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Oswald Tay

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Hi, I am relatively new to plumbing, and I am simply wondering it the design in the picture below is acceptable in terms of venting and wet venting? I am renovating an old basement half bathroom.
The drain for the sink is already taken care of, but I am just not sure about the drainage and venting when adding a shower to an existing toilet drain pipe. The piping will be under the concrete slab which I will cut into.

Could someone please clarify if this plan is acceptable, if it is not, what should I change?
I live in New York State, and this is the code about wet venting:

912.1 Horizontal Wet Vent Permitted
Any combination of fixtures within two bathroom groups located on the same floor level is permitted to be vented by a horizontal wet vent. The wet vent shall be considered to be the vent for the fixtures and shall extend from the connection of the dry vent along the direction of the flow in the drain pipe to the most downstream fixture drain connection to the horizontal branch drain. Each wet-vented fixture drain shall connect independently to the horizontal wet vent. Only the fixtures within the bathroom groups shall connect to the wet-vented horizontal branch drain. Any additional fixtures shall discharge downstream of the horizontal wet vent.
[BATHROOM GROUP. A group of fixtures consisting of a water closet, lavatory, bathtub or shower, including or excluding a bidet, an emergency floor drain or both. Such fixtures are located together on the same floor level.]
[HORIZONTAL BRANCH DRAIN. A drainage branch pipe extending laterally from a soil or waste stack or building drain, with or without vertical sections or branches, that receives the discharge from two or more fixture drains or branches and conducts the discharge to the soil or waste stack or to the building drain.]

Plumbing design.jpg
Plumbing design.png
 

Reach4

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IPC does not like your horizontal dry vent. Instead, run the shower trap arm to the wall where you vent, and then turn and join the toilet, which gets wet-vented.
 

Oswald Tay

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IPC does not like your horizontal dry vent. Instead, run the shower trap arm to the wall where you vent, and then turn and join the toilet, which gets wet-vented.
Thank you!
I forgot to include the wall that will be between the shower and the toilet.
If I'm understanding your reply correctly, would the following design be acceptable then?
 

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Reach4

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Yes. You would want to make the connection with the toilet 3 inch with a wye. I think you want 3 inch between the vent and the toilet soil pipe, because a wet vent should be up-sized above the 2 inch that a shower drain would need if it was not wet venting something. And I think you want a 2-inch vent.

I am thinking the venting could be via a 3x3x2 sanitary tee on its back, and a bushing on the 3-inch inlet matching the 2-inch from the shower trap. I am not a plumber.
 
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Oswald Tay

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So, I have a 3" pipe which is the drain for the toilet, a 3" to 2" wye off the toilet pipe, then the shower drain and the vent going up the wall are all 2" pipes.
And you are saying that the vent in the wall must be at least 3" away from toilet drain pipe.
Is all that correct?
 

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This is my idea, and I don't have room to have a vent for the sink tie into the main stack, so I would just use a mini-vent, like the example below:

Plumbing design (2).png


minivent-sample-2.jpg
 
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Reach4

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Ok, thank you.
Also, if I wanted to actually tie in a sink into this drainage system under the floor, could I just add it in with a wye off the 3" toilet pipe, and a mini-vent device at the trap?
Adding a bathroom sink (lavatory) can simplify it. The lavatory vent can be the only vent.

Use blue in wall sloping down at least 1/4"/ft, or pink which drops into the floor via long sweep.

I don't know if cleanouts are needed, but one under the lavatory seems wise.
 

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wwhitney

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I think you want 3 inch between the vent and the toilet soil pipe, because a wet vent should be up-sized above the 2 inch that a shower drain would need if it was not wet venting something.
That's not correct. Both the IPC and UPC allow a 2" wet vent to carry 4 DFUs of drainage, versus the 6 or 8 DFUs (respectively) for a 2" horizontal drain that is not a wet vent.

Cheers, Wayne
 

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That's not correct. Both the IPC and UPC allow a 2" wet vent to carry 4 DFUs of drainage, versus the 6 or 8 DFUs (respectively) for a 2" horizontal drain that is not a wet vent.

Cheers, Wayne
https://wabo.memberclicks.net/assets/pdfs/Plumbing_Venting_Brochure_2018.pdf says
What is a horizontal wet vent? A horizontal wet vent is a horizontal branch drain pipe that has been increased in size larger than what is normally required by Section 710. This increase in pipe size allows for a large air space above the maximum probable waste flow level in the pipe so as to provide for adequate venting airflow in the same pipe as the waste flow.​
My thinking is that a shower drain must be at least 2 inch. To upsize 2 inch would call for 3 inch. No?
 

wwhitney

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Well, as it happens, the IPC allows a 1.5" drain for a shower up to 5.7 gpm of shower heads, in which case a 2" shower drain is already upsized (but a good idea, and the minimum UPC size for a shower drain, and the minimum IPC size for a 2-4 DFU wet vent).

But if you read the various sections, the only one that refers to upsizing is the UPC's section on vertical wet vents. Otherwise, the rule is that the allowable DFUs through the wet vent are less than for a drain only. Sometimes that results in having to bump up one size; sometimes it doesn't.

If the shower in the OP's second post is 5.7 gpm to 12.3 gpm, then the IPC considers it 3 DFUs and requires a minimum 2" drain size. But as that is under the 4 DFU limit on a 2" wet vent, it can be a wet vent without upsizing.

