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Plumbing Vent Question

Rich Kontny_3
Rich Kontny_3 Member Posts: 562
Part of our exam in Wisconsin is a drawing along with sizing. You would flunk that test as 2' is overkill for a lav. 2' will handle 8 DFUs (drainage fixture units) a lav is one dfu. This simply is not cost effective and not needed.

What would you do undera pedestal lav with 2"? (that would be one ugly install as space is very limited.)

Search for: Wisconsin Dept of Commerce Safety and Buildings then go to plumbing codes.Wisconsin has excellant charts for waste and vent, water, storm, sewers and even a chart for horizontal waste arms.


  • In Canada anyway,

    we are not "allowed" to drop down from the trap then vent. It must be a "vertical(45 or above) continuous waste & vent" same level as the trap, most inspectors don`t like fit 45s,,(no S-traps),,,had an old (retired) plumber today argue that point, what say you?
    The old 4" CI drainage piping had alot of air movement, but not code acceptable nowadays. Thoughts?

  • greg_47
    greg_47 Member Posts: 9

    i am not sure i undestand your question. the reason for not allowing a trap arm to change elevation is you are making a s-trap that could siphon out the trap. Water closets are a exception to this rule because there trap is in the fixture and utilize this siphon action to operate. You also mention the verticle or 45deg, this is usually in reference to the vent pipe and where it originates. this is used to select fittings and insure no "s-trap" could be formed.

    does this help?
  • Mark Hunt_4
    Mark Hunt_4 Member Posts: 68
    Greg has it

    google "hydraulic gradient".

    Mark H
  • Love ya guys,, try this,,

    100% vertical drain, basin rough-in(usually) 18" high, within 5`, proper fitting allowance to basin trap(horizontally), can a TY be used, or is a wye & fitx45 illegal?

  • rich pickering
    rich pickering Member Posts: 277

    Has to be a ty. The drop of the trap arm cannot exceed the diameter of the pipe. So, 1 1/2" pipe, the trap arm cannot be longer than 5' before it connects to a vent. A y and 45 would have the drop greater than 1 1/2".

    If you use 2" for the trap arm you can go 7'.
  • greg_47
    greg_47 Member Posts: 9
    trap arm lenghts

    in washington and idaho state, our trap arm lengths for 1.5 and 2 inch are 3'6" and 5' (per the UPC)

    "i see your trap arm is bigger than mine"
  • Are you sure Rich?

    I think "rule of code" is 5` max.(within fit allowance & fall) for basins or other P-trap fixtures(depending on the fixture units), and remember stack loads come into play here too. 3" or 4" syphonic fixtures bring circuit venting to mind,,, have to dust-off my code books, been awhile.

  • Don \"Grumpy\" Walsh
    Don \"Grumpy\" Walsh Member Posts: 184
    Proof at last

    So, size really does matter!
  • rich pickering
    rich pickering Member Posts: 277

    Assumption is 1 1/2 trap arm. But you can use 2". I've used it on a tub to avoid running an extra vent.
  • Rich Kontny_3
    Rich Kontny_3 Member Posts: 562
    Our code is very clear about this

    We cannot exceed 6'0 from crown of trap to point of vertical discharge with a horizontal 1.5" waste arm at 1/4" pitch. If we go from horizontal to vertical you should not use a wye as it defeats the whole water hydraulics of this ruling. Using a wye on the vertical would use up the 1 1/2"s before you start the horizontal branch!

    What we are trying to do is not negate the vent by exceeding the inside diameter of the pipe in the horizontal run. If you used a wye you would have to pull another vent off the horizontal arm and tie it back in to the stack a min. of 38" above finished floor.

    All about plus and minus pressures within a drainage system. Minuses create vaccuum that syphons traps and allows sewer gas in the living space (not a good thing)

    Great question though! Do they allow air admittance devices (like Studor vents) in Canada?

  • kevin_60
    kevin_60 Member Posts: 38

    In Jersey 1.5" 5 ft to vent. 2" 8 ft to vent. NO wye off arm it has to be a t wye they say because vent is below trap weir with wye and st. 45. Also any drain line with more than 1 fixture has to be long sweep fittings.
  • Excellant Rich K,

    That is our code too, but we are limited to 5ft horizontally from "crown" of the trap to vertical CW&V tie-in point,,, our fitting allowance is 1-90 or 2-45s, but the fall is not to exceed the pipe diameter,,, of course any other drainage from above(upstream), trashes the TY installation!
    Just thought I would bring-up a different subject!
    Thanks again Rich.
    BTW-I think you are referring to what we call "cheater-vents",,, have I used them? never LOL,,, besides they only vent 1 way.

  • Hi kevin,

    I understand your code & point here,,, I actually never considered running 2" to a basin to gain that extra 3ft,,,perhaps I`ll look into that more closely,,thanks, but how would that work at your reduction point? do you install a 2in trap, then reduce to 1-1/4 at the PO?,, if you use an 1-1/4 or 1-1/2" trap does that defeat the purpose of fall from trap to vent using 2"?,, up-here the trap must be the same size as the waste pipe.

    Trade school for me was many years ago!

  • kevin_60
    kevin_60 Member Posts: 38

    You should use a 2' trap and reduce at the tail piece but i usually install a 2 x 1 1/2 bushing in the 90 at the wall because a 2" trap is hard to fit in small vanities, Pedistal bases or even wall hung basins. Because the drain on most basins are too close to the wall to fit a large 2" trap.
  • Think I understand kevin,

    up here they call that "circuit venting", and that can only be done on syphonic fixtures(I think),, been awhile LOL!

  • Ron Gillen
    Ron Gillen Member Posts: 124
    Drainage from above?

    What does that mean, drainage from above trashes the TY installation? Cheater vents now legal under Canadian National Code, never used one, never will! Sewer gas can kill! Don't know how to impress this on the young guys.
  • Cool your heels Ron,

    I never said that about cheater vents, they are NOT code!
    "Drainage from above", only meant an upper floor, then you should(or must) use a wye,,sheesh!
    Don`t get all defensive now!!

  • Rich Kontny_3
    Rich Kontny_3 Member Posts: 562
    If we oversize

    we have to intall a cleanout. So we would revent the fixture within the 6' rule. On a lav 5 or 6 ft makes little or no difference as lavs are considered only one DFU (drainage fixture unit)

    Studors actually work fine and I prefer them on islend kitchen sinks with loop vents as they are below the flood level rim of the fixture. A kitchen sink with a disposal and a partialy plugged or fully plugged drain will fill the vent as the 3/4- 1hp disposal will push the waste over the top of the loop and plug it eventually.

    We have very limited use of Studors per code and never on a positive pressure drain (wash machine standpipe etc) We did have a major controversy re: Studor vents as some clowns used 59 of them in one motel. They of course failed often as positive pressures were encountered often.
  • Rich Kontny_3
    Rich Kontny_3 Member Posts: 562

  • bob young
    bob young Member Posts: 2,177

    oversizing also prevents self-scouring action
This discussion has been closed.