which is a correct practice in sanitary pipe routing in the following case as given in the image.

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Narasimha

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We are in the process of designing a waste pipeline and are seeking guidance on the best practice for connecting two WC units that are positioned back to back. Could you provide insight or recommendations for this scenario?
need advice.png
 

Jeff H Young

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one on the right is better with 2 seperate wyes the left picture would get you called out by inspector most likely for having flat branches it might be prohibited to have the doulble wye on side less than 45 degrees not sure on that , but if its at 1/4 inch per ft fall then branches would be an absolute fail
 

Tuttles Revenge

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Neither of those drawings take venting into consideration.

A double WYE on its back is gernerally not allowed as Jeff mentions because while flat and even with grade, each branch enters the main line without proper grade unless another 45 is added to offset this. Which is not how this drawing is drawn.

The second drawing allows each fixture to enter the main line with proper slope at each point of the fittings.
 

Jeff H Young

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I think Wayne calculated out how much fall the wye would need in order to provide fall for the 2 "flat branches" to have a minimum of 1/4 inch per foot. But I have no idea if it would actually be legal because could be some other technicality prohibiting use of a double wye in horizontal fasion, I kinda think it should be prohibited but uncertain . I avoid it like the plague.
In a nutshell I dont think anyone experianced really likes the left side drawing as being better
 

Tuttles Revenge

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Our counties chief inspector issued an official "Opinion" on double wyes on their back. Basically the opinion was that if the run is graded (which gives the inlets some slight grade as Wayne calculates) and if street 45's are inserted to aim perpendicularly to the run, and with a twist, they will create enough slope to offset the very slight section of flat section in the fitting as to be negligible.

I honestly don't think I've ever installed it, but definitely would if it gave me an advantage on a job.
 

Jeff H Young

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I dont know why or where a street 45 figures in to this and why any fitting would be required between the wye and the closet bend other than to reach desired location
 

Reach4

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I dont know why or where a street 45 figures in to this and why any fitting would be required between the wye and the closet bend other than to reach desired location
They are trying to add some slope as soon as possible.
 

Jeff H Young

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Without adequate venting it is not a working system.
good catch it will need venting, but thought he was asking if the drainage works Id say only under certain conditions would the left side drainage be compliant 1/4 inch per ft on the main and double wye wont cut it.
 

wwhitney

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I think Wayne calculated out how much fall the wye would need in order to provide fall for the 2 "flat branches" to have a minimum of 1/4 inch per foot.
If the barrel is pitched, and you achieve the best case that the two branch inlets of the double wye are at exactly the same elevation, then those branch inlets are never completely "flat". Their pitch will be 1/sqrt(2) = 70.7% of the pitch of the barrel. So if you want their pitch to be at least 1/4" per foot, you need to pitch the barrel at least sqrt(2)/4 = 0.3535" per foot.

Of course, getting the two branch inlets at exactly the same elevation is tricky; if you're half a degree off on the barrel rotation, one inlet is more pitched, the other inlet less pitched. So it's still delicate, and pitching the barrel a little more would provide a margin of error.

I think the point about putting street elbows in the branch inlets is that you can now rotate that joint to ensure that coming into the elbow you have at least 1/4" per foot slope. So if you were slightly off on the double wye's pitch or rotation, the error would be just for the length of the branch inlet portion of the double wye fitting.

Cheers, Wayne
 

Jeff H Young

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If the barrel is pitched, and you achieve the best case that the two branch inlets of the double wye are at exactly the same elevation, then those branch inlets are never completely "flat". Their pitch will be 1/sqrt(2) = 70.7% of the pitch of the barrel. So if you want their pitch to be at least 1/4" per foot, you need to pitch the barrel at least sqrt(2)/4 = 0.3535" per foot.

Of course, getting the two branch inlets at exactly the same elevation is tricky; if you're half a degree off on the barrel rotation, one inlet is more pitched, the other inlet less pitched. So it's still delicate, and pitching the barrel a little more would provide a margin of error.

I think the point about putting street elbows in the branch inlets is that you can now rotate that joint to ensure that coming into the elbow you have at least 1/4" per foot slope. So if you were slightly off on the double wye's pitch or rotation, the error would be just for the length of the branch inlet portion of the double wye fitting.

Cheers, Wayne
thanks wayne I guess i remember correctly laying the wye flat with minmum fall clearly would not comply with enough fall for the branches . im getting a little technical because Ive had inspectors pick apart the smallest of things unless youve got gobs of fall exceeding the (absolute mathamatical minimum) by giving it over 3/8 " to 1/2 inch per foot on the barrel Id see it as a possible/ likely fail .
yea if you have short pups glued in the wye and they have back fall its a fail but putting a street 45 or 22.5 might cause an optical delusion and get passed inspector or we could just plumb it right and not worry always allow for fall and you never get in trouble
 
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