Two toilets on 4" drain

Can I do this, and will it work properly? The (blue) drain is 4" pipe where it attaches to each closet flange and ties into the main stack which goes up through the roof. The black fitting is a combo-wye. There are no other vents (besides the main stack, obviously). The rightmost toilet is 6 feet from the main stack.

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Whether it is correct or not depends on your local code, but it is NOT a good installaton, because flushing one toilet could create a suction in the line compromising the other one's trap seal.

I've seen a home with a bath up and a bath down.
When the upper toilet is flushed, the water in the lower toilet moves.
That isn't normally the case when they have both been vented.
There are positive and negative forces when water drops down a pipe.

If that drawing shows it on the horizontal, I still don't like it.
No venting, and you have a stack from upstairs which will give you problems too.
Why no venting? If you have problems with that, which by the way I would never as a plumber have done, it's going to be a venting issue.
It's pretty easy to tell. install the toilets, flush them and see if the water stays in the bowl. If the water level in the opposing bowl is lowered after a flush, then it's not right.

Sounds like I need to add a vent in there just above the combo-wye, then.

Adding a vent won't make any difference. There is nothing really wrong with your lay out and if you are using 4" you won't have any problems.

question

I am hoping we can pursue this thread a bit in the spirit of learning more about how venting works and why. If I recall correctly, in previous threads about venting it was explained that the primary purpose of venting is to protect the trap seal. It would seem that this example offers a good illustration of a trap seal that would be compromised by one fixture flowing past another without a vent. HJ and Terry seem to support this and yet Tom, another seasoned veteran who has demonstrated a wealth of knowledge about venting states the introduction of another vent would 'make no difference'.

Tom, can you discuss this further? Why would an additional vent make no difference? Why would the present layout have no or little effect on the stability of the trap seal of the second toilet? Why would 4" lines improve the situation if no additional vent is introduced?

Thank you in advance!

Because at no time will a 4" pipe be totally full to the top of the pipe and therefore there is an air space that allows the drain to aspirate. Though we are all familiar with the water level bouncing in a toilet from time to time, it takes some pretty dramatic conditions to actually pull enough water from the trap to break the seal.

I thought I would follow up here just in the nature of better understanding the layout of my initial post. I have since completed it, with both bathrooms tiled and toilets installed, so I can describe how it's working.

The toilets are both Kohler 1.28 gpf models. The plumbing shown above the black combo wye above is 4" PVC. The plumbing shown below the black combo wye is 50-year old 4" cast iron.

Flushing the toilet on the left does not affect the toilet on the right.

Flushing the toilet on the right quickly by pressing the lever and releasing will bounce the water around in the toilet on the left. Flushing the toilet on the right by pressing the lever and holding it until it empties the tank, will siphon about 2/3 of the water from the toilet on the left. With 1/3 of the water left in the left toilet, any amount of continued flushing (long or short flushes) from the right toilet will move the water slightly, but will not continue to siphon it. The trap seal never is broken, but it will become lower after a big flush, as I just mentioned.

I would assume that practically, this is not a problem. If I had to do it again, I would have installed another vent for the rightmost toilet, because it's somewhat annoying to go #2 on a toilet with about 1 - 2 pints of water in the bowl.

When a toilet bowl isn't full to start with, it often doesn't flush properly the next time it is used.

Especially with a "low volume flush" toilet, if the bowl is NOT full when it is flushed, the first flush's water is used EXCLUSIVELY to refill the bowl. Then it has to be flushed a second time to do an actual flush. Your installation is a good example of a DIY job which followed faulty advice. BUT installing a vent between the combo and the right toilet would have eliminated the "bouncing" condition, but might NOT have eliminated the problem since it would still flow past the right toilet which DID NOT have a vent to prevent the situation.

Update:

Turns out both toilets are siphoning one another. Neither will siphon the other to where it breaks the trap seal, but to the extent where only the minimum amount of water necessary to maintain the seal is left. Flushing a toilet with such a small amount of water basically produces a lot of air bubbles and a weak flush.

How do I fix this right, guys? Saw a number of other threads on this forum where someone suggested that a toilet having a 2" or 2-1/2" output would never fill a 4" pipe to where it would siphon another fixture. Clearly, this is false, given my experimentation here.

HJ, I read your last reply, but I think you may have meant "...since it would still flow past the LEFT toilet which..."?

Thanks.

EDIT: If there is any other solution which wouldn't involve breaking up tile and concrete (another type of toilet, etc), please advise.

I have a 2 family home that has 2 toilets (back to back bathrooms on the second floor) connected within a foot or so to a 4" horizontal branch. There is NO vents for the toilets.......There is no problems and never has been. I have lived here for 40 years.....The 4" horizontal branch is about 5 feet long and the tubs and sinks also connect to it down stream of the toilets. Those fixtures are vented since I redid some of the plumbing myself. The main stack goes straight up and thru the roof. It drops to the basement and exits the house at about 3 feet above the basement floor level...

I have replaced some of the pipes and redid a lot in PVC where ever I could. Main stack is cast iron and in good shape.....Smaller drains for the sinks and tubs were/are galvanized......and I replaced anything I could get to.....

I have a new Toto Vespin on one side and it is the last fixture on that branch....Works perfect.....and does not cause any issues on the other side when it is flushed....As I said the toilet for the opposing side is about a foot down stream on the branch.....And it was plumbed using cast iron Sanitary tee type fittings.....not Y's........and that is also what was used where the vertical changes to horizontal and exits the building thru the block wall.......Plumbing in 1950 was not so good.....but it all works....

If I could I would draw a layout of what is there but I don't know how to do that and post it on here.....It would never be up to code under the rules used today....but it does work fine and always has.....