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Thread: Adding a utility sink - Will this work?

  1. #1
    DIY Junior Member Sylvester's Avatar
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    Default Adding a utility sink - Will this work?

    Hi everyone. Here's another "can I do this" thread.

    I want to add a utility sink into my basement. Ideally, it will be positioned next to the washer and dryer, and I would like to tie it into the existing drain line from the washer discharge. It currently has 2" Sch. 40 PVC washing machine standpipe and P-trap, going to a Sani-Tee and drain with an AAV on the top, like this (pardon my horrendous drawing):

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    I want to add the sink between the washing machine and the Sani-Tee. I'm thinking Sch. 40 P-trap, trap arm going to a 90 elbow that connects to a combo wye added to the existing drain line. Like this:

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    Will this work? Is it legal? And will it vent properly? I'm not sure if I'd have to add another AAV between the sink trap and the combo wye. Thanks!

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
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    The sink trap will get sucked dry when the WM drains.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  3. #3
    Test, Don't Guess! cacher_chick's Avatar
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    What you have drawn is not permitted. You can install another sanitary tee over the existing one and run your sink to it. Both the washer and the sink should be 2" pipe, and the trap to vent distance cannot exceed 5' if it is under UPC.

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    DIY Junior Member Sylvester's Avatar
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    Cacher_chick, is it not permitted to tie in to the existing WM drain, period? Or just because of the siphoning issue?

    I clearly see what you mean now, Jadnashua.

    Another concern I have is, looking at things now, I might not have the vertical clearance I need anyway. The bottom of the P-trap might be at the level of the existing horizontal drain run. Could I flip-flop the arrangement, and do this?
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    In the Trades Gary Swart's Avatar
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    You have not one but two illegal S traps. Vents must be in the trap arm prior to the arm turning downward.

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    Forum Admin, Expert Plumber Terry's Avatar
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    Putting the sink above the washer works best.
    The washer will always siphon a sink if the sink is below.

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    Moderator & Master Plumber hj's Avatar
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    They are NOT "S" traps unless they are connected to the vertical line with Ys or combos. The sink can connect to the horizontal line if the trap is PROPERLY vented. You cannot connect the sink the vertical line UNDER the washing machine, unless you properly vent the sink line to prevent both positive and negative pressures, which means, but definition, you cannot use an AAV.
    Last edited by hj; 02-01-2014 at 07:13 AM.
    Licensed residential and commercial plumber

  8. #8
    DIY Junior Member Sylvester's Avatar
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    Quote Originally Posted by Gary Swart View Post
    You have not one but two illegal S traps. Vents must be in the trap arm prior to the arm turning downward.
    Forgive me, my morning coffee is still trying to work in. I thought the S-trap was defined by having too short of a trap arm (not 2x the diameter), not necessarily by its venting.

  9. #9
    Test, Don't Guess! cacher_chick's Avatar
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    Gary is mistaken. You should be able to lower the sanitary tee for the washer so that the washer trap is lower, and the sanitary tee for the sink will go above the washer's. You do not want a major drain over the top of a minor one.

  10. #10
    DIY Junior Member Sylvester's Avatar
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    Quote Originally Posted by hj View Post
    They are NOT "S" traps unless they are connected to the vertical line with Ys or combos. The sink can connect to the horizontal line if the trap is PROPERLY vented.
    To connect the sink to the horizontal line, could I vent the trap arm with an AAV? Given the location, I do not have another way to vent.

  11. #11
    Test, Don't Guess! cacher_chick's Avatar
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    You could do that, but both of the AAV's need to be installed higher than the flood rim of the higher fixture. Not doing so will cause the AAV's to fail if a backup of laundry suds comes up the vent.

  12. #12
    DIY Junior Member Sylvester's Avatar
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    Ok, great. Now, if I did it the other way - basically as I drew most recently but with the sink draining ABOVE the washer - that will be a wet vent of the washer, yes?

  13. #13
    Test, Don't Guess! cacher_chick's Avatar
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    Correct- it is a vertical wet vent, which is a common practice with a washer and utility sink.

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