Why do I need to vent this washer/sink drain?

Discussion in 'Plumbing Forum, Professional & DIY Advice' started by AllenEllis, Apr 20, 2010.

  1. AllenEllis

    AllenEllis New Member

    Messages:
    4
    Location:
    Pittsburgh PA
    Hi all,

    I'm re-doing a laundry area and I think I have a clever trick to venting my washer and sink, but I want to see what you guys think:

    [​IMG]

    As I understand it, the purpose of venting is to allow air to escape from the pipes as water tries to flow down. But because of the way the washer's drain hose rests inside the drain pipe, there is an opening, and therefore, two-way airflow.

    So as water drains from the sink, air can be pushed out through the top of the washer's drain, and vice versa. The p-trap under the slab prevents sewer gases from coming up either opening. So in this case, is there even a need for a vent through the roof?

    The complications I forsee are:
    1) If the sink is full and plugged, the washer may not be able to drain and may overflow
    2) Draining water from the washer might flow into my sink a little
    3) If you drain a lot of water down the sink at once, might there be enough pressure for it to come out the washer's drain? Would a sanitary T help prevent this?

    Any other thoughts?

    Thanks.

    Last edited by a moderator: Apr 20, 2010
  2. TristanP

    TristanP New Member

    Messages:
    10
    Location:
    Delaware
    Posting this before Terry does

    [​IMG]

    Maybe an AAV above the T would make things less awkward. You need a P trap at the bottom of the washer drain before it goes horizontal.
  3. AllenEllis

    AllenEllis New Member

    Messages:
    4
    Location:
    Pittsburgh PA
    Lol, yep I've seen that image posted in lots of other threads I found. I see lots of diagrams like that with roof vents, but I'd like to know whether I can get away without that given that there is airflow in this case.

    Why is it necessary for the washer to have a P-trap before the T? Isn't its function just to prevent sewer gases? And if so, why doesn't the p-trap in the slap accomplish that?

    Thanks again.
  4. nukeman

    nukeman Nuclear Engineer

    Messages:
    711
    Location:
    VA
    You are not allowed to double trap a fixture (like your sink in the diagram).

    Too much height from the trap to the top of the standpipe can cause the trap to siphon. This is why they want the washer trap above the floor and why there are limits on how tall the standpipe can be.

    The washer discharge will pull the water out of the sink's trap

    [​IMG]
    Last edited by a moderator: Jun 2, 2010
  5. AllenEllis

    AllenEllis New Member

    Messages:
    4
    Location:
    Pittsburgh PA
    I was under the impression that you can't double-trap in a closed system, but if there is airflow between the traps (as in this case), it's ok. That's not true?

    Right, 18-48" is what I've read the standpipe height should be. The p-trap is about 8-10" below the concrete, so my total height would be 20-25".

    What about an AAV above the T as Tristan suggested? I know those are considered cheap workarounds, but would it work?

    Thanks again, I appreciate all your help :)
  6. Redwood

    Redwood Master Plumber

    Messages:
    7,450
    Location:
    Connecticut
    The reason for that picture is that it is done correctly...
    I would suggest getting your drawing closer to that picture...
  7. nukeman

    nukeman Nuclear Engineer

    Messages:
    711
    Location:
    VA
  8. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    27,258
    Location:
    Cave Creek, Arizona
    It is NOT a "clever trick", you have merely created a convoluted trap riser. The trap is NOT vented, and because of where it is located, it can be subjected to other forces besides siphonage. The trap under the sink is useless, even though your washer pipe would vent it. Air would NOT flow through the sink trap, unless it had lost its seal from siphonage, which would only happen if your drain did not vent it. The reason for the vent is NOT because the washer riser will not allow air and water to flow in both directions. The hose does not fit so tightly that it could not happen, but that has NOTHING to do with the requirement for a vent anyway, so it is irrelevent. You are NOT a plumbing hydraulics expert, so do not quit your day job, and leave piping designs to the people who know how to make them work.
  9. AllenEllis

    AllenEllis New Member

    Messages:
    4
    Location:
    Pittsburgh PA
    Thanks everyone for your help.
  10. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    27,258
    Location:
    Cave Creek, Arizona
    quote; Thanks everyone for your help.

    I hope that is sincere, and not sarcasm. We try to keep people from making mistakes when they try to do tasks they have absolutely no idea what they are doing.
Similar Threads: need vent
Forum Title Date
Plumbing Forum, Professional & DIY Advice Need to install a PV Supervent on a tight system Nov 7, 2014
Plumbing Forum, Professional & DIY Advice Do ALL drains need venting? Aug 15, 2014
Plumbing Forum, Professional & DIY Advice Need help with adding tub drain and vent Aug 10, 2014
Plumbing Forum, Professional & DIY Advice Need advice on vents for sink and tub Aug 3, 2014
Plumbing Forum, Professional & DIY Advice Do I need a vent here? Jun 24, 2014

Share This Page