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Thread: Flat Vent

  1. #1
    In the Trades Tom Sawyer's Avatar
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    Default Flat Vent

    I would like to clear up some confusion here regarding the practice of flat venting. I fully realize that in some states it is perfectly legal and acceptable to inspectors. I want to show you why it should not be though, and that just because a thing can be done it is not always the best reason to do it that way. Take a look at the picture here. This is an acceptable ( to some ) method of venting a water closet. The wye fitting which is being used for the vent has been rolled up so that its run inlet is above the center line of the horizontal drain. So far so good. Where this goes wrong is the horizontal pipe that heads back to the wall parallel to the WC closet bend and then it runs up the wall and presumably to another vent or through the roof. At any rate, that horizontal piece of pipe can and will eventually fill with toilet paper, tampons, condoms and poo and it will plug up thereby negating it's intended use. Better that the vent continue to rise vertically until it has reached a point (6" above the highest fixture served which is the lav) and then it can go horizontal because it won't be subjected to all the nasty stuff that can clog it up.
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    Last edited by Tom Sawyer; 07-07-2011 at 07:35 AM.

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    Moderator & Master Plumber hj's Avatar
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    "Stuff" will only get into that flat vent when the toilet drain line plugs up, but once it gets into that pipe, unless there is some way to dislodge it, it will stay there. That is why flat vents almost ALWAYS are required to have some other fixture, such as a sink, connected to it to keep it flushed out. A cleanout would be a less than desirable alternative because you would NOT use it unless you KNEW the vent was plugged.

  3. #3
    In the Trades Tom Sawyer's Avatar
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    Yes but a fixture attached makes it a wet vent and in it's current configuration you would never know if it was plugged or not because the toilet is going to flush just fine anyway because it does not need a vent to operate.

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    Moderator & Master Plumber hj's Avatar
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    You are addressing a flat vent, and the normal configuration for a flat vent IS a wet vent to eliminate the problem you have discussed.

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    In the Trades Tom Sawyer's Avatar
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    HJ, you are probably from an area where you just don't run into a true flat vent. Massachusetts allows them and up until a couple years ago Maine did also. I think it is confusing though to call a flat vent a wet vent. Lord know enough plumbers get confused over what constitutes a proper wet vent.

  6. #6
    Expert Plumber plumber2011's Avatar
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    As HJ said, "That is why flat vents almost ALWAYS are required to have some other fixture, such as a sink, connected to it to keep it flushed out". I see no issue with flat venting as long as the horizontal run is as short as possible AND a sink and/or tub are connected...see image.
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    Writing, constructionDIY Member Yersmay's Avatar
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    Default A question

    Tom, I'm curious about something you said in a few posts up in this thread. You said that a toilet does not need a vent to operate. I guess this makes sense because a toilet evacuates via a siphon, so I suppose a toilet is a big brother to an 'S' trap. Would a toilet have an even more powerful flush without a vent? If a toilet has a built in weir which keeps the sewer gasses out of the house, why then is a vent required on toilets? Does the vent have less to do with the toilet itself and more to do with keeping things moving in the waste line downstream of the toilet? Thanks in advance! Interesting thread!

  8. #8
    George the Plumber Gsalet's Avatar
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    have you ever put your finger over the end of a straw that was in a soda then removed the straw from the soda? notice how the soda stays in the straw, now release your finger and see how the soda drains from the straw. without a vent the toilet might flush but would not drain.

  9. #9
    In the Trades Tom Sawyer's Avatar
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    Quote Originally Posted by plumber2011 View Post
    As HJ said, "That is why flat vents almost ALWAYS are required to have some other fixture, such as a sink, connected to it to keep it flushed out". I see no issue with flat venting as long as the horizontal run is as short as possible AND a sink and/or tub are connected...see image.
    The diagram you have there is not a flat vent it is a wet vent. You guys need to understand your definitions a little better to avoid confusion. A true flat vent is not washed at all which is why it is not allowed in most places. I see no issue with wet venting, it is pretty much standard procedure.
    Last edited by Tom Sawyer; 07-07-2011 at 05:09 AM.

