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Thread: Island Sink Venting

  1. #1
    DIY Member techinstructor's Avatar
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    Question Island Sink Venting

    I came across the method for venting an island sink while browsing threads on vent issues. I'm so glad I found this. I had previously given up on trying to properly vent my kitchen sink because of the double window above it and had planned to use a Studor vent. However, I think the island sink method will solve all the issues I have and be a much better option. I check the NC Code and found that this method is allowed for kitchen sinks. However, I do have some questions.

    Here is a picture showing basically what I want to do. I saved the picture from another site and have added labels.



    1. Does the bow vent (the highest point under the cabinet) have to be higher than the bottom of the sink?

    2. What kind of 90 elbow is at the top of the bow? (Regular or sanitary 90, as opposed to a vent 90 or long sweep 90)?

    3. Where do the cleanout(s) for the vent go? NC Code says this:
    Cleanouts shall be provided in the island fixture vent to permit rodding of all vent piping located below the flood level rim of the fixtures. Rodding in both directions shall be permitted through a cleanout.
    To accomplish this it seems like I would need a cleanout in both locations shown on the drawing.

    4. Is there a maximum horizontal distance the foot vent can go before making the turn to vertical? I need to go between 6 and 7 feet.

    5. What type of 90 elbow is used where the foot vent turns to vertical?

    Thank you for your help.
    Last edited by Terry; 08-10-2011 at 06:50 PM.

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    In the Trades Tom Sawyer's Avatar
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    Ok so here is where everyone gets a chance to jump on me because I am always saying how much I hate AAV's and that I will fire anybody working for me that puts one in............with one exception; and the island vent is that exception. I truely feel that I would rather have an AAV there than a loop vent, and the reason why is that even though clean out's must be installed on the horizontal leg (your diagram does not show them but code does require it) if this vent plugs nobody will ever know that it is plugged. I think that an AAV makes more sense here than that convoluted mess of unnecessary fittings. However, many times all of this mess can be cured by simply upsizing the drain lateral to 3" and essentially stack venting the sink into the 3" If I remember correctly as I don't have a copy of the code handy, you can go something like 13' with 3" before hitting a vent. Please correct me if the distance is off.
    [B]No, plumbing ain't rocket science. Unlike rocket science, plumbing requires a license[B]

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    Moderator & Master Plumber hj's Avatar
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    The elbow, and it does not have to be a long sweep, (in fact, I usually use a short turn "vent" elbow), has to be as close to the countertop as possible. You do need a cleanout in the vent somewhere, but usually above the floor level. Using a 3" line is NOT an alternative everywhere, and when it is allowed it is a "combination waste and vent", not a stack vent. If you understand the physics of a 'yoke vent", you would know that the vent riser is redundant.
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    Quote Originally Posted by Tom Sawyer View Post
    However, many times all of this mess can be cured by simply upsizing the drain lateral to 3" and essentially stack venting the sink into the 3" If I remember correctly as I don't have a copy of the code handy, you can go something like 13' with 3" before hitting a vent. Please correct me if the distance is off.
    This is an interesting idea, but I need help understanding exactly what you mean. Sorry, I'm totally inexperienced with this. I had originally included a vent on top of the vertical stack that exits the house. I assume that this would be the stack vent. When I showed my plans to my inspector he told me I didn't need that vent. (I had a vent from my washer, lav, and tub that led into one vent going out of the roof. I originally planned to join vents from 2 kitchen sinks into that line, but I opted for AAV's because of the windows - until now considering the island vent for the kitchen sinks.) Since I no longer have any fixture vents near the stack, I'm wondering if the inspector would want me to put the stack vent back in....but I digress.

    So if I'm understanding this correctly, if I up-size the horizontal sink drain to 3" (one sink is already on a horizontal 3" drain), and add a stack vent (one sink is about 12 from the stack; the other is less than 2 away), then I would need not need any other vent for the sinks. I would have to drop vertically from the height of the trap to connect to the horizontal drain which runs under the floor joists. Is there a way to do this without creating an S-trap?

