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Venting question

Sorry this isn't steam, but I need plumbing advice. Two 2" vent stacks take their air from a single 4" soil stack ("Soil Stack One"). A third 2" vent stack takes its air from a second 4" soil stack ("Soil Stack Two"). The actual venting done for fixtures by these vent stacks is: East Vent Stack - 11 dfu, Center Vent Stack - 25 dfu, West Vent Stack - 8 dfu. You can see that the Center Vent Stack is over the limit of 24 dfu per each 2" vent stack.

Both soil stacks are cracked. I engineered a means to to eliminate Soil Stack Two to cut repair costs. This means that I must get air to Vent Stack 3 by other means. Please consider the following two proposals:

Scheme 1: Join East and West Vent Stacks to create a larger combined Vent Stack with a total of 19 actual dfu venting (Center Vent Stack remains separate with 25 dfu venting.

Scheme 2: Join all three Vent Stacks to create one combined venting network with a total of 44 actual dfu venting. In this arrangement, two 2" vent pipes would penetrate Soil Stack One in the attic. Can this arrangement handle 2 x 24 dfu = 48 dfu? Or is the venting capacity instead limited to only 24 dfu because the two Soil Stack penetrations are on the same Soil Stack?


  • Venting

    Both schemes will work as long as you provide at least one full-bore soil stack (i.e. 4") and size the vents properly.  Per the Uniform Plumbing Code, vertical vent sizing is:

    2" - 24 fixture units

    3" - 84    "       "

    4" - 256  "        "

    If it were me, if those cracks are just superficial and not structural, patch them.
    Often wrong, never in doubt.
  • icesailor
    icesailor Member Posts: 7,265
    Cracked vent/soil stacks:

    Assuming that the structure had plumbing drain, waster and vent installed as a "system", the system depends on the other parts to work properly. Just because you have decided what the fixture drainage unit requirements, doesn't mean you can eliminate parts of the "system". The cracked soil pipes are usually old 4" that is over 70 years old. If it is cracked, you should replace it. I replace it all the time.

    Were you planning on leaving the cracked soil pipes in the wall and connected to the DWV system? How will you keep sewer gas out of the structure" Usually, the cracked pipes are the soil stacks and not the vent stacks. Vent stacks don't leak water like soil pipes do.

    The pipes are what is the weak point. They crack. The fittings are usually very thick and of better CI quality.

    Don't do a HA job. Fix it right or don't fix it at all and get someone who will.
  • Schvenzlerman
    Schvenzlerman Member Posts: 39
    Answer + Question

    Ice - The cracked soil stack that is to be eliminated served only one fixture. That fixture will be relocated close to another soil stack that will take its drainage. The vent stack that served the cracked soil stack will be disconnected from it. This disconnected vent stack  will then be connected to one of the other soil stack's two vent stacks.  The reason why is  because it runs down to the basement to vent the drainage system where it exits the structure.

    Let me now ask a hypothetical question. Assume that a structure has a single 4" soil stack. The owner wants a boat-load of fixtures.  Each vent stack that you install for the owner must be 2" pipe and MUST be attached to the soil stack in the attic (with a 4x2 fitting). What is the maximum venting in dfu that you can achieve under this scheme? How many vent stacks do you install to obtain this maximum dfu?
  • meplumber
    meplumber Member Posts: 678
    Cross Sectional Aggregate Area.

    In the IAPMO, the cross sectional aggregate area of the vents must equal the area of the sewer line exiting the building.

    Are the fixtures back vented?
  • icesailor
    icesailor Member Posts: 7,265
    Venting Question:

    I live in Massachusetts. We do not use the IAPMO. We have our own.

    Every case is different. But, the #1 rule is that you have a full size pipe of no less that 3", connected to the main building drain, that runs full size through the full length of the building and through the roof. You can vent any individual fixture with 1 1/2". You can use "wet venting" as long as the vent is 2" and and one fixture is floor mounted. Ie" Bathtub, shower, toilet or floor drain. There are fixture unit values to drainage but you can not put more than three toilets on a 3" soil stack or building drain or more that three bathroom groups on a 3" soil stack or building drain. But you can put 144 toilets on a 4" horizontal drain.

