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Question DetailsAsked on 2/2/2018

What toilet brands work best in a back-to-back toilet installation situation?

3rd floor condo; the soil stack is a shared standard T pipe...not a wye pipe fitting.

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1 Answer


I can't see that the type of toilet makes any difference because the same waste is coming down the line, except maybe with a macerating toilet - which has a built-in sewage grinder pump, but as far as I know those are always pressurized systems, with the toilet shredding the waste and then pumping it, under pressure, up to an above-ground sewage pipe to discharge into. Used in areas where the toilet is below sewer level or sewer is susceptible to backup from outside, and they do not want to put in a separate sewage grinding lift pump.

I guess a pressurised flush toilet (which uses air pressure or a pump to supplement the flushing power) would work better, but they also tend to be noisy and to have catastrophic overflow events if the toilet plugs up, and with your plumbing I can see a distinct possibility of it blasting your waste right through the Tee and out the neighbor's toilet - not a particularly neighborly thing to do. Plus toilet cost is around $500-1000 range for a run-of-the-mill one as opposed to a normal $100-350 range for a normal gravity toilet, and they can be more of a maintenance hassle.

For your case, two toilets should NOT feed into a standard Tee like this - whether the outlet stack or branch pipe runs "horizontal" (actually typically at a 2% downhill slope) or vertical from the center of the Tee.

As you apparently have found up, flushing one toilet can cause solids to go into and partly block the outleet of the other toilet, and can even cause partial backup of the waste into the other toilet.

Of course, if this is new constrution then the plumber should correct it at no cost - and I would suggest having the rest of the plumbing checked for oddball things too, and maybe ask the building inspector for a special reinspection of the plumbing because of it.

The alternatives (pay no attention to sizes shown, I just picked clear photos of the of fitting - diameter per code, usually 3 inch coming from toilets):

1) not allowed in many areas for toilet lines or in others allowed only if the outlet goes straight downward from the Tee, is a sanitary Tee where as you can see (with some models) the internal passages from the side openings of the Tee both curve towards the outlet opening with solid plastic blocking direct flow from side to side - preventing direct flow from one side of the Tee to the other side. Generally used for cleanout use only, with the "Tee" opening position on the top side. A true cleanout tee has direct flow passage between the two opposed openings (the through-going route), plus the curved sections from the Tee opening to allow a snake or router or jetting tool to go around the bends into the main pipe more easily - from the outside they can look the same, you have to look inside to see if the opposing openings have a straight-through passage or not.

2) For tight conditions, a double sanitary tee like below is normally used - again, not allowed in some areas if the outlet is basically horizontal as is common in a bathroom seetup with basin and tub/shower also coming into the stack branchline before it goes into the vertical stack. The "top" opening in this photo would be used for a cleanout plug if accessible, or capped off if not, or used for inlet of flow from a basin or tub/shower if the toilets are not the upstream most fixtures on that branch, with the "downflow" direction being in the direction the curved parts go - downward in this photo. Can be used on "horizontal" branch or vertical stack.

3) If there is room, I prefer to use separate wyes from each toilet, independently branching into the branchline or stack about a foot or more apart, with this type of fitting, which has a "street" or "sweep" or "long turn" entry into the pipe to promote smooth flow and reduce tendency to blockage. Can be used on "horizontal" branch or vertical stack.

4) an improvement over a normal sanitary tee, if space allows but separate wyes are not feasible due to restricted available space or (like in your case) would be too expensive to access and put in, is a double street or sweep sanitary tee like this -

5) it is always a good idea, and mandated by code in some areas, to have the wye located so it is coming in on the "top" of a "horizontal" branch line, so the liquid is entering from the top rather than the side of the line - promotes clearing out of the solids, and prevents backup and accumulation of soilds coming from "upstream" in theopening or arm of the wye or tee. Especially in situations where there are two toilets feeding into the same branch line - every effort should be made to make the entries from the "top" of the line, not the side - meaning the branch line would be in the subfloor or under a slab in most cases.

Obviously, Plumbing is the Search the List category for this type of work, to find well-rated and reviewed contractors to do the work. Or if you want the plumbing, and any floor/ceiling/wall opening up and repair/repainting work to be under one contractor, then a Remodeling - Kitchen and Bath contractor would be the category.

Repair charge - depends of course on howmuch has to be opened up to replace the fitting if the pipe is not exposed, whether or not the fitting were close-coupled so adjacent fittings have to be rerplaced too or the hubs reamed out to allow glueing in the new pipe or if there is enough straight pipe between them to allow cutting and glueing a new fitting on, how tight the work confines are, and the repair cost to close up the access hole. But I would be very surprised to see it at less than $300 or so (generally only if fully exposed in an open garage or open between-the-joists situation) for the plumber and probably more in the $400-600 range ifthe pipes are in floors/walls - possibly more if he has to deal with incoming flow from upstream or 4th floor + water sources flowing through that line.

Plus typically $250-600 range for handyman (or with textured finishes, a drywall contractor) to repair drywall, then get it repainted.

And of course, the work and payment would have to be coordinated with the neighbor, and possibly with a condo association or manager because you are undoubtedly talking about the piping being joint responsibility property.

Good Luck

Answered 2 years ago by LCD

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