected. I was planing of going pass the 3 inch cast iron then turn 90 degrees to go behind the low wall and conect on the oposite side where the 2 inch venting is. That's why I was asking about going below the highest fixture.

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