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    DIY Member techinstructor's Avatar
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    Quote Originally Posted by hj View Post
    The elbow, and it does not have to be a long sweep, (in fact, I usually use a short turn "vent" elbow), has to be as close to the countertop as possible. You do need a cleanout in the vent somewhere, but usually above the floor level. Using a 3" line is NOT an alternative everywhere, and when it is allowed it is a "combination waste and vent", not a stack vent. If you understand the physics of a 'yoke vent", you would know that the vent riser is redundant.
    Thanks for the tip about the elbow. I would prefer to use a "vent elbow" and save space. As for the alternative 3" line.... I assumed this would be a form of a "wet vent" which is allowed, but if it is a combination waste and vent, then I would not meet this limitation because my sinks ARE located along a wall.

    912.2.1 A combination waste and vent system is limited to sinks, dishwashers, floor sinks, indirect waste receptors, floor drains or similar applications where the fixtures are not adjacent to walls or partitions. It consists of the installation of waste piping in which the trap of the fixture is not individually vented.
    I guess I don't understand the "yoke vent" physics. I thought the purpose was to provide an air chamber and if that air comes from the vent riser, then I don't see how it would be redundant. Not questioning your logic here.... I just don't quite understand.

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    DIY Member techinstructor's Avatar
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    Default Another island vent questions

    After dry fitting the parts together, I discovered that on one of my sinks, it would work best for the bow or yoke vent to be upstream of the drain. I wondered how this would affect its operation.

    Here is a picture.


    Can I do this or should I flip the whole bow vent to the left side of the sink? If I do then the foot vent will have to be longer because it need to go to the right.

    Also, this sink drains into the main horizontal branch downstream from the bathroom. Do I need an additional cleanout below the trap? Seems like the cleanout at the trap would be sufficient, but I've seen this on another drawing so I wondered.

  7. #7
    Moderator & Master Plumber hj's Avatar
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    Normally, all the piping under the floor would be 2". The physics of the yoke vent are this. While the sink is draining, the vent has absolutely no function. When the flow slows to the point where siphonage COULD occur, then the flow in the "horizontal drain line" is too small to fill the pipe, so the air in the upper quadrant of the pipe would be available to flow through the yoke to prevent siphonage. Therefore, the lateral vent pipe is cosmetic and is just there to make inspectors happy,

    Last edited by Terry; 08-11-2011 at 04:40 PM.
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    In the Trades Tom Sawyer's Avatar
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    If you are within distance you can run 3" up through the cabinet base and drop the sink directly in.
    [B]No, plumbing ain't rocket science. Unlike rocket science, plumbing requires a license[B]

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    Moderator & Master Plumber hj's Avatar
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    quote; drop the sink directly in.

    You MIGHT want to fit a trap in there somewhere.
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    Default Update on venting option

    I had a chance to show the drawings to my plumbing inspector. He said that the "foot vent" and the vent riser would have to be "wet," i.e. also a drain from another fixture. Fortunately, since HJ had explained the yolk vent physics, I was able to suggest that I simply eliminate that part of the system since it was redundant anyway. He said that would be fine. WOO HOO. That made the job so much easier. Since I'm eliminating the riser, we're going to add a cleanout on the downside of the vent, just in case it's ever needed. This is what it will look like:



    The other thing he reminded me about concerning the rest of my plan was to NOT have more than 4 45-degree bends without a cleanout. I had to "redesign" some of my fixture drains to eliminate some bends, but as we're starting to dry fit this together, it's all working out well and the design has actually improved. He also clarified the definition of an S-trap. According to him, as long as the trap arm length is a minimum of 2 times the pipe diameter before it turns from horizontal to vertical, then it is NOT an S-trap. (Both this inspector and another one who I consulted earlier had recommended not putting a vent on the tub drain since would be adequately vented from the lav vent. I had struggled with how to intersect the drain with the horizontal branch without creating an S-trap. So now that issue is solved)

    Just wanted to again express my thanks for the advice on this board. It has been most helpful.


  11. #11
    In the Trades Tom Sawyer's Avatar
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    Hate to toss a fly in the ointment here but I believe that NC will allow you to use an AAV and though a few may argue with me and lord knows that nobody hates the damn things as much as I do, I still feel that an AAV is far superior to any configuration of an island vent or a loop vent both of which are prone to having the vent plug up and require stupid amounts of pipe and fittings.

