Double wye/18 bend requirement for back-to-back Toto installation

[Gleno]

I have replaced an existing off-brand low-flow toilet with a Toto 654 MF dual flush model. The reason for the replacement has been the tendency of the existing toilet to clog. The opposing toilet does not have the clog tendency. The closet bend and main soil stack are clear of soils. However, my drain snake returned with a wax residue that could only have come from the opposite toilet wax gasket. This indicates to me that I have a double sanitary T/ sanitary cross for the opposing toilet's junction with the main soil stack.

The installation instructions of several models of Toto toilets state that the double sanitary T fitting is not suited for opposing toilets. Rather, they require a Double Wye/1/8 bend .

My first question: While I can easily understand the preference of the Double Wye under most circumstances, is there hope for a workable installation here without a rebuild of the existing soil stack?

All plumbing in this building is concealed behind finished walls. The bathrooms in question are on the second floor, and the joist system is 2x10 material, which should offer 9-1/2" of vertical location space for a Wye retrofit. The building was built in the 60's, and the DVW system is all copper pipe. At the roof, the vent pipe is 4" material.

My second question: Is a retrofit remotely possible by access from below?

I have a vision of cutting away the existing Sanitary Tee, mission clamping a short section containing the Double Wye and probably two new closet bend lengths, all below the floor level. If the Sanitary Tee configuration exists, the clogging issue is not going away anytime soon.

Would an Old School toilet be a possibility here?

 

[Terry]

1960's with copper DWV is most likely a sancross

The toilets will work, but they may push water from the opposing bowl when flushed. Is that an issue for you now?

 

[Sponsor]

 

[Reach4]

I would just try it. The main problem I have read is that the quick pressure change sloshes some water out of the toilet on the other side. I am not a plumber.

If you make a change, there is something between a double wye and a double sanitary tee: a double fixture fitting.

How would it work to block one port of the existing double sanitary tee, and pipe the new toilet to a wye below the existing double sanitary tee? Maybe use a good 45 degree output closet flange.

Remember before cutting, you should support the pipe. A riser clamp may help with that.

 

[Jadnashua]

Newer toilets try to make do with less water, so they are designed to use techniques to speed up the outlet waste. They generally either do that with a siphon jet or pressure assist. That creates enough velocity to literally suck the bowl dry to evacuate the waste. Older, higher volume toilets flushed much slower, so the fitting requirements weren't as strict.

Worst case, it flushes some waste into the other toilet (probably a rare situation), but moving the water in the opposing toilet is fairly common, and it can cause the level to drop as it rocks the bowl water...some sloshes over the weir that holds it in. A toilet that is not properly filled may not flush well when next used.