Vent size for 3" horizontal drain from water closet

Discussion in 'Plumbing Forum, Professional & DIY Advice, Tips & ' started by techinstructor, Aug 6, 2011.

  1. techinstructor

    techinstructor New Member

    Messages:
    81
    Location:
    North Carolina
    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. SacCity

    SacCity In the Trades

    Messages:
    189
    Location:
    Sacramento, CA
    Minimum vent size for a toilet is 2"
    Michael
  3. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,279
    Location:
    Cave Creek, Arizona
    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.
  4. Tom Sawyer

    Tom Sawyer In the Trades

    Messages:
    3,071
    Location:
    Maine
    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.
  5. techinstructor

    techinstructor New Member

    Messages:
    81
    Location:
    North Carolina
    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.


    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.
  6. Tom Sawyer

    Tom Sawyer In the Trades

    Messages:
    3,071
    Location:
    Maine
    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.
  7. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,279
    Location:
    Cave Creek, Arizona
    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.
  8. supermattthehero

    supermattthehero New Member

    Messages:
    78
    Location:
    virginia
  9. Tom Sawyer

    Tom Sawyer In the Trades

    Messages:
    3,071
    Location:
    Maine
    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.
  10. techinstructor

    techinstructor New Member

    Messages:
    81
    Location:
    North Carolina
    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. Tom Sawyer

    Tom Sawyer In the Trades

    Messages:
    3,071
    Location:
    Maine
    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.
  12. phaedrus123

    phaedrus123 In the Trades

    Messages:
    24
    Location:
    ny
    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.
  13. Tom Sawyer

    Tom Sawyer In the Trades

    Messages:
    3,071
    Location:
    Maine
    Well even 3" through the roof changes depending where you are. down south you can poke 1-1/2 through
  14. phaedrus123

    phaedrus123 In the Trades

    Messages:
    24
    Location:
    ny
    guess you dont have freezing problems like we do.in ny they want min 3inthru roof and extend a minimum of 18 inches
  15. Winslow

    Winslow Plumber

    Messages:
    450
    Location:
    Hawaii

    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.
Similar Threads: Vent size
Forum Title Date
Plumbing Forum, Professional & DIY Advice, Tips & Contractor mistakes? Wet vent, downsized line? Jun 7, 2014
Plumbing Forum, Professional & DIY Advice, Tips & Can you combine smaller pipes on a vent branch to reach the min. 1 1/4" size? May 23, 2014
Plumbing Forum, Professional & DIY Advice, Tips & Vent size two toilets Jan 3, 2012
Plumbing Forum, Professional & DIY Advice, Tips & 1 final vent termination size? Jun 20, 2011
Plumbing Forum, Professional & DIY Advice, Tips & What size vents? Apr 26, 2011

Share This Page