Bathtub dry vent

[Canada_DIY_Plumber]

I've been asking a few questions and have greatly appreciated the pointers. I'm putting in a bathtub on a concrete slab. The bathtub will have a dedicated dry vent (not part of a wet vent system.) I'm having a hard time figuring out how to get this vent into the plumbing wall given the rule that I am not supposed to have any horizontal sections of vent until I get above the overflow level of the tub. I tie into the sewer line at the far end of the tub so my preference is to avoid a 90 degree bend if I can avoid it.

Here are two options. Blue is drain, green is vent, black is outline of tub area, and grey is outline of plumbing wall.

The red option gives me a nice straight drain run from the tub drain to where I enter the main sewer line. However, since the vent taps in at the far end of the p-trap, I have a height challenge to get it back to the plumbing wall without having a horizontal section of vent pipe.
In this youtube video, at 4:45, it appears that they have a horizontal section of vent pipe to deal with this issue. Is that allowed?

The purple option solves that issue by setting the p-trap parallel to the plumbing wall. However, then I have to make a 90 degree turn (and some additional turns further down the drain) to meet up with the sewer which seems suboptimal.

Any thoughts on which would be preferred (or another option entirely)?

 

[wwhitney]

Note that with a p-trap, the outlet elbow doesn't have to be in the same plane as the u-bend, you have a degree of freedom there. So we could call the red option 6/6 as seen from above in your drawing, the u-bend is at 6-oclock, and the outlet elbow is also at 6-oclock. And the purple option is 9/9.

Have you looked at 11/6 or 1/6? The u bend would be pointed (almost) towards the plumbing wall, but then the outlet elbow would point away from it towards the sewer. [12/6 would cross itself.] Then if you use a wye for the vent takeoff immediately after the outlet elbow, does that configuration get the wye close enough to the wall so that the vent can rise into it without going horizontal?

You could also consider something like 12/7:30 and using a 45 degree elbow after the wye (better than the 90 used in the purple). Also, bear in mind that you can roll the wye off upright if necessary, up to 45 degrees is generally OK; at a roll of 45 degrees off upright, you'd need a 60 to go vertical in the wall.

Cheers, Wayne

 

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[Canada_DIY_Plumber]

Thanks. That makes sense. I will have to mock it up in the pit tonight. If I use a 2" trap, I will need to make sure my vent comes off at least 4" after the trap, right? I've always been a bit confused about that. I get the need for a developed trap arm that is at least twice the trap diameter when you are dropping into a vertical drain (to prevent it becoming an s-trap) but it seems like that would be less applicable when the vent is coming up off a long section of horizontal drain.

 

[wwhitney]

Yes, but the 4" is measured horizontally from the downstream vertical inside wall of the u-bend to the point on the wye where the branch inlet opening first breaks the inside wall of the barrel. So I would think that you may only need 1" or so of pipe showing between the trap outlet elbow hub and the wye inlet hub; with the pieces in hand, you can measure what's required.

Also, the vent only needs to be 1-1/2", and a tub is allowed to be on a 1-1/2" drain. So if it were critical to making things fit, you could use 1-1/2" fittings on the drain path. It could still be useful to upsize the drain to 2" just after the wye.

All my comments are based on US codes, but I don't expect any difference from Canada, not sure.

One more option just in case it helps: if you have the available elevation, in your purple drawing, the 3 way joint with the two blue (drain lines) and green (vent) line, that fitting could be a san-tee with the barrel at 45 degrees off plumb and the side inlet horizontal for the trap arm. The top inlet of the san-tee would be pointed towards the wall for the vent to rise up at a 45 into the wall. And the outlet of the san-tee would hit a 45 for the drain to go back horizontal. If minimizing the elevation loss is important, you could put a street 45 into the outlet of the san-tee (or use a street san-tee into a regular 45).

Cheers, Wayne