    Your ideas should be run by a code enforcing person/plumbing inspector to see what they have to say. I think that they would require you to repair/replace the broken pipes.

    I find that cracked soil pipes are like fleas. There's always more. If this is a commercial building, and they will allow PVC, its a lot easier to replace it than to screw around with patching and re-routing. 
  • Schvenzlerman
    Schvenzlerman Member Posts: 39
    cross-sectional aggregate

    If I understood you correctly, you're saying that a 4" sewer main connection would mean that a structure could have a maximum of two 2" vent stacks.

    Yes, the fixtures are back vented, if you mean vented on the away side of traps.

    To answer Ice, City code (Philadelphia) requires structures exceeding 3 stories to have cast iron pipe. Not sure why.
  • meplumber
    meplumber Member Posts: 678
    That would be a minimum of 2-2' vents.

    Not uncommon for structures above 3 stories to require cast iron.
  • meplumber
    meplumber Member Posts: 678
    sorry, 2'

    Ice is correct that it must have a continuous stack vent. 
  • Schvenzlerman
    Schvenzlerman Member Posts: 39
    Maximum & Minimum

    Okay, so there must be a minimum of two 2" vent stacks if the structure's connection to the sewer main is 4". What is the maximum number of 2" vents that one could install in that illustration?
  • icesailor
    icesailor Member Posts: 7,265
    2" vents

    And it takes 4-2" pipes to equal a 4" pipe.
  • Schvenzlerman
    Schvenzlerman Member Posts: 39
    Actually a little less...

    It's actually not quite four 2" pipes:

    2" pipe OD: 2.375

    - wall: .154

    = 2.221 ---------- (2.221*2.221) * .785 = 3.87 sq. in.

    4" pipe OD: 4.5

    - wall: .237

    = 4.263 ---------- (4.263*4.263) * .785 = 14.265 sq. in.

    14.265/3.87 = 3.69 2" pipes

    My home actually has five 2" vent stacks. MePlumber wrote that, 'the cross sectional aggregate area of the vents must equal the area of the sewer line exiting the building.' If so AND were the sewer main connection correctly sized, the connection would be:

    3.87 sq. in. * 5 = 19.35 sq. in. for five 2" vents

    square root of (19.35/.785) = 4.96" sewer main connection

    My guess is that there is nonetheless a 4" connection to the sewer main. I'll call the water and sewer dept. on Monday and find out. This is a circa 1905 home.
  • icesailor
    icesailor Member Posts: 7,265
    Venting questions:

    It's not the OD, it's the ID. The water doesn't flow on the outside of the pipe, it's flowing on the inside.

    The area of a 1" circle is .7854.

    The area of a 2" circle is 3.141.

    The area of A 4" circle is 12.566.

    The area off a 16" circle is 201.06

    Divide by 4 and you will come back to .7854

    Massachusetts Apprentice Plumbing Math 101

    All hydronic piping and flow are based on these numbers.
  • Schvenzlerman
    Schvenzlerman Member Posts: 39
    Thanks for the numbers correction

    Very much appreciated. I still have five 2" vent stacks, so I'll still call the water dept. to solve the mystery concerning whether the sewer main connection is larger than 4".
  • icesailor
    icesailor Member Posts: 7,265
    Building drain

    What;s the question?

    If it is a "normal" building, it has a 4" cast iron building drain. Maybe it transitions to 6" going through the wall in the basement. It doesn't matter about your cracked soil pipes, you must replace them or completely replace the whole sanitary drain, waste and vent system and bring it up to code, today's code.

    There's never enough time to do it right but always time to do it over.
  • jonny88
    jonny88 Member Posts: 1,139

    you need to run a continuos 4" stack from your basement through to yuour the base of the stack you can cut in a 4x2y and start a 2"vent stack.this 2" vent stack can vent all your fixtures on each floor,just make sure you connect the vent on each floor at 42".at the top of your waste stack it becomes a vent stack and at this point you can tie in your 2" vent stack.dont patch.icesailor is right do it once and be done with it.
  • rlaggren
    rlaggren Member Posts: 160
    Just to clarify


    Do you have any 4" plumbing pipes going through your roof? After reading your posts I'm not absolutely sure. You did say you had five 2" pipes penetrating the roof but that does not mean you have no 4" penetrations. I'm not understanding your terminology fully.