    HJ, yes a trap does still need to be installed.
    [B]No, plumbing ain't rocket science. Unlike rocket science, plumbing requires a license[B]

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    We considered an AAV and yes, they are allowed in NC, but I don't like the idea of having to replace it every few years, but thanks for the suggestion. If the loop vent becomes a problem with clogging we can always switch to an AAV at a later time.
    Last edited by techinstructor; 09-02-2011 at 08:39 PM.

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    Quote Originally Posted by techinstructor View Post
    I had a chance to show the drawings to my plumbing inspector. He said that the "foot vent" and the vent riser would have to be "wet," i.e. also a drain from another fixture. Fortunately, since HJ had explained the yolk vent physics, I was able to suggest that I simply eliminate that part of the system since it was redundant anyway. He said that would be fine. WOO HOO. That made the job so much easier. Since I'm eliminating the riser, we're going to add a cleanout on the downside of the vent, just in case it's ever needed. This is what it will look like:



    The other thing he reminded me about concerning the rest of my plan was to NOT have more than 4 45-degree bends without a cleanout. I had to "redesign" some of my fixture drains to eliminate some bends, but as we're starting to dry fit this together, it's all working out well and the design has actually improved. He also clarified the definition of an S-trap. According to him, as long as the trap arm length is a minimum of 2 times the pipe diameter before it turns from horizontal to vertical, then it is NOT an S-trap. (Both this inspector and another one who I consulted earlier had recommended not putting a vent on the tub drain since would be adequately vented from the lav vent. I had struggled with how to intersect the drain with the horizontal branch without creating an S-trap. So now that issue is solved)

    Just wanted to again express my thanks for the advice on this board. It has been most helpful.

    Unless the horizontal drain is oversized then you will have problems with that configuration. As drawn in the picture there is no way for the line to vent properly, that is the purpose of the foot vent. Depending on the height of the vertical drain the hydraulic jump at the point of connection between vertical and horizontal will cause problems.
    It is absurd to suggest that the foot vent needs to be wet. That inspector is not familiar with that type of island venting. Here in Hawaii they will not let us use an island vent unless the sink is a true island. If it locates on any type of wall then we need to vent it in the traditional manner, boxing it out if necessary. Although eliminating the foot vent will save you on materials, it does serve a vital role in the operation of the sink drain. Additionally all the wye fittings under the floor have to be 2", only the vertical vent above the top combo can be 11/2, according to the UPC. Another point worth mentioning is another difference between your two drawings. In post #6 the drain is coming off the wrong location, it has to be after the vent fitting as it is drawn in the other examples.

    Just as a matter of general info the purpose of a yoke vent is to prevent excessive pressures from building up in stacks over 10 stories high. Unless your hose is at least 10 stories high you wold never have a yoke vent.
    Last edited by Winslow; 09-02-2011 at 10:05 PM.

  14. #14
    In the Trades Tom Sawyer's Avatar
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    How will you know if your loop vent is plugged?

    Do you plan on cleaning it out say 3 or 4 times a year?

    A qualtiy AAV should last a good 10 years or so and takes a couple of minutes to replace.

    With an AAV you won't have half of your cabinet full of pipes.

    An AAV will cost less than the pipe and fittings for an island vent.
    [B]No, plumbing ain't rocket science. Unlike rocket science, plumbing requires a license[B]

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    Moderator & Master Plumber hj's Avatar
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    1. Properly installed, there is almost NO reason for a loop vent to become plugged in the first place. The lateral vent COULD get plugged it the main drain were consistently getting plugged and backed up, but then that would indicate a deficiency in the plumbing ELSEWHER.
    2. I would hope a properly installed drain system would NOT plug up 3 or 4 times a year. AND it would have to happen much more than that to plug the loop vent even ONCE a year, or maybe once ever few years.
    3. If you cannot tell when a loop vent is plugged, HOW would you know when to replace an AAV?
    4. A "relief" vent stabilizes and eqializes pressures in multistory building.

    Last edited by Terry; 01-26-2012 at 05:27 PM.
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