Cheers, Wayne
 
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Oswald Tay

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Well, as it happens, the IPC allows a 1.5" drain for a shower up to 5.7 gpm of shower heads, in which case a 2" shower drain is already upsized (but a good idea, and the minimum UPC size for a shower drain, and the minimum IPC size for a 2-4 DFU wet vent).

But if you read the various sections, the only one that refers to upsizing is the UPC's section on vertical wet vents. Otherwise, the rule is that the allowable DFUs through the wet vent are less than for a drain only. Sometimes that results in have to bump up one size; sometimes it doesn't.

If the shower in the OP's second post is 5.7 gpm to 12.3 gpm, then the IPC considers it 3 DFUs and requires a minimum 2" drain size. But as that is under the 4 DFU limit on a 2" wet vent, it can be a wet vent without upsizing.

Cheers, Wayne

Thank you for all the advice!
The shower that I will be buying says it's around 1.75 gpm.
But I want to be as safe as possible with the drainage system, so do you think I can have the shower drain and the wet vent from the lavatory all be 2"?
Or is there a maximum on the pipe size?
Also, I am assuming the dry vent that ties back into the main vent stack (which goes out the roof) is a 2" pipe, correct?
 

wwhitney

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If your drain connectivity is "single lav drain joins shower, lav/shower drain joins WC," and you are under the IPC, and you want an AAV under the lav to dry vent the lav and wet vent the shower and WC (which is allowed if you have an atmospheric vent elsewhere), then the minimum pipe sizes would be:

lav drain to lav/shower wye: 1.5"
shower drain to lav/shower wye: 1.5"
lav/shower wye to WC wye: 2"
WC and downstream: 3"

But making the shower drain 2" seems like a good idea to me. Upsizing the lav drain to 2" I'm neutral on, but I would disfavor drilling alot of 2x4 studs to make that happen.

In your plan with a dry vent pulled off the shower drain in the wall between the WC and shower, which wet vents the WC, the minimum sizes are 1.5" for the shower drain, 1.5" for the vent, and 2" from the vent takeoff to the WC wye. But again I'd make the shower drain 2"; the vent can stay 1.5". The lav trap still needs a separate vent, but the drain can stay 1.5" and it can connect in anywhere.

Cheers, Wayne
 

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The big construction help, vs what I was thinking, is the wye into the toilet can be done with a 3x3x2 wye.
 

Oswald Tay

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If your drain connectivity is "single lav drain joins shower, lav/shower drain joins WC," and you are under the IPC, and you want an AAV under the lav to dry vent the lav and wet vent the shower and WC (which is allowed if you have an atmospheric vent elsewhere), then the minimum pipe sizes would be:

lav drain to lav/shower wye: 1.5"
shower drain to lav/shower wye: 1.5"
lav/shower wye to WC wye: 2"
WC and downstream: 3"

But making the shower drain 2" seems like a good idea to me. Upsizing the lav drain to 2" I'm neutral on, but I would disfavor drilling alot of 2x4 studs to make that happen.

In your plan with a dry vent pulled off the shower drain in the wall between the WC and shower, which wet vents the WC, the minimum sizes are 1.5" for the shower drain, 1.5" for the vent, and 2" from the vent takeoff to the WC wye. But again I'd make the shower drain 2"; the vent can stay 1.5". The lav trap still needs a separate vent, but the drain can stay 1.5" and it can connect in anywhere.

Cheers, Wayne

This is extremely helpful!
So you are saying that I could use an AAV, and that would act as a vent for the entire bathroom group?
I also have the ability to do a 2" vent that ties back into the main stack, but of course that would be a lot of hole drilling...
Which option do you think is better?

Another thing, what is required in terms of sloping for the lav drain. That will be in the wall, then go into the floor to connect to lav/shower wye.
Is there a requirement as to what angle the lav drain has to come into the shower drain, because it is acting as a wet vent?
 

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The big construction help, vs what I was thinking, is the wye into the toilet can be done with a 3x3x2 wye.

I am also wondering what material I should use for the system, is PVC ok (I see a lot of people using it for plumbing under the concrete slab), or is ABS better?
Again, I'm new to plumbing...
 

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Which option do you think is better?
Better as in work for years and not give odors? Real vent. Better as in easier? AAV usually.

Trap arm slopes 1/4 inch per foot until the vent. After the vent, it can slope at that slope or more -- even vertical.
 

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Better as in work for years and not give odors? Real vent. Better as in easier? AAV usually.

Trap arm slopes 1/4 inch per foot until the vent. After the vent, it can slope at that slope or more -- even vertical.

Thank you.
The sketch below is standing facing the lavatory.
If I understand you correctly, the blue pipe going down to the wye in the shower drain can be any angle even vertical, correct?
But the small bit of p-trap arm that connects at the light blue circle via a sanitary tee has to be sloped 1/4" per foot, correct?
Untitled drawing (1).png
 

wwhitney

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Yes.

To clarify, the trap arm (from trap outlet to vent connection) needs to be sloped at least 1/4" per foot, but also may fall no more than one pipe diameter. That's to avoid the possibility of siphoning the trap. So you could slope the trap arm at more than 1/4" per foot, but that reduces how far you can go before hitting the one pipe diameter drop limit.

Cheers, Wayne
 
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