  10. #10
    In the Trades Tom Sawyer's Avatar
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    Quote Originally Posted by Yersmay View Post
    Tom, I'm curious about something you said in a few posts up in this thread. You said that a toilet does not need a vent to operate. I guess this makes sense because a toilet evacuates via a siphon, so I suppose a toilet is a big brother to an 'S' trap. Would a toilet have an even more powerful flush without a vent? If a toilet has a built in weir which keeps the sewer gasses out of the house, why then is a vent required on toilets? Does the vent have less to do with the toilet itself and more to do with keeping things moving in the waste line downstream of the toilet? Thanks in advance! Interesting thread!
    There is so much misunderstanding about what vents are for, even by experienced plumbers. Vents protect the trap seal. Without traps, there is no need for a vent at all. Toilets are self siphoning and incorporate a full S trap in their design. A toilet will flush just fine with no other vent than the atmosphere pushing down on it's open bowl. To prove this point I attached a working toilet to close to 60' of 3" pvc with no vent at all and the pipe pitched at 1/8" IPF. The toilet flushed perfectly every time. we filled it with wet paper towels, it still flushed perfectly every time and the reason is because the toilets bowl is the vent. Then again ANY fixture will drain just fine with no vent for the same reason. If a plugged vent is causing the fixture to drain slowly it is because the drain itself is plugged not the vent.

  11. #11
    In the Trades Tom Sawyer's Avatar
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    Quote Originally Posted by Gsalet View Post
    have you ever put your finger over the end of a straw that was in a soda then removed the straw from the soda? notice how the soda stays in the straw, now release your finger and see how the soda drains from the straw. without a vent the toilet might flush but would not drain.
    That is just plain not true at all.

  12. #12
    Expert Plumber plumber2011's Avatar
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    Yet here at this thread (click below) where I drew up a WET VENT (post #9) you called it a FLAT VENT (post #10), Tom?? In fact, that is what started this thread...isn't it? Yet all I was proposing in that thread was to roll the wye up as shown above and use the tub to wet vent the toilet...again, you called it a flat vent???

    http://www.terrylove.com/forums/show...-no-wall-space

    I think it's you that need to get things straight...

    PS: I also disagree completely with your assessment of venting as presented here, Tom! *SHRUGS*....dunno, just dunno!
    Last edited by plumber2011; 07-07-2011 at 05:32 AM.

  13. #13
    In the Trades Tom Sawyer's Avatar
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    Quote Originally Posted by plumber2011 View Post
    Yet here at this thread (click below) where I drew up a WET VENT (post #9) you called it a FLAT VENT (post #10), Tom?? In fact, that is what started this thread...isn't it? Yet all I was proposing in that thread was to roll the wye up as shown above and use the tub to wet vent the toilet...again, you called it a flat vent???

    http://www.terrylove.com/forums/show...-no-wall-space

    I think it's you that need to get things straight...

    PS: I also disagree completely with your assessment of venting as presented here, Tom! *SHRUGS*....dunno, just dunno!
    Take another look at your post #9. The section from the vertical vent to the tub wye in particular. That flat section of pipe there is not washed by any fixture above as drawn and is therefore a "flat vent"

    That you disagree with my assessment of venting comes as no suprise to me. You disagree because you really do not understand venting. To qualify my statements though. I have been a master for 37 years now. I teach plumbing apprentice classes as well as licence recertification seminars. I was a state plumbing inspector for a few years and local plumbing inspector as well. I currently teach plumbing at a high school technology center and hold a masters in plumbing design and education.

  14. #14
    Expert Plumber plumber2011's Avatar
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    Again, you are mixing terms here. That is a wet vent....period!

    I'm not going to go into exactly how many engineering degrees, or prosthetics and electrical degrees I have...not important to prove anything to you. You also can't insult me or even upset me as your opinion is not importanmt to me, Tom. Suffice it to say, again, that I disagree with you...completely!

  15. #15
    In the Trades Tom Sawyer's Avatar
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    NO it is not a wet vent. The flat section that rises to vent is NOT washed by any fixture avove therefore it is NOT a wet vent.

    Nobody is trying to insult you here at all. Enlighten perhaps but not insult. You can disagree all you want, that's your right but before you do I suggest a little trip through your code book will change your mind.

    The title of this thread is "Flat Vent" not wet vent. There is a difference and that difference is shy I started this thread.
    Last edited by Tom Sawyer; 07-07-2011 at 06:05 AM.

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