I have just completed installing a new dual sink vanity in an upstairs bathroom. I opended up the wall behind the vanity to redo the plumbing as it was formerly a one sink vanity. My problem now is the sinks drain slowly (with hot and/or cold opened up the bowls gradually fill). It doesn't make any difference if one or both sinks are running. If I completely remove the stoppers from the drains both sinks will drain perfectly. I have verified there are no obstructions at any point in the drain pipes. My suspicion is, it is either a venting problem or perhaps the stoppers simply do not allow the water to pass efficiently enough (American Standard faucets and drain stoppers).

Drain plumbing design is as follows: I removed all the old drain plumbing from the single sink. Using the original 1 1/2" drain pipe in the wall I attached a "w" shaped pvc wye pipe to run pipe to each sink drain and the vent (vent goes straight up to attic from this location, I have verified it is unobstructed). The left and right openings on the wye are at 45 degree angles to the center opening. The wye (and vent) in the wall is located equally between the two sinks. To get the drain pipe from the wye to each sink there is about 16" of pvc pipe (going up from the wye at a 45 degree angle) then a 90 degree elbow to get through the wall into the vanity, then a couple inches of straight pipe in the vanity, and then the p-trap. All pvc pipe is 1 1/2"; the American Standard provided drain pipe connected to the sink is 1 1/4" (used a reducer in the p-trap to connect the drain and the p-trap).

As noted above, with the sink stoppers completely removed, the sinks drain to perfection. Is this possibly a venting issue and with the stoppers fully removed the drains are in effect getting additional "venting" from the sinks themselves? Or is it possible the shape of the sink bowls in combination with the way the stoppers fit just doesn't allow for efficient enough draining (vanity top is nothing special, just an off the shelf dual bowl vanity top from Home Depot)?

Other info - the old single sink drained without issue; the bathtub and toilet in this bathroom drain without issue. The sink, toilet, and shower in the adjacent upstairs bathroom drain without issue.

Thank you in advance for any suggestions.

Well Skorch, Yes, This sounds like a venting problem.
Lots of information but not what we needed to know. The type of lavatories that you installed. Were they regular lavatories or were they vessel sinks.
Bye the way, you went to a lot on extra work to drain the two. All you could have done is install a contentious waste, either center or end outlet, (see images). And used the old trap. Regards, tom

Regular sinks, not vessels; under vanity top mounted. Agreed, drain plumbing was definitely more complicated than need be, but using the existing trap/plumbing wasn't an option due to the vanity used.

Do your lavatories have overflow holes? Is there any gurgling as the sinks drain? Are the stoppers set up high enough for good drainage? Ifc you go to drain one lavatory and pull the stopper out on the other one will it began to drain normally?
Back to you, Tom

1. No overflow holes in sinks
2. No gurgling, can hear water draining through pipes, but no discernable gurgling
3. Stoppers are not adjustable, but they do not appear to be lower/higher than in my other bathroom sinks. I did just try a different stopper/drain pipe from another bathroom (also an American Standard product) and had same slow draining problem.
4. stopper in or out has no impact on draining of other sink (positive or negative)

Is it possible the venting is too low because the wye is about 12 inches below the trap and horizontal pipe as opposed to at the same height? I could redo the 90 degree elbows coming through the wall and add additional venting at this point (at same height as horizontal pipe and trap) if it would help.

Quote:

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Stoppers are not adjustable, but they do not appear to be lower/higher than in my other bathroom sinks. I did just try a different stopper/drain pipe from another bathroom (also an American Standard product) and had same slow draining problem

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What do you mean
"stoppers are not adjustable? Aren't your pop-up like this one? (see image) The holes in the clevis or the clevis screw will adjust the height of the stopper.
Let me know. Yes, this is a venting and air lock problem. The lack of overflow holes doesn't help either. Try this. Fill a bowl a pop the drain. Now ttake a object and begain to slooooly lift the stopper. When does the bowl began to drain normally? Regards, Tom