Back to Back Toilet with Double Fixture Still Stealing Water

[chihotdog13]

My home had cast iron 3" with back to back toilet. It used a 4 way cross, and I don't remember water being stolen with new toilets. PVC was upgraded and a 4 way cross installed, but noticed water was being stolen. I've read many posts in this forum on this top, and learned that a double fixture fitting was needed to help with that. Once has since been installed, but now, the toilets are still stealing water. The horizontal length from center of flange to the main stack is 17" and the other is 16". The vent is clear because some of that cast iron was replaced with PVC as well. Also, both toilets go from 4" to 3" with a closet elbow, then straight horizontal into stack, stack being 3".

I'm guessing the only way to fix this would be to separate the two, so that you stack a Long Turn Wye on top of the another long turn wye. I don't think my issue is water shooting up the other side, but the siphon air action stealing water.

Instead, would it be possible to place another two long 90 bends in one side, so that the air takes more time to reach the toilet?

Current: vertical 4" > closet elbow (4" reduce to 3") > 3" horizontal into double fixture tee

Proposed: vertical 4" > close elbow > short 3" horizontal > 90 long turn > 3" short vertical > 90 long turn > 3" horizontal into double fixture tee

Thanks

 

[Terry]

For back to back, a double wye fitting.
A double fixture fitting is not enough to prevent the water from skipping over the fitting.
No double santee
No double fixture fitting
Yes on a double wye with 45's

 

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[chihotdog13]

So I read a bunch of posts on here but there seemed to be conflicting info on whether to use a double fixture or double wye.
I appreciate the quick response!

 

[Terry]

So I read a bunch of posts on here but there seemed to be conflicting info on whether to use a double fixture or double wye.
I appreciate the quick response!

My video above is with the double fixture fitting. It makes the bowls lose water. I first noticed that happening in the 90's.
I had one toilet pulled and the toilet in the other bathroom was flushed. I'm there watching water rush into the side I'm on. That rush of water going up the other arm is pushing water up in the bowl, causing the water to slosh over the top of the high bend. Each time the other toilet is flushed, it raises the water in the other bowl, and each time that happens, water sloshes out of the bowl.

Both Kohler and TOTO say to use a "DOUBLE WYE"

With the correct fittings, it won't happen.

 

[chihotdog13]

Ok the I just have misunderstood all the posts. I appreciate the help thank you! This forum and you are very knowledgeable. Is it possible to sticky this or another post for back to back installation?

 

[Jeff H Young]

Whats wiered is kohler and toto got thier toilets to get a upc stamp even though they dont perform properly on a legaly plumbed home.
Im not totaly up on code but has upc code evolved to require the double why configuration on back to back.?
Based on practice I would never plumb back to back w double fix. fitting though I used to all the time it was normal just wondering if its a fail on inspection ? I think any approved w/c should work on aprooved plumbing rough in.

 

[Reach4]

Whats wiered is kohler and toto got thier toilets to get a upc stamp even though they dont perform properly on a legaly plumbed home.

I am thinking that there is a UPC provision that you have to follow the manufacturers' directions, and those directions say to use double wye for back-to-back if using those toilets.

 

[Jeff H Young]

I am thinking that there is a UPC provision that you have to follow the manufacturers' directions, and those directions say to use double wye for back-to-back if using those toilets.

When you rough in a toilet you follow plumbing code shouldnt need to build a house for every darn specific toilet . if a toilet wont work with standard plumbing codes it shouldnt pass upc and get a stamp in my opinion. that said Im not sure on code on this matter if its been changed just to satisfy quirky poor manufacture desighns that dont work properly. it seems you shouldnt change codes to satisfy manufacture but with the low fush requirements etc perhaps no one can build a toilet that works properly without reppiping the waste system

 

[Jeff H Young]

basicaly if I buy a toilet I should bust open walls and check to see how the house is plumbed?

 

[chihotdog13]

So then, is the alternative to back to back (if you have the room) is to stack either WYE's w/ 1/8" bend (Single), or a combo tee (single) on top of each other?

 

[chihotdog13]

basicaly if I buy a toilet I should bust open walls and check to see how the house is plumbed?

yea that is kinda dumb tbh