Back to Back Toilets: Double Fixture vs Double Wye

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SoConfused

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Hello - I know this subject has been covered numerous times on this forum and I have read and re-read the threads but am still confused. I am a VERY INexperienced DIYer. I had a cast iron drain pipe rusting through so I replaced all remaining cast iron with PVC. This is probably more than I should have taken on with no experience but I'm a widow with limited resources so I do whatever I can myself. I thought I had replaced everything with the same fittings but now when I flush one toilet water splashes up in the other toilet then siphons out. From the forums I understand that I have used the wrong fitting in the cross. (Double San-Tee) What I don't understand is whether I can use a double fixture fitting or if it has to be a double wye. I have read both answers on the forum.
I'm attaching before and after pics. As you can see I have very little vertical room to work with.
I don't know if it will be clear in the pics but the vent pipe is a 3-inch cast iron pipe that I've connected to the PVC with a fernco. The toilets are both Cadet 3 style, American Standard with 3-inch flush valves.
So should I use a Double Wye with 45s (built in, not street 45s attached) or a Double Fixture fitting??
I appreciate any help you can give me!

CastPipes.jpg
PVC Pipes.jpg
 

Terry

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A double wye, not a double santee, and not a double fixture fitting.
Or in your case, wye off the main line for one, and use a single santee where the double is and run it to "one" toilet.


anytimece-703-0-drainage-02.jpg


The compacted air pushes water out of the opposing toilet bowl, making the water in the bowl slop over the high bend and out.

back_to_back.jpg


back_to_back_kohler.jpg


wet_vent_upc_back_to_back.jpg
 
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Reach4

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From the forums I understand that I have used the wrong fitting in the cross. (Double San-Tee)
I presume the toilets feed the two side ports on the double santee.
What is the middle (top) port of that fitting doing?

If that were connected to a roof vent, even with your wrong fitting, I doubt that you see your symptom, because pressure could not build.

An AAV would not help.

Also, what is that pipe coming into the 3-inch on the left side of the picture? I am wondering if you could stop using (cap) the left port on your double santee, and instead join the flow with that toilet farther downstream.
 

SoConfused

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I presume the toilets feed the two side ports on the double santee.
What is the middle (top) port of that fitting doing?

If that were connected to a roof vent, even with your wrong fitting, I doubt that you see your symptom, because pressure could not build.

An AAV would not help.

Also, what is that pipe coming into the 3-inch on the left side of the picture? I am wondering if you could stop using (cap) the left port on your double santee, and instead join the flow with that toilet farther downstream.
The top part of the double santee is attached to a 3 inch roof vent. I've checked to be sure that vent is clear. The left side of the santee goes to one of the toilets. The toilets are not technically "back" to "back". They are just adjacent to each other and share a drain and vent.
 

SoConfused

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A double wye, not a double santee, and not a double fixture fitting.
Or in your case, wye off the main line for one, and use a single santee where the double is and run it to "one" toilet.

The compacted air pushes water out of the opposing toilet bowl, making the water in the bowl slop over the high bend and out.

If I Y off the main line for one of the toilets and eliminate the double santee, how does the toilet I Y'd off the main line get to the vent?
 

Terry

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If I Y off the main line for one of the toilets and eliminate the double santee, how does the toilet I Y'd off the main line get to the vent?

A toilet bowl is designed to siphon. The 3" line at max will be 1/3 to 1/2 full allowing air over the top of the water, allowing air to the vent.
A toilet uses a 2" vent, and you have 3" there.
 

wwhitney

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Looks like the fitting below the double san-tee is a quarter bend with low heel inlet? If so, that's akin to a san-tee, and is not be used with the low heel inlet horizontal, the low heel inlet has to be vertical. The fitting should be a combo.

What is the configuration upstream of that fitting? If it's a single dry vented bathroom fixture, then it could wet vent the righthand WC, which could drop straight down to a combo. The left WC would be vented via a single san-tee in place of the double san-tee (which could be a street san-tee if necessary, as a combo is taller than a quarter bend).

Otherwise, what's the vertical clearance between the two 3" horizontal lines on the left side of the double san-tee/quarter bend with inlet?

Cheers, Wayne
 

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Looks like the fitting below the double san-tee is a quarter bend with low heel inlet? If so, that's akin to a san-tee, and is not be used with the low heel inlet horizontal, the low heel inlet has to be vertical. The fitting should be a combo.
Cheers, Wayne

Right, the 3x2x3 is the wrong fitting there. It's a vertical only fitting. The poop lands straight down and goes both directions, blocking the uphill 2" line. I've had to cut those out before and it was disgusting.
The 2" line should come in with a wye downstream of a "long turn 90".
And yes, if the lav is dry vented, it could also be used to wet vent a toilet.
 

