drain pipe size and venting sink

Discussion in 'Plumbing Forum, Professional & DIY Advice' started by barryw, Mar 15, 2007.

  1. barryw

    barryw New Member

    My kitchen sink drain runs from one side of the house to the other where it connects to the base of the stack vent. The total distance is about thirty feet.
    I have two questions:

    1. Is the current 1.5" pvc pipe large enough or should I replace it with a two inch pipe? It drains a double sink with garbage disposal.

    2. The only vent it has is basically a loose drain connection from a washing machine that was once installed (rigged) in the kitchen for the previous owners, if you want to call that a vent.
    I'm going to replace the pipe. Over that 30 feet, does it matter where I tie in a vent assuming it needs a vent. I want to vent it as close to the stack as possible.
    Hope that makes sense.

  2. jimbo

    jimbo Plumber

    San Diego
    You have a problem here. The maximum length of the trap arm...the pipe from the trap to the vent connector....under the UPC is 42" for 1½" pipe, and 60" for a 2" pipe. You should fix that/
  3. leejosepho

    leejosepho DIY scratch-pad engineer

    I had a similar situation before a complete re-do of our drain plumbing, and here is my own take on the matter:

    First, an 1-1/2" sink trap cannot possibly let enough water through fast enough to completely fill a sloping 2" line and keep it that way "to the last drop". Therefore, a vent to protect that trap from being siphoned dry by the sink discharge going through it is but marginally necessary, at most. However, and since "code" seems to require one, you might simply install one of those "AAV" one-way check vents.

    Second, "to vent it as close to the stack as possible" is neither a good idea nor necessary, and for two reasons:

    1) If the drainage from the sink actually *could* suck its trap dry, a vent at the other end of the line would not stop that;
    2) That stack should already be well-vented by other means, making another vent right next to it and in an incoming line redundant.
    Last edited: Mar 16, 2007
  4. can't be, as written.


    you must be mistaken. perhaps you meant to write it the other way 'round.

  5. Randyj

    Randyj Master Plumber

    Should there be a partial obstuction it will cause siphoning of the trap if you do not have the vent very close to the trap....preferably on a tee at the end of the trap....or very close to it.
  6. leejosepho

    leejosepho DIY scratch-pad engineer

    Ah! I had not thought of that.

    So much for my "marginally necessary", eh?!

  7. barryw

    barryw New Member

    Ok I found a vent. There is a vent beneath kitchen sink running into the wall.
    I missed it because I checked the attic and basement when looking at the plumbing. The problem, I can't find any place where the vent ties into the stack, and there are no other vent pipes running through the roof.
    So, suppose the vent is there and it is vented properly, are there any issues with the 1.5" drain pipe that runs about thirty feet to the base of the stack vent?
    Thank you!
  8. Winslow

    Winslow Plumber


    The drain should be 2", at least that is what is required by the UPC. Especially with a disposer on the line.
  9. barryw

    barryw New Member

    Alright. I will change the pipe and do what I have to do to get the vent right.
    Thanks again.
  10. kk

    kk New Member

    Eastern CT
    Trying to learn something here... supposing Barry's sink didn't have a proper vent - what would the correct thing to do be? Being as it's 30 feet away from the stack, how do you run a vent line? With an existing structure it seems like it would be close to impossible to run up and over to the vent stack, and running a new vent stack up in the kitchen could be a problem too.

    What do you do?

  11. hypothetic

    hi keith!

    i think lee joseph 0 mentioned AAVs. Studors.

  12. leejosepho

    leejosepho DIY scratch-pad engineer

    The situation with the drain line for our washing machine was like that, and I have resolved that dilemma to my own satisfaction by placing the trap right at the stack and running a sloped 2" drain line to there. The washer sits below the middle of an upstairs room and is not on an outside wall. So, and even if this is not "in the books" or whatever, I simply did what it took to make a safe, working drain.
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