Need help with back to wall plumbing configuration plz.

[Erick Didonato]

Pictured is our kids bathroom which we are going removing the old toilet flange to install a back to wall toilet with a behind the wall water tank for that toilet from Swiss Madison. Currently the bay where our vent stack is where the water tank for toilet needs to be. I have access to remove the roof stack to go in the next bay over though my question is how do I pipe this so that I can pick up the back to wall toilet in one bay , vanity sink drains in another bay all while taking the vent out of the next bay? Sorry for any confusion. Currently the pipe is 3” and my thought was to put a wye in the pipe in the floor joist and to go through one floor joist to pick the vanity sinks and reconnect to the stack which will be in that next bay though how do I vent the toilet? Thank you in advance for your help.

 

[wwhitney]

"Back to wall" is a new term to me, but it's plumbed like a wall hung toilet (rear discharge), it just sits on the floor like a regular toilet.

Your drain path for the toilet should all be in the current joist bay, you'll just have the appropriate 90 to mate with the toilet's rear discharge, then a LT90 to go horizontal into the joist bay.

Your photo shows two small diameter horizontal drains, what is the lower one? That's the one uncertainty in my suggestions.

Hopefully you can fit a 3" x 1-1/2" (or 2" if I am misjudging the size in the photo) san-tee between the two 90s on the toilet drain path. That would bring in the lavatory (and the other fixture) and wet vent the toilet. So the lavatory vent take off would be an upright combo in the stud bay to the right of the toilet. The vent could rise up in that stud bay and cross over to meet up with the existing vent. You don't need a full 3" vent under the IPC, so you can just keep the roof termination at 3" (good for cold climates, avoids having to work on the roof) and run the smaller vent to meet up with it at the top of the wall or in the attic.

Cheers, Wayne

 

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[Erick Didonato]

"Back to wall" is a new term to me, but it's plumbed like a wall hung toilet (rear discharge), it just sits on the floor like a regular toilet.

Your drain path for the toilet should all be in the current joist bay, you'll just have the appropriate 90 to mate with the toilet's rear discharge, then a LT90 to go horizontal into the joist bay.

Your photo shows two small diameter horizontal drains, what is the lower one? That's the one uncertainty in my suggestions.

Hopefully you can fit a 3" x 1-1/2" (or 2" if I am misjudging the size in the photo) san-tee between the two 90s on the toilet drain path. That would bring in the lavatory (and the other fixture) and wet vent the toilet. So the lavatory vent take off would be an upright combo in the stud bay to the right of the toilet. The vent could rise up in that stud bay and cross over to meet up with the existing vent. You don't need a full 3" vent under the IPC, so you can just keep the roof termination at 3" (good for cold climates, avoids having to work on the roof) and run the smaller vent to meet up with it at the top of the wall or in the attic.

Cheers, Wayne

Appreciate the feedback Wayne. I’m trying to understand what you described in regards to the 90 coming out of the toilet to the San-tee connection. Can you by any chance draw that out or explain that a little better please. The two 1 ½ pipes where from the double vanity sink.

 

[Tuttles Revenge]

He's talking about this style wall hung concealed tank toilet.

 

[wwhitney]

Yeah, so that lower elbow in the picture Tuttle was kind enough to post, if it's high enough off the floor that you can fit a 3" x 2" san-tee above the floor below the elbow, and if there's room to get a 2" pipe past the carrier (doubtful now that I look at it), then you can do that, with a LT 90 below the san-tee to turn into the joist bay and connect to the existing 3" drain. You could then do a layout like in the picture below, where green = 3", red = 2", blue = 1-1/2", (and brown lines = framing I erased). I probably have the double fixture fitting in the wrong stud bay but you can move it over to where you want. Likewise the upper horizontal red line (vent) can be any height at least 6" above the flood rim of the lavatories. (And the vent could be 1-1/2" under the IPC, although I drew it as 2".)

If you can't get around that metal carrier, then instead of using a 3x2 san-tee in the wall, you can use a 3x3x2 wye (or combo) horizontal (flat, 2% slope) in the joist bay and bring the lav drain in that way (and that might be preferable for other reasons, don't know). The 2" combined lav drain would go down into the floor system with a LT90 and then turn to drill through 1-3 joists to hit that 3x3x2 wye or combo.

Cheers, Wayne

 

[wwhitney]

if there's room to get a 2" pipe past the carrier (doubtful now that I look at it)

Although, with a back to wall toilet, there should be no need for a metal carrier, as the toilet just sits on the floor. So I'm unclear on what the in wall portion will look like.

Cheers, Wayne

 

[Terry]

With the carrier in the wall, you would wye off for the lavs and the venting.
A 3x2 wye would work in most cases, unless you really need a 3" vent up through the roof there.

 

[Erick Didonato]

With the carrier in the wall, you would wye off for the lavs and the venting.
A 3x2 wye would work in most cases, unless you really need a 3" vent up through the roof there.

Thank you so much for the replies gentlemen. Can I do the wye 3x2 horizontally in the floor joist to the vanity sinks and then up to roof sink? I do not have enough room below the toilet 90 to put a San-tee (suggested by Wayne prior).

 

[Terry]

You can go horizontally with a wye to the next joist bay, pick up the two bathroom sinks with 2" or larger, and that will also wet vent the toilet.
2" would be enough for the lav/wet vent portion.

 

[Erick Didonato]

You can go horizontally with a wye to the next joist bay, pick up the two bathroom sinks with 2" or larger, and that will also wet vent the toilet.
2" would be enough for the lav/wet vent portion.

Awesome thank you so much.