Venting question while converting from single to double vanity.

[Dgeist]

Greetings, all. I'm converting my kids' bathroom from a single to double vanity. The DWV stack is CI in the left corner of the room. Above is vent to roof only. The current lav drain is centered in the wall and is 1.5" copper soldered to a 1.25" brass trap at the fixture drain. The wier is JUST below the downward bend on the stack, so it drains, but there's not much room to work moving it any farther away from the drain.

My thought is to simply cut out the whole shebang back to the CI tee and put in a larger 2" PVC horizontal drain arm to the new drain stubs spaced equally in the double vanity. I'm concerned about backflow between the sink traps with this setup if I depend on simple slope to clear the grey water. Am I overthinking that?

I can alternately install separate horizontal arms for each sink and have one of the flanges come out of the wall a bit lower OR just make the slope a bit steeper and pull a dedicated vent vertical up near the far drain stub and bring that up and over to the CI stack above the lav's spill line.

Thoughts?

See attached photo for current state. Second photo is an example of the dedicated vent I'm talking about.

 

[Reach4]

Second photo is an example of the dedicated vent I'm talking about.

Is that your stuff, or an example from somebody else? I see what I think are two weird things in that picture.

I marked those as A and B in the attached picture. I threw some more letters in, in case you wanted to reference something else.

 

[Sponsor]

 

[Dgeist]

@Reach4 , sorry for not being clearer. What you labeled as "G" is what I'm talking about adding in the single-to-double vanity photo in the first post (so I can increase the horizontal slope and still provide airflow). I picked a photo with a lot more going on...

The "weird things" you're talking about is actually the top of the vent from a basement bathroom group beneath that was added a couple years back. It ties in the vents for pieces in that bath through walls and some soffit to be able to snake up there. No other way to vent the basement other than ripping apart a ton of walls above it. I would have done that little bit with 4 elbows a bit differently, but ironically, the pro did that part while I was at lunch. That stack has nothing above but air. The 4" CI Vertical to PVC to the far left has a toilet above it, so it was no-bueno for venting. I would have replaced ALL the CI, but there was a little bit that I had to leave until I demo the kitchen behind it. Couldn't reach it all to get it out. It's strange looking but it works and passed inspection.

Back to the original question with the lav stubs and one stack: Can I get by with gravity feed alone as long as the diameter and slope allow air to the weir or should I plan on putting in an additional dry vent like in the "weird things" photo?

 

[wwhitney]

If you're going to cut into the cast iron, consider the option shown here (as modified in the subsequent discussion):

Single to Double Sink - Vent to Right

Cheers, Wayne

 

[Dgeist]

@wwhitney , thanks for the link. I hadn't considered incorporating a vertical drop on the individual lavs but that's a good idea that'll keep the "flow-by" from possibly impeding the downstream sink. I may be able to re-use the stud holes and just raise the drain holes a few inches (need to measure) to get the orientation right. I assume the 2 dry vents just need to be above the sink level (or more correctly a few inches above it)?

After some trial and error, I have found which sawzall blades work well on CI in previous projects.

 

[wwhitney]

Vents can go horizontal or be combined at 6" above the flood rim level of any associated fixtures.

Cheers, Wayne

 

[Tuttles Revenge]

Some things to consider first before determining that you can use the existing stud holes. What height is your cabinetry and counter top? How deep of sinks? and what style of sink stopper - Lift rod pop up or spring loaded clicker. Each of those will determine whether you can convert your existing height drain to work with the example provided in the link above. As you can see, each of those drains stub outs is now Above the horizontal drain. Your sinks need to be able to accommodate that height.. or conversely your drain needs to accommodate your design for the sinks when finished. You can see in that provided link photo, that the new horizontal drain has been lowered to maintain the same height of finished stub out.

 

[Dgeist]

@Tuttles Revenge, thanks. I'm in conversation with the cabinet maker and countertop folks to answer those very questions. Ironically the existing single drain is perfectly centered and right at 18" from finished floor (and working great for 50 years). If only my family wanted there to be just one sink...

 

[wwhitney]

@Tuttles Revenge, thanks. I'm in conversation with the cabinet maker and countertop folks to answer those very questions. Ironically the existing single drain is perfectly centered and right at 18" from finished floor (and working great for 50 years). If only my family wanted there to be just one sink...

If it's 30" or less between the two sink basin drains, you could plumb them with a single trap, like a double kitchen sink is plumbed.

Cheers, Wayne

 

[Reach4]

If it's 30" or less between the two sink basin drains, you could plumb them with a single trap, like a double kitchen sink is plumbed.

I don't find lavatories with left-side and right-side drain variants. Somebody is missing a market maybe.

 

[Dgeist]

Just wanted to drop a note and say thanks for the input on my project. After finalizing the dimensions on the vanity, I was able to knock out the stubs this weekend. Here's a before and after:

 

[Terry]

From vertical to horizontal, long turn 90, wyes, or combos.
A combo is basically a wye with a 45.

 

[Dgeist]

From vertical to horizontal, long turn 90, wyes, or combos.
A combo is basically a wye with a 45.

Thanks, Terry. That was my one (hopefully forgivable) sin. I couldn't source one with a sweep with the right dimensions that would fit in tight quarters without completely hacking up the wall. I know it's not technically approved, but since it is a lav and the two 1.5" drains expand into a 2" in that fitting, and that the sink on the right has a decent slope and will flush the the one to the left, I think it'll be alright. After seeing the hairballs and sludge in the old 1.5" copper, it almost HAS to be better than that.