Relief Vent a Trap Arm for a Fixture Drain serving as Wet-vent

Discussion in 'Plumbing Forum, Professional & DIY Advice' started by ghaun, Mar 27, 2021.

  1. ghaun

    ghaun Member

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    All,

    I am hoping that you can help me with a scenario. In order to wet-vent a tub, I have a vanity sink which would be pressing the length of its trap arm. Is it possible to relief vent that trap arm, so that the vertical wet-to-dry for the wet vent is still present for the tub? Due to floor joists, soffits, etc. The pipes really can't move from these positions. Thoughts?

    Bath-Vanity.jpg
     
  2. PlumbNuts

    PlumbNuts SC Licensed Plumbing Contractor

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    What you have drawn should work fine. You could move your relief vent a little closer to the tub than pictured (stay within maximum trap arm distance); it may save on materials.
     
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  4. ghaun

    ghaun Member

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    So, here is a slight modification which would allow for me to stay within a soffit and the floor joists without plowing through studs. From a physics standpoint, it seems fine. The horizontal vent for the tub is wet and is washed and the sink is also dry vented. Do the different drainage planes matter, given the fact that each one has a dry vent connection and that the horizontal vented part is wet and washed?

    Thank you.

    Bath-Vanity_Mod.jpg
     
  5. wwhitney

    wwhitney Well-Known Member

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    What's the shortest that you can make the bottom vertical segment between the two horizontal drains? And could you make that short vertical offset with a 22.5 and a LT90 over some horizontal length, rather than with a short vertical segment?

    If yes to the latter question, then I say there is no question that you can eliminate the lefthand dry vent. Since starting at the sink san-tee, there is just one vertical region and one horizontal region, as usual. Actually, I think you could eliminate it in the first diagram in this thread, but I understand that is subject to some debate, because the wet vent portion is horizontal - vertical - horizontal.

    Cheers, Wayne
     
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  6. ghaun

    ghaun Member

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    Wayne,

    Thank you!

    The white pipe at the top of the drawing is a vent pipe already in that position, so ultimately, I have to connect there. There is a cathedral ceiling and that is the only place that I can go up.

    To answer your other question with regard to the 22.5, etc. I could do a 22.5 with a LT90, but that wouldn't eliminate the planar concern from a venting perspective, which is why having that vertical vent is advantageous. Since the horizontal wet-vent portion is still washed, I think that should be fine, even though, I don't see anything in the code with regard to what I have drawn. I wish I could better classify. The way I have it drawn now, I could almost classify this as being wet-vented by the vanity, with the vanity trap arm having an offset with a vent at that offset location, which alleviates that concern; otherwise, it would be an s-trap.

    Additionally, the pipe which the tub drains into is in a soffit below the joists. The perpendicular pipe that heads over to the vanity is in a joist bay. Either way, I would still need to connect to the vent directly above.
     
  7. wwhitney

    wwhitney Well-Known Member

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    What planar concern? Do we have any code or engineering evidence that says that a horizontal wet vent at 22.5 degrees is worse than a horizontal wet vent at 1.1 degrees (2%)? Note that IPC 915.2.1 on the slope of Combination Waste and Vent specifically calls out a maximum slope of 4%. The lack of any corresponding requirement in IPC 912 says to me that there is no upper bound on the horizontal slope.

    Cheers, Wayne
     
  8. ghaun

    ghaun Member

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    Wayne, perhaps I didn't understand your desire with the 22.5 / LT90. What were you suggesting? The slope of the pipe has to do with the amount of air space left relative to the weir of the trap. That is why I have both vents.
     
  9. wwhitney

    wwhitney Well-Known Member

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    The right hand vertical vent is your vent for the sink. All trap weir concerns end at that point.

    The only reason for the left hand vent is if you feel that the upper horizontal segment isn't allowed to be part of a horizontal wet vent for the tub (I'm aware of no such prohibition). Then you want the wet vent to end at the upper terminus of the lower vertical segment, and you add the left hand dry vent to serve the tub.

    One possible reason to feel that the upper horizontal segment isn't allowed to be part of a horizontal wet vent is if you take the position that a horizontal wet vent should have only one region of horizontal drains, possibly preceded by a vertical segment (e.g from a lav san-tee). As opposed to being horizontal - vertical - horizontal (your first drawing) or vertical - horizontal - vertical - horizontal (your second drawing).

