Venting a double vanity - converting from single

See attached picture.

I have a single drain pipe that was used for a single vanity sink. I am in process of installing a double vanity. How do I go about adding a second drain line. I am concerned that if I just run a line through the wall to the new drain loaction that when one sink drains it will suck the water from the trap of the other.

1-1/2 PVC is the existing drain.

I am unsure what the drain and vent set-up is as it is under the floor. All I have easy access to is the single drain pipe. (I could access the floor by cutting the subfloor as I am installing a new floor).

The white drawing is my proposed plan to tee off of the existing drain (keep a pitch) and add a trap. But, I think I will have a vacum problem with the drains as there is no vent after the traps.

Any suggestions/drawings would be appreciated.

The way that drain is plumbed, it will siphon the trap anyway.

Where is the vent here?

You could run another line over for the second lav, but either way, they should have vents.

If you can't run a vent through the roof, at least add a AAV (Studor Vent) after the p-trap.

draiin

There is no way to do that properly with your present piping because the second sink will always have to drain past the first one. A secondary vent between the two drain openings will help, but the proper way would be to lower the horizontal pipe so each drain connection could drop into it.

Thanks for the reply. I do not want to vent through the roof, so I will need to go with the AAV option.

Does it matter where I put the AAV with regard to each trap? Do I need just (1) AAV?

One AAV could be used if the two lavs are on either side, using a fixture cross.

If like hj mentions, coming up from below for each lav, then you would use two AAV's.

The tee for the AAV must be at the level of the trap arm.

https://ipscorp.com/plumbing/brands/studor/

drain

Terry said:

One AAV could be used if the two lavs are on either side, using a fixture cross.

If like hj mentions, coming up from below for each lav, then you would use two AAV's.

The tee for the AAV must be at the level of the trap arm.

http://www.studor.com/DesignCriteria.pdf

Click to expand...

He could also use one central drain riser with a double fixture cross and two drain arms and a single AAV, but it would still require that the horizontal line be lowered.