Drain Waste Vent options — the good, the bad, the ugly.

Discussion in 'Plumbing Forum, Professional & DIY Advice' started by 157, Dec 10, 2020.

  1. 157

    157 Member

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    Remodeling a second-story master bath in a 1920's house that looks like it was redone in the late 90's — hopefully by an unwitting homeowner (and not a licensed plumber) because it was vented out the side wall, half behind the window shutter:

    [edit: added for clarity] The PVC vent sits on top of a PVC drain stack that runs down the outside wall into the basement where it connects (rather poorly) into the main cast iron drain.

    Inside
    (also note the buried box)

    [​IMG]

    [​IMG]

    Outside
    [​IMG]
    [​IMG]

    Over in the opposite corner, the original cast-iron stack is still there, with the main bath on the other side of the wall currently tied in:
    [​IMG]
    [​IMG]
    [​IMG]

    So it seems like I essentially have two options here.

    1. Reroute the PVC that's currently going through the side wall, bring it up through the attic, and punch a hole in the roof with a new vent. Challenges here is that the attic is partially finished (original plaster/lathe), so I would have to be breaking through some walls to fish it through. The placement of the bathroom would make the vent end up either directly on the front pitch of the house (see picture below), or it would have go up and over — ending up within a few feet of the existing cast-iron vent. Also, it is what it is, but I'm not particularly excited about putting a new hole in the roof.

    [​IMG]

    2. Tie into the existing cast-iron stack. Obvious challenges here are having to break into the cast iron, but make certain I can reconnect the currently operational tie-in with the main bath. The position of the flange/wye is a bit tricky since it's pretty well embedded in the joists/wall. Seems like one of those projects that could go sideways real quick.

    I realize this is mostly a judgement call, but I wanted to hear from those who've had experiences with both of these options, and hear your thoughts on what would be the most sensible option (or, maybe even some better ideas)?
     
    Last edited: Dec 11, 2020
  2. Jeff H Young

    Jeff H Young In the Trades

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    go up wall tie into 4 inch cast. no new hole in roof
     
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  4. 157

    157 Member

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    The one thing I failed to mention (I'll update my original post) is that the PVC that vents out the side wall has its own drain stack that runs down the outside wall and ties into the cast iron down in the basement. This means if that I was to use the cast iron just for venting, (I believe) it would make the cast iron vent too far away from the PVC drain stack (over 12 feet to run all the way across the room).

    I may just be misunderstanding your suggestion... you may be saying to just run everything over to the cast iron and tie it all in, drain and all?
     
    Last edited: Dec 11, 2020
  5. Jeff H Young

    Jeff H Young In the Trades

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    No run 2 inch vent up wall and connect to the cast iron. btw what's the purpose of the 2 inch PVC that connects to the 3 inch horizontal? Nothing is supposed to tie into that line. But if it was vented you wouldn't even need the pipe that goes out side wall, it could be eliminated
     
  6. 157

    157 Member

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    The whole PVC setup had a bathtub, vanity and toilet connected. What you are seeing in that top picture is the toilet flange (connecting into the horizontal 3”) and then the 2” coming in is what the vanity and tub were connected into. I’m replacing all of, with the exception of the vent — that is... until I opened up the wall and saw what was there,

    The problem I see with using the PVC drain line but then tying into the cast iron for venting is that, from where the drain drops down through the wall, up and over to the cast iron is over 12’. From what I’ve been reading, that’s way too far for a proper vent?
     
    Last edited: Dec 11, 2020
  7. wwhitney

    wwhitney Well-Known Member

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    12' not a problem for a dry vent. Perhaps you are thinking of the limitations on the trap arms?

    Cheers, Wayne
     
  8. Jeff H Young

    Jeff H Young In the Trades

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    The vent can do what I said. But that other stuff cant tie in like that . Between the vent at the wall and the toilet flange the 3 x3 x2 wye has to go You didn't show a tub or lav how are those vented? Maybe you need more pictures but the one that shows the PVC looks wrong
     
  9. 157

    157 Member

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    Hey Wayne, entirely possible I'm mashing up requirements when I was thinking it would have to be a wet vent. I added a image below that illustrates the layout...
     
  10. 157

    157 Member

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    Hey Jeff. Since the toilet (and all the other fixture) are being moved, all of those current tie-ins with the PVC stack are being reworked. Here's an illustration of what new fixtures are going in and their approximate position in the space. I took a guess at interpreting how it all may work: I could keep the PVC drain stack, but run a new vent up and over to the cast-iron stack — is that pretty close to what you've been talking about?

    [​IMG]
     
  11. wwhitney

    wwhitney Well-Known Member

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    The vent length in the ceiling isn't a problem, but the wet venting is not correct.

