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Thread: Vent size for 3" horizontal drain from water closet

  1. #1
    DIY Member techinstructor's Avatar
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    Default Vent size for 3" horizontal drain from water closet

    My water closet empties into a 3" horizontal drain (using a combo wye for the change from vertical to horizontal). I plan to add a vent, connected with a wye, about 7' downstream from that point. What is the correct pipe size for this vent? NC code says the vent must be at least 1/2 the diameter of the drain pipe so I figured 1 1/2" would work and meet code. Also, am I correct that the distance to the vent on a 3" drain cannot be more than 10'? The slope of the horizontal drain is 1/4" per foot.

    Thanks for your help. Just want to be sure.

  2. #2
    In the Trades SacCity's Avatar
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    Minimum vent size for a toilet is 2"
    Michael
    Michael
    Sac City Plumbing
    http://SacCityPlumbing.com

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    Moderator & Master Plumber hj's Avatar
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    IF that combo is UNDER the toilet, then we might have to know what is upstream from it to determine whether you even need it, if that is the correct location for the vent or if it has to be somewhere else.
    Licensed residential and commercial plumber

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    In the Trades Tom Sawyer's Avatar
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    Quote Originally Posted by SacCity View Post
    Minimum vent size for a toilet is 2"
    Michael
    Is that a California thing because we can wet vent two entire bath groups off a single 1-1/2" vent.

    Codes vary but toilets do not need a vent to operate correctly. Toilets are full siphoning fixtures and the atmospheric pressure above the bowl is sufficient.
    [B]No, plumbing ain't rocket science. Unlike rocket science, plumbing requires a license[B]

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    DIY Member techinstructor's Avatar
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    Quote Originally Posted by hj View Post
    IF that combo is UNDER the toilet, then we might have to know what is upstream from it to determine whether you even need it, if that is the correct location for the vent or if it has to be somewhere else.
    HJ,
    The combo is directly under the toilet and there is nothing but a cleanout upstream from there. The 3" horizontal drain has another 3" branch drain that joins it about 6' downstream from the toilet. The intersecting drain runs perpendicular to the main horizontal drain. I was going to add the vent to the drain approx 1 - 2 feet downstream of the intersection - a location that allows me to easily direct the vent upward into the wall.


    Tom Sawyer -- Codes vary but toilets do not need a vent to operate correctly. Toilets are full siphoning fixtures and the atmospheric pressure above the bowl is sufficient.
    I'm in North Carolina. My inspector told me that I didn't need this vent, which was actually closer to the toilet when he originally looked at the plan. I assume he was thinking it could all be wet vented via the lav vent, which is at the end of the horizontal drain that runs perpendicular to the branch that the toilet is on. However, I've read that every fixture needs a vent so I'm trying to do what's right. So is this just wasted effort, or a little insurance on all working properly?

    Thanks for the input.

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    In the Trades Tom Sawyer's Avatar
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    Again, toilets don't need vents....ever. They are designed to empty the entire contents of the bowl and the trap with every flush. The trap is a full s trap. Under the latest version of the IPC, the trap to vent distance for a water closet is unlimited. So yes, it is wasted effort.
    [B]No, plumbing ain't rocket science. Unlike rocket science, plumbing requires a license[B]

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    Moderator & Master Plumber hj's Avatar
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    Toilets do not NEED vents, but most codes REQUIRE them. A vent past the intersection with the sink drain would be cosmetic and have no function as far as the toilet is concerned. The lav drain, SHOULD be adequate to fulfil the toilet's venting requirements.
    Licensed residential and commercial plumber

  8. #8

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    Hi,

    If toilets don't need vents, then why does HJ infer that I need one here?

    http://www.terrylove.com/forums/show...n-4-quot-drain

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    In the Trades Tom Sawyer's Avatar
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    You have to check your state and local code. If you are under the IPC, no local vent is needed but most UPC states still want one withing prescribed distances.
    [B]No, plumbing ain't rocket science. Unlike rocket science, plumbing requires a license[B]

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    DIY Member techinstructor's Avatar
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    Thanks a bunch HJ and Tom Sawyer. I will eliminate the toilet vent since it is not needed and it is not required here by code.

    I have one more question. The lav vent will intersect with the vents from the washer and tub before it exits the roof. I read somewhere (don't remember where) that the when vents are combined the size of the pipe should increase, so I planned to increase the pipe from 1 1/2" to 2" at the first intersection and then continue with 2" until it exits the roof. The vents from the tub and washer are 1 1/2". Is this correct or should I leave it at 1 1/2" all the way through the roof.

    Also, I'm curious as to why some codes require a toilet vent when one is not needed. Just curious.

    Thanks again for your help. It is most appreciated.

  11. #11
    In the Trades Tom Sawyer's Avatar
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    Vent size is determined by the dfu of the fixtures served and in your case 1 1/2 is plenty large enough but, it's hard to find 1 1/2 roof flashings so increasing to 2" is probably the ticket.
    [B]No, plumbing ain't rocket science. Unlike rocket science, plumbing requires a license[B]

  12. #12

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    some say min 2 inch vent some say 1.5 inch.if toilet were 4 in vent would be 2in.half the diameter.where i work in nyc toilet would require 2in vent however as i am learning about wet venting and getting advice from the above plumbers i would listen to them.also minimum of 3in going thru roof.

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    In the Trades Tom Sawyer's Avatar
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    Well even 3" through the roof changes depending where you are. down south you can poke 1-1/2 through
    [B]No, plumbing ain't rocket science. Unlike rocket science, plumbing requires a license[B]

  14. #14

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    guess you dont have freezing problems like we do.in ny they want min 3inthru roof and extend a minimum of 18 inches

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    Plumber Winslow's Avatar
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    Quote Originally Posted by Tom Sawyer View Post
    Is that a California thing because we can wet vent two entire bath groups off a single 1-1/2" vent.

    Codes vary but toilets do not need a vent to operate correctly. Toilets are full siphoning fixtures and the atmospheric pressure above the bowl is sufficient.

    The vent also prevents trap siphonage, which will occur without a vent.

    Some codes like MA only require 1/2 the trap arm size. Other codes, like the UPC requires a 2" vent for a water closet.

    The trap to vent for a toilet is 6' max according to the UPC.

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