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Reach4
This has nothing to do with the venting. It's water skipping across the double fitting.
I noticed this in the 90's
TOTO and Kohler have it in their instructions. It's not the venting, it's the fitting, which needs to be a "Double Wye".
I now run them separately to prevent the air pushing the water out of the bowls.

Pushing, now repeat that three times. Pushing, Pushing, Pushing.
 

SoConfused

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A toilet bowl is designed to siphon. The 3" line at max will be 1/3 to 1/2 full allowing air over the top of the water, allowing air to the vent.
A toilet uses a 2" vent, and you have 3" there.
Are you saying I don't need to worry about venting the second toilet? I'm sorry for the questions but this is all very new to me.
Also, a friend's husband suggested leaving the double santee and Ying into the elbow below the toilet on the left and dropping it straight down to the drain line from there. Theoretically allowing it to still vent through the double santee but eliminating the pressure build up from the toilet on the right. Not sure if that makes any sense at all. He's more experienced than me but not a plumber.
 

Terry

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SoConfused,
I have to say though, the work you did looks really nice. And you understood what was happening to the water in the bowls using the new toilets with the 3" flush valves. You're way ahead of the game and I know you will get good results with the next tweaks. And the rest of us get to learn some new things.
 

SoConfused

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You look to have a shielded coupling above the double santee now.
I would drop the double off and use a single santee there.
Alot of the conversation is so far above my head that I don't understand it. I'm plenty intelligent but don't know plumbing concepts or terminology. But here's what I think I understand about what has been said so far.
I need to get rid of the double santee and replace it with a single santee coming from one toilet.
When I do that the fitting that goes from the single santee to the drain line needs to be a "wye downstream of a "long turn 90""
What would I do with the second toilet? Terry, when you said that it has a 3 inch line and only needs a 2 inch, are you saying I don't need to vent the second toilet?
 

Reach4

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I need to get rid of the double santee and replace it with a single santee coming from one toilet.
Or re-route one of the toilets so they are not sharing a path until later.

I am thinking that is what Terry was thinking in #11.
 

wwhitney

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If you tell us what is upstream on the 2" line, and the vertical clearance available between the upper and lower 3" lines on the left, then I'm happy to provide one or two drawings of the various options discussed.

Cheers, Wayne
 

SoConfused

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Thank you for your kind words and encouragement in #12 Terry! And thanks to all who have responded. I think I understand the concept of what I need to accomplish. But I'm still foggy on the venting question on the second toilet. I'm also wondering IF I could fit a double wye fitting where the double santee fitting is would that take care of the issue or am I still gambling? (I know I would still need to change out the fitting below the double wye) Seems like that would be fewer things for me to cut out and try to replumb.
 

Terry

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The area of a 2" pipe is 3.142 inches
The area of a 3" pipe is 7.069 inches
If you have half of the 3" pipe to vent with, that would be plenty. I can vent three bathrooms with a 2" pipe.

By the time you add a double wye, and then a long turn 90, it's too far down.
 

SoConfused

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If you tell us what is upstream on the 2" line, and the vertical clearance available between the upper and lower 3" lines on the left, then I'm happy to provide one or two drawings of the various options discussed.

Cheers, Wayne
Upstream on the 2 inch line are the rest of the drains in the house. Furthest to closest are washer, kitchen sink/dishwasher, tub & bathroom sink. Let me see if I can find the distance between the lower 3 inch lines without going under the house.
This pic shows the next drains upstream. I have since fixed where the tub drain had no fall. The only thing upstream of this is the kitchen/laundry.

PXL_20210622_013511962.jpg
 

SoConfused

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The area of a 2" pipe is 3.142 inches
The area of a 3" pipe is 7.069 inches
If you have half of the 3" pipe to vent with, that would be plenty. I can vent three bathrooms with a 2" pipe.

By the time you add a double wye, and then a long turn 90, it's too far down.
OK, so I don't have room to do the double the double Y and I do NOT need to worry about tying the second toilet into the vent. Correct?
 

wwhitney

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By the time you add a double wye, and then a long turn 90, it's too far down.
From the Charlotte catalog, if you have a 3" street 45, going into a 3" double wye (not available street), with a 3" street LT90 in the outlet, the center-line drop is 11.4", so you'd need to have 7.9" clear between the upper and lower 3" lines.

If you don't like the street fitting with the street end upstream, add 1.6" to end up at 9.5" clear between the two 3" lines. And to use a combo, you'd need another 3.5" clear, or 13" between the 3" lines.

Cheers, Wayne
 
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