    The lav is the highest p-trap, and it needs a vent at the san-tee in the wall, either a dry vent run up to the ceiling to connect to a vent through the roof, or an AAV (assuming you are under the IPC).

    Then for a wet vent, the upstream most joint between two fixture drains has to include the dry vented fixture, and other wet vented fixtures can join in downstream of that. That dry vented fixture then wet vents the other fixtures. So if your tub and lav were swapped, and the lav were dry vented, your diagram of green drain pipes would be OK. The yellow existing vent would not be required (as isn't likely to be useful regardless).

    Because your lav is closest to the existing drain, though, you have a little challenge. You could run the lav drain to the right, join up with the tub drain, turn towards the bottom of the page, meet up with the shower drain, then turn towards the existing drain, meet up with the toilet, and then hit the existing drain. But those two 90 degree turns aren't great.

    If the left-right distance from the tub and shower p-traps to the upstream most horizontal lav drain section is less than 8', then under the IPC you can use a 2" trap for the tub, and run the tub trap arm to the left to the horizontal lav drain before the wye. The shower would do the same and hit the combined lav/tub drain before the wye. Then the toilet would come in last. If the distance left right is over 8', you could move the lav san-tee in the wall to the right, and have a lav trap arm in the wall.

    You could dry vent the tub by having it's p-trap outlet point towards the wall with two windows at a 45 degree angle; do a dry vent take off in that wall; and then turn 90 degrees back towards the bottom left. That way the shower and toilet could meet up with the tub drain and be wet vented by the tub. You'd have two dry vents to join in the ceiling (or you could use an AAV for the lav).

    Those are a few of the options.

    Cheers, Wayne
     
  12. wwhitney

    wwhitney Well-Known Member

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    My last post was a bit verbose, so I just wanted to highlight the wet vent design rule that you need to comply with: the upstream most joint between two fixture drains has to include the dry vented fixture (that will wet vent everything downstream).

    Cheers, Wayne
     
  13. 157

    157 Member

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    Rough sketching some interpretations of what you are suggesting...

    Something like this?

    [​IMG]

    Not totally sure about the 45 degree angle outlet, but in general order of operations... like this?

    [​IMG]
     
  14. 157

    157 Member

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    Thanks for having mercy on me. :D It made me wonder... if I just decided to ignore the PVC stack and brave tying in to the existing cast iron, would this "solve all my problems"?

    [​IMG]
     
  15. wwhitney

    wwhitney Well-Known Member

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    1) Yes, as long as there's space and all the trap arms are within their limits (IPC 6' for 1-1/2" trap arm, 8' for 2" trap arm; achieving those lengths requires a slope of exactly 1/4" per foot, no more no less).

    (Also, it's good to minimize elbows as much as practical, e.g. turn the toilet output 45 degrees CCW, so you can use a 45 downstream instead of a 90. The toilet drain line could be in line with the existing drain and hit a combo for combining with the lav/tub/shower)

    2) Yes, except you could eliminate alot of those elbows by running some drains at a 45 angle.

    3) Yes, that works well (in terms of code compliance, no comment on performance); IPC does not require the toilet to be the last fixture. Upstream of the lav/toilet combo, everything (including the vent) could be 1.5".

    Cheers, Wayne
     
  16. 157

    157 Member

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    Thanks so much Wayne. Really appreciate you taking the time to slog through all this. You mentioned "no comment on performance" — is that particular layout one that may have performance implications (vs the other options), or were you just making a general statement?
     
  17. wwhitney

    wwhitney Well-Known Member

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    The latter. I don't have the experience to know how things perform (plumbed two houses total). I just read and try to understand the plumbing code.

    Cheers, Wayne
     
  18. 157

    157 Member

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    Got it. Thanks for your input!
     
  19. Jeff H Young

    Jeff H Young In the Trades

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    I'm glad I brought this up the out side vent wasn't needed.
    I was thinking tying in low waste on cast iron might be a burden instead of using PVC for the waste . but the plan works
     
  20. wwhitney

    wwhitney Well-Known Member

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    A related question: what size are the floor joists, and which way do they run? Prescriptively, you can't drill a hole for a 3" drain line through a 2x10 or smaller joist. So your choice of layout may be driven by routing concerns on the 3" drain.

    Cheers, Wayne
     
  21. 157

    157 Member

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    Ha. You are always one step ahead. I spent today realizing my plan won't work. The joists (1.75" x 9.25" x ~13') run lengthwise, so the 3" toilet drain would have to run across the wrong way. Between the windows, chimney, door position etc, it took me forever to figure out a layout that "worked"... but I guess not. *sigh*

    [​IMG]
     
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