Having a similar problem here... Customer had two 1.6 GPF TOTO toilets installed back-back, replacing two 3.5 gpfs that worked fine. The new TOTOs siphoned each other and when not in use, lost large amounts of water in their bowls. TOTO condemned the use of the "double estabrook" which is a fitting used in back-back installations for years. TOTO suggested re-piping using double wyes. I'm a Master plumber with 30+ years experience and that solution would have been costly and seemed a bit far-fetched... We got up on the roof and determined that the 3" vent stack/stack vent was clear, no obstructions. The toilets flushed fine as did a 3.5 gpf in the basement. The main drain was clear and the septic tank was recently pumped. The customer decided to change toilets. We removed one of the TOTOs & installed a Kohler... Same thing happened. Occasionally the bowls drain overnight...When thew bowls go low sewer gas enters the dwelling... We have to figure this out-- In Massachusetts a double estabrook is used 90% of the time in back-back installations. Double Wyes are not legal and cannot be part of a venting technique called "stack venting"... I'm beginning to agree with TOTO that in back-back installations the new style toilets will not work properly when piped using an estabrook or double TY. This is going to be a major problem going forward for many homes that have back-back baths on second floors. It is not impossible to change the piping, just time consuming and expensive. Going forward the codes may need to be changed regarding back-back toilet rough-ins...

Vinnie Sharon Plumbing & Heating Norton, MA

Hi Vinnie and welcome to the Plumbing pageat at AskMeHelpDesk.com. You piggybacked on an a 3 year old dead thread so I gave you one of your own. In the future, before you post, look in the upper left hand corner for the date.
Since you're a MA. Resident I'll ley Massplumber answer this one. Regards, Tom

HI Vinnie...

You said, "We removed one of the TOTOs & installed a Kohler... Same thing happened"... so the issue doesn't seem to be the Toto toilet itself, right... Or did I miss something?

Have you talked to Jimmy or Kevin over at Republic Plumbing Supply (Toto distributer @ Norwood branch) to see what they are hearing on this? I know I haven't had any such issues with any of my back to back installations... Hmmm, curious, for sure!

And, just like you, I'm having trouble with this statement, "Occasionally the bowls drain overnight"... odd stuff you've presented!

Hey! I'm glad to chat back and forth on this... check with my local people if you like, but I'm having trouble blaming the toilets here, you know?

Let me know more, OK?

Mark

In Ohio using double Y or double TY is not legal because vent comes off lower than openings receiving discharge from toilets. In Ohio you must use a cross with vent opening higher than openings receiving discharge. I have used this installation many times with Kohler or Toto toilets without a problem. In Mass. This sounds like a standard rough in without a problem so it doesn't sound like this is the problem if this is SOP in Mass. But under this circumstance the vent maybe causing the problem or IF there is a fixture discharging above the double Y is pulling a trap.This may just be a new problem using 1.6 toilets which seem to have more of a blow out effect than a flush. I have 1.6 in my house back to back with no problem roughed in on a cross. Good luck with this problem and would be interested in hearing the solution.

I echo Puffmugs's comments. Code-wise, same applies in California: we have to use Cross-San-T for back to back installations.

Just thought I might chime in... Milo

In Massachusetts when piping back-back bathrooms we are allowed to use a double sanitary tee which a lot of us wrongfully call a double TY, or a double Estabrook fitting which is a double sanitary tee with up to two two-inch inlets which enter the fitting at the same level as the 3". I've seen other configurations that were passed by inspectors using two sanitary tees on their backs in between two 90 degree elbows, one at the top of the waste stack and the other used for the second closet bend. One of the sanitary tees picks up the toilet closest to the first 90 and the second one becomes the continuation of the 3" stack. I've always questioned this type of installation, but I've seen it done.

With the code required 1.6 GPF toilets the back-back piping used with 3.5 gpf toilets may not be adequate, at least TOTO believes it is not. The above described situation would have me believe they could be onto something...

Similar to what has been said by the plumber in Ohio "In Ohio using double Y or double TY is not legal because the vent comes off lower than openings receiving discharge from toilets-" we in Massachusetts enforce the same engineering concepts in our code. It appears though, our codes may have been written with 3.5-5.0 gpf toilets in mind and now we may be forced to rethink our back-back piping requirements with the newer 1.6 gpf toilets and now the 1.28s.

What are the options? Re-piping back-to-backs may be necessary if we find the standard 1.6 toilets installed back-back malfunction, allowing the introduction of sewer gas to the dwelling. It could end up being a race to see which manufacturer can guarantee a toilets performance in a back-back installations using double Estabrooks or double sanitary tees. But- if retooling is not in the future perhaps the use of another fitting may prove advisable. There are double Wyes with inlets center of the fitting and also combination Wye & 1/8 bends with inlets in their branches. It would depend on their ability to prevent trap seals from being lost due to siphonage or back pressure. All drainage waste and vent (DWV) piping is designed and installed to prevent just that- the loss of traps seals and the admittance of sewer gas to the dwelling. As a licensed plumber I think I can speak for all of us when I say "We're doing a pretty good job. You don't hear of it."

I'm going to keep making calls after the holidays, hopefully getting the help of a mechanical engineer in the process. It just may take a qualified engineer or a member of the MA State Plumbing Board to get the right answer...

Hi Vinnie:
I have looked up the Estrabrook fitting that you are using and in Ohio, at least that would not be a legal fitting, but I see how it would serve the purpose you are using it for. If the top of the cross is used for a vent with nothing discharging above it, and the 2" openings are used as vents (which seems like overkill) there is no reason to lose a trap seal, but if the 2" openings are used as waste I see a possibility of pulling a trap seal, but since this is something that you have never had a problem before with I don't see this happening and the 1.6 toilet seems to be the culpit. Like you said this sounds like something for a engineer to solve. Would be very interested in hearing the solution. Thanks.