Does every stack require a stack vent

Discussion in 'IPC Plumbing Code Questions' started by ghaun, Mar 12, 2021.

  1. ghaun

    ghaun Member

    Joined:
    Mar 12, 2021
    Location:
    West Chester, PA
    All,
    I have a question regarding the concept and requirement for a stack vent for every stack. I live in PA and my township stipulates the 2003 IPC and updates thereafter.

    I have a 3" stack that extends as a stack vent through the roof and that serves as a drain for a large shower on the first floor and a bathroom on the second floor. All of those fixtures also use the stack vent as their vent through the roof, as well as another vanity, tub, and laundry group which connect to the vent above the second floor bath.

    In addition, there is another stack (less than a branch interval), which serves as drainage to the building drain for a double bathroom group (vents separately through a 2" vent through the roof), another toilet and vanity (separate 2" vent through the roof), and the other vanity and tub (use the other 3" for vent).

    In total, I have 3 vents through the roof, a 3" and (2) 2", which equate to more than the 4" building drain.

    I really am just confused on the need to vent each stack as a separate requirement to venting all traps.

    I have attached a graphic and will work to update it and possibly post more pics if necessary.

    Thank you for your help!
     

    Attached Files:

  2. wwhitney

    wwhitney Well-Known Member

    Joined:
    Mar 17, 2019
    Location:
    Berkeley, CA
    I didn't take the time to fully visualize your description, but let's see if this helps. From the 2015 IPC:

    https://up.codes/viewer/pennsylvania/ipc-2015/chapter/2/definitions#stack

    "STACK. A general term for any vertical line of soil, waste, vent or inside conductor piping that extends through at least one story with or without offsets."

    "STACK VENT. The extension of a soil or waste stack above the highest horizontal drain connected to the stack."

    "VENT STACK. A vertical vent pipe installed primarily for the purpose of providing circulation of air to and from any part of the drainage system."

    So one way to vent the fixtures draining through a stack is with a stack vent (assuming various other requirements are complied with). But it's not the only way, you can have a drainage stack that doesn't extend above the highest horizontal drain connected to the stack; the top of the stack might just be an elbow for that highest horizontal drain. The fixtures could be individually vented away from the stack with the vents combined in the ceiling above and rising through the roof. That would not be a stack vent, as it is not the extension of a waste stack.

    Somewhat confusingly, that combined vertical vent would be a vent stack, but it might not be a stack (e.g. for a top story bathroom, the combined vent doesn't rise extend through a full story, so it's not a stack).

    Cheers, Wayne
     
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  4. ghaun

    ghaun Member

    Joined:
    Mar 12, 2021
    Location:
    West Chester, PA
    Wayne,

    Thank you for your response. That was my understanding as well, but then I saw this article which described the need for both, given my interpretation of the paragraph entitiled "FIXTURE TRAP VENTS" on page 5 of https://cob.org/wp-content/uploads/aspe-vent-systems.pdf

    ...and the "VENT STACKS" paragraph on page 4.

    It seems to talk about two needs. One is the need to equalize pressure from rapidly falling water and air movement in stacks, ultimately relieving sewer gases and pressures possibly affecting traps, and the second, being the need to have vents for fixtures to ensure the traps are not siphoned by themselves or competing fixtures.

    I had originally thought that the fixture vent capabilities would alleviate both and still believe this to be the case, unless we are talking about high drops (5 branch intervals or more), where vent stacks become a requirement. ...which don't pertain to my scenario.

    Thank you,

    Greg
     
  5. wwhitney

    wwhitney Well-Known Member

    Joined:
    Mar 17, 2019
    Location:
    Berkeley, CA
    That document is targeted towards engineers and may be useful in explaining some of the reasoning behind venting code provisions. I read the two sections you mentioned, but based on what the plumbing codes require, I infer that the comments in the "VENT STACKS" section only apply to that "5 branch intervals or more" situation.

    I.e. putting a stack vent on every drainage stack may be the best practice, but for low rise buildings we can get sufficiently good performance without doing that.

    Cheers, Wayne
     
  6. Reach4

    Reach4 Well-Known Member

    Joined:
    Sep 25, 2013
    Location:
    IL
  7. ghaun

    ghaun Member

    Joined:
    Mar 12, 2021
    Location:
    West Chester, PA
    Reach4,
    I am familiar with that brochure. Thank you! All of my vents are dry with the exception of the horizontally wet-vented toilets for two bathrooms and the stack-vented toilet on the top floor.

    My question had to do with some statements that I had read where every stack must be vented through a transition to a stack vent, in addition to ensuring that every individual trap is vented. I have one 4" stack that continues from the building drain to the roof, where it transitions to a 3" stack vent and one other 4" stack which goes vertical from the building drain for 7' to accept the drains from many horizontal branches where the individual constituent fixtures are all individually vented. This stack does not continue through the roof with a stack vent.
     
  8. Reach4

    Reach4 Well-Known Member

    Joined:
    Sep 25, 2013
    Location:
    IL
    So the word stack can be a thing that means different things to different people.

    Looking at your drawing, it was not clear that there was a separate vent pipe and drain pipe passing through the floor under the new construction. So if that is covered, I think you are good.
     
  9. ghaun

    ghaun Member

    Joined:
    Mar 12, 2021
    Location:
    West Chester, PA
    Reach4,

    I apologize, I should have called that out better. I have a vent for the shower running right next to the 3" stack. That vent is a 2" pipe which I colored in yellow in the drawing, so that it would be distinguishable. Unfortunately, the drawing shows much smaller than uploaded.

    Thank you!
     
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