Vent reroute for single to double sink conversion

[Travis45]

Hi everyone,

I am looking to convert a single sink to a double, and need some advice after opening up the wall to assess the situation. Turns out there is a 2" vent right in the way of where I want to run the left sink drain. My plan now is to reroute it with 90s (or 45s to ensure it could drain rainwater) , to the left in between the next studs.
My main question is, is it acceptable to reduce this 2" to a 1 1/2", just for the reroute section or does that violate code? The reason for the reduction is because there is also a stud pair clocked 90° to the left of the next stud and there is not much room for the vent reroute. I will have to notch out that stud a bit, but it does not appear load bearing, but just a "frame" around a furnace vent, goes just up to the ceiling and not to the roof.
Also, can the drain on the left be installed without an upper portion venting up and tieing in to the main vent? In other words, just tieing into the original vent section?

I've attached a photo to show the layout, and a sketch to show what I'm thinking as an option (note" the blue tape shows where the 1.5" vent ties into the 2"vent) . I'd really like to get some feedback, as I have done some minor plumbing diy but nothing like this (yet). Had a plumber come and quote me $6k for this, just roughed in. That is well out of my budget, so either I tackle this myself OR I just buy a new single vanity and try to sell/return the double I already have sitting in the garage....

Note: This is an upstairs bathroom, nothing above it. I beleive the 2" vent connects to the half bath on first floor.

Thanks!

 

[John Gayewski]

You can't reduce the section you want to re route, but you can re route it as drawn.

You don't need the vent your have drawn on the right.

Does the 1.5 inch pipe your using as a drain change to 2"at some point? The problem. Is size wise a 1.5"pipe can take both vanities as a vertical drain, but when it changes to horizontal it should upsize to 2" to be technically correct.

The fitting you need to use to change to a double lav drain should be a double fixture fitting.

 

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[Travis45]

Thanks for the tips. Good to know that the extra vent loop isn't required, I watched a few videos online and some didn't have that but some did, so I was unsure what was technically correct. Less work, works for me

As for the current drain I am tapping into, all I see is 1 1/2" unfortunately. It's possible it turns into 2" below the floor somewhere, but I have no idea. So worst case scenario, I assume it's 1 1/2 and how bad is that? Is it just a code violation or is it risky?

I'd like to use the correct double fixture fitting, but I don't think they make it in a 1 1/2 (vertical), I just see 2" and up. Would a 1 1/2" double sanitary tee be horrible to use instead? Seems like the main disadvantage is not being able to snake downward, after reading up in this thread: https://terrylove.com/forums/index....xture-tee-or-double-sanitary-tee.22539/page-2

Alternatively, how much worse would it be to use the 2" double fixture tee but reduce to 1 1/2?
Is one of these to "not so great" options better than the other?

 

[John Gayewski]

A double tee would be almost impossible to clean out. You could make up for it by having a cleanout accessible on the vertical pipe.

If i were you I wouldn't be afraid to use the 1.5"pipe to drain both lavs. Just be sure your lavs are 1.0gpm or less.

 

[John Gayewski]

Instead of doing all of the work to re route that 2" I think i might just come through the wall at a 45° angle right next to the 2". Then line up with the drain outside of the wall, but inside of the cabinet where it won't be seen.