    The point of my suggestion of a 22.5 elbow followed by a LT90 to get back to 2% slope was to eliminate the second vertical segment in your last drawing. That makes it a simple vertical - horizontal arrangement, which I suggest would definitely eliminate the need for the left hand vent.

    On the other hand, the left hand vent does no harm, so if you prefer to add one more vent than you need, no problem.

    Cheers, Wayne
     
    Last edited: Mar 29, 2021
  10. ghaun

    ghaun Member

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    Wayne, I have thought some more about this and this horizontal offset is almost like a single stack system with an offset, except there is only one branch below and above. The vent to the left would be the offset vent. So the concept of horizontal wet-vent (the branches) to vertical to horizontal and back to vertical has been seen in the code. One could argue that the bathtub is a horizontal branch below the offset, so the offset vent would be required. Either way, I think that I may have found a parallel concept within the code.
     
  11. PlumbNuts

    PlumbNuts SC Licensed Plumbing Contractor

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    What you have drawn out here should work fine for what you are trying to accomplish.
    Git R Dun
     
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  12. ghaun

    ghaun Member

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    PlumbNuts, thank you. I am putting it into execution:) Thanks.
     
  13. ghaun

    ghaun Member

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    So, I realized that there was a part of my plan which I did not like. I had a common vent downstream to serve the shower and tub, before I planned on extending to the sink for the wet vent. That common vent would not work, as I would now have two fixtures on one arm. The shower was on the lower and the tub (and now vanity sink) would be on the upper. This would not work. I could have turned the entire thing into a wet vent like we originally had, but I had some concerns when now combining those common vented fixtures into one wet vent with a 7.5DFU shower. I have the dry-vent there any way at the end which would have served the common vent, so I looked at this more and figured that I could make this more like a circuit vent and probably achieve better venting. See my attached picture. I have a dry vent at the most upstream area too. I realize that the dry-vent for a circuit vent is placed between the two most upstream fixtures. That last fixture is designed to flush the dry vent. In my scenario, my last fixture cannot enter at a horizontal, thus I vented it separately, but it does still serve to flush the dry-vent. Let me know your thoughts. I have a relief vent; though, not necessary. ...but it will give some extra protection.

    Thank you!

    Bath Circuit Vent.jpg
     
  14. wwhitney

    wwhitney Well-Known Member

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    Where are you getting 7.5 DFUs from for the 6 head shower? How many gpm for all the shower heads together?

    For the IPC, Table 709.1 says that a shower that can put out 12.3 gpm to 25.8 gpm is 5 DFU and requires a 3" trap. The supply plumbing to such a shower would be pretty impressive.

    Personally, with such a shower, I'd be inclined to dry vent it and keep it separate from the sink and tub until the stack. I.e. raise up the shower trap arm and let the vent on the right be the shower dry vent, and let the tub and sink come in lower. The sink wet vents the tub as previously discussed.

    Your latest diagram would still be a considered an acceptable wet vent if you deleted the leftmost and right most dry vent, so it's certainly fine and certainly would work, whether you call it a circuit vent or wet vent or whatever.

    Cheers, Wayne
     
  15. ghaun

    ghaun Member

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    Wayne,

    I had not seen that sizing for GPM for showers. That helps tremendously. I had estimated 15 GPM worst case. It would likely be more like 12. So, the 5 DFU is much better than I was estimating. I had planned on a 3" trap anyway, so that is good. That is one of the original reasons that I had these separated at first. Without separating them and having the dry relief vent at the right side (downstream end) of the picture, I was hoping that would be sufficient. That is one of the reasons that I have the relief vent at the downstream end. My goal was to simply have a purely dry vent at the upstream end, instead of the vertical wet vent with a vanity. Do you see anything wrong with the way that I have it drawn, given having the relief vent or would you still split it out? Either way, I have to stay in that vertical plane there, as I have a wall.

    With regard to supply. I have this zoned and have (2) 199,000 BTU tankless heaters for this side of the house, where the shower would be split into 2 zones.
     
  16. ghaun

    ghaun Member

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    West Chester, PA
    Wayne and All,

    Given my updated DFU, would you still split it out separately? Either way, I will still have the vent at the right on the drawing. The question is whether it is purely an individual vent or also serving as a relief for both the shower and tub with just one drain vs two stacked. Also, the vent at the left-hand side of the picture is already there. By separating out the drains, that connection would serve only the tub as a wet-vent.

    Thank you!
     
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