Toilet directly on top of soil stack - Can I use this DWV fitting? (4" wye w/ 2" side inlet)

[nelson_rg]

Hello. I'm a DIYer working on an old house bathroom renovation project. The attached image/drawing is my current plan (sorry, some of the spatial relationships are a bit wonky on my isometric drawing). Any big no-nos happening here? My main hang up right now is wondering if I can use this fitting below the toilet with the wye and the side inlet (despite the price of it... $117... ugh). There’s another version of this fitting with a tee branch instead of the wye (which I see referred to in many ways: cottage tee, Wisconsin tee, apartment tee, compact tee, bungalow tee, etc), but I feel like a tee so close to the toilet isn’t ideal. So this fitting with the wye branch (which will connect to the main vent) seemed like the answer.

Other info to know:
–The toilet sits directly on top of the soil stack.
–All current pipes and connections are the original cast iron. The only cast iron I will keep is the soil stack below the toilet and the DWV fitting. Above that point, I will plan to use PVC.

Thank you for taking a look! I appreciate any help/feedback that I can get.

 

[wwhitney]

That side inlet is definitely not a drainage pattern, so you can not do what you show.

But, you can easily use wet venting, with this pattern starting at the lav:

2x2x1-1/2 san-tee for the lav ; 2" pipe and fittings for the lav drain to enter the floor; 2" horizontal combo to receive the tub drain ; 2" horizontal pipe ; 3x3x2 combo with barrel vertical for the WC.

The 2" vent on top of the first san-tee would go through the roof. The tub trap arm is from the tub trap to the 2" horizontal combo; if the trap and trap arm are 1-1/2", it is limited to 42" in run and 1-1/2" in fall; if they are 2", it is limited 60" of run and 2" of fall.

Now, the UPC (which MN uses) also requires the total vent area through the roof to be at least equal in area to the minimum building drain size. So depending on what is going on elsewhere, you can abandon the 4" vent through the roof, or you may still need a 3" or (unlikely) 4" vent through the roof. How many WCs in the entire house, and what other vents through the roof?

Cheers, Wayne

 

[Sponsor]

 

[nelson_rg]

Thanks for the reply, Wayne!

Got it, yeah I had wondered about that side inlet since it kinda looks like there's a sharp ledge on the inside and not a gradually sloped down surface as you'd like to see for drainage.

Let me see if I'm following what you're saying:
On the 3x3x2 combo wye (like this? Or I might go with the 4x4x2 since I'm connecting to the 4" cast iron below), the 2" combo wye connecting in to the 2" pipes and fittings provides enough air space to vent the toilet and wet vent the sink + tub/shower. That said, in my drawing I can eliminate those 4" fittings behind the toilet that connect back to the 4" vent. The 2" vent coming out the top of the lav san-tee now becomes the solo vent outlet for the bathroom and the 4" vertical vent in my drawing can also be eliminated. Is that right?

Elsewhere in the house:
–There is 1 WC on the 1st floor (the bathroom described above is on the 2nd floor)
–The 2" vent for the 1st floor WC routes in another part of the house, comes up in the attic, and then connects in to the current 4" cast iron vent just before the 4" pipe goes out the roof.
–There is just 1 vent going out the roof for this house.

If I could reduce that vent exiting the roof to 3" with the bathroom vent below being 2", that would be nice since the current 4" vent is not very friendly to my framing plans. In this scenario, I can just install a 2" to 3" pipe increaser up in the attic, correct?

Thank you!

 

[Reach4]

https://www.revisor.mn.gov/rules/4714.0906/ says

906.7 Frost or Snow Closure. Vent terminals shall be not less than 2 inches (50 mm) in diameter and shall not be smaller than the required vent pipe. Any change in diameter shall be made inside the building not less than 12 inches (305 mm) below the roof in an insulated space and terminate not less than 12 inches (305 mm) above the roof.

 

[nelson_rg]

Good to know—thanks for this info @Reach4 !
I will be able to make that 2" to 3" transition on the vent pipe more than 12" from the roof and in an insulated space.

 

[wwhitney]

Let me see if I'm following what you're saying:

I marked up your drawing below. Blue is 1-1/2", Green is 2", Orange is 3". The tub trap and trap arm could be increased to 2" if you need the extra length allowance that affords, or if you just like the idea of a 2" tub trap.

Elsewhere in the house:
–There is 1 WC on the 1st floor (the bathroom described above is on the 2nd floor)
–The 2" vent for the 1st floor WC routes in another part of the house, comes up in the attic, and then connects in to the current 4" cast iron vent just before the 4" pipe goes out the roof.
–There is just 1 vent going out the roof for this house.

Surely there is at least one other vent that combines with the 4" vent, such as for a kitchen sink or a clothes washer?

If so, then combining the two 2" vents (one from each WC) and the 3rd vent into a single 3" roof penetration suffices to satisfy UPC 904.1, as you have only two WCs, so the minimum size building sewer is 3".

Chapter 9 Vents: Vents, 2020 Minnesota Plumbing Code | UpCodes

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I don't think combining just two 2" vents into a 3" roof penetration complies, as the area of a 3" vent is 2.25 times the aggregate area of two 2" vents.

Cheers, Wayne

 

[nelson_rg]

Thank you for the mark-up—very helpful! And all understood.

Ah yes, sorry. The third vent comes from the basement utility sink, catches the kitchen sink on the way through the 1st floor, and then links up over the bathroom fixture vents before the merge to the stack vent.

I appreciate the help and pointers @wwhitney !