    AFAIK whether you are required to have a "full bore stack", ie. a pipe the same size as your main drain going through the roof, depends on your local jurisdiction. Your five 2" vents may meet code no problem. But if that is what you have (no 4" penetration), then you _must_ retain all five 2" vent penetrations. Whether you need to run them all the way to your main drain under ground individually or whether you can combine them on the way down with 3" or 4" drains (stacks) you will need to check carefully with your inspector. The written code is just a guide - the inspector's word is the law and anything he says supercedes whatever _you_ (or anybody else) may think the written code says.

    Also AFAIK in residential, 4" through the roof will suffice regardless of whether your building has a 6" main drain. But that needs checking.

    Normally when running vents we "header early and header often". Ie. we combine vent piping as soon as possible to reduce the number of pipes we need to fight through the roof. That's what all those rules about fixtr units tells - how quick we can combine into one pipe. The only single fixture that has to start out w/a 2" vent is the toilet; all others are usually run in 1-1/2" until the fixture count exceeds (what is it...senior moment here) eight, I think. Sometimes old houses ran everything in 2" because that was more convenient material-wise with bell&spigot.

    CI is required in taller building because it doesn't conduct fire like plastic.

    I'm not clear what part of your stacks are cracked, but Ice is pretty sure right: Repair (ie. replace) the cracked CI. Don't wag your whole design with that tail.

    How many vents into a 4" stack? Anything up til you reach the fixture limit - 256 units. But know your code for wet venting and check ahead with your inspectcor. Not all jurisdictions allow it and it has special rules.

    disclaimer - I'm a plumber, not a heating pro.
  • Schvenzlerman
    Schvenzlerman Member Posts: 39
    More clarification...

    East soil stack - 4" through roof, has two 2" vent stacks that attach in attic

    Center soil stack - 4" through roof, has one 2" vent stack that attaches in attic

    West soil stack - 4" through roof, has one 2" vent stack that attaches in attic

    Kitchen drain stack - 2" drain is vented through roof by one 2" vent stack

    I'm replacing all of the cast iron except for the single cast iron vent stack. It appears to be in good shape and it's all dry except for one run that will be replaced.
  • icesailor
    icesailor Member Posts: 7,265

    Not to belabor a point but, if you are going to the trouble of replacing all that you say, replace the last vent stack. It's only the 4" CI that I have ever seen crack. Like I said before, cracks are like fleas. Where there's one, there's more.

    To my retarded way of thinking, it's like going to the hospital, having major major surgery, and being stitched up. A week or so later, you go to have the stitches out. But the doctor decides to leave a couple of stitches in because he doesn't feel like removing them.
  • Schvenzlerman
    Schvenzlerman Member Posts: 39
    the cast iron vent stack

    Again, the 2" cast iron vent stack is dry except for one horizontal leg. So it's probably in good shape with that exception. I have to put a tee on it to vent a sink in a new location, so I'll be able to inspect its interior condition.

    By the way, I learned today from my water dept. that our connection to the City's sewer is 6". That explains why I have five 2" vent stacks when the soil stacks are only 4".

    What I find most confusing about vent stacks is understanding length limitations. The original design of the home's vent stacks was as follows:

    East vent stack: 14 dfu and 48' length (this will change to 11 dfu)

    Center vent stack: 8 dfu and 48' length (this will change to 17 dfu)

    West vent stack: 7 dfu and 48' length (this will change to 8 dfu)

    North vent stack: 6 dfu and 48' length (this will change to 9 dfu)

    Kitchen vent stack: 1 dfu and 48' length (no change)

    I have found no charts that plot dfu against 2" vent stack lengths.

    We're getting rid of one soil stack because we are moving a toilet that will instead drain to a different soil stack. In consequence, we plan to join the East and West vent stacks by adding a 15" length of horizontal pipe between them. Sounds okay to join the two vent stacks?
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