Single to double vanity conversion plumbing

[Kmpcfp]

I am planning to switch out a single sink vanity to a double sink vanity. I need help with the drain/plumbing layout. This is the existing set up with the single sink.

The new sink will be added on to the left. The two sink drains are speced to be 29 1/8" apart. The wall behind the vanity is a load bearing wall, so I would like to avoid messing with that. The existing 1 1/2" drain line comes from a wall to the right. Ideally I would like to have the drain line up against the rear of the cabinet for storage considerations, also with the ptrap going more toward the rear of the cabinet.

What is the "best" way to install, and/or are there any other considerations I should keep in mind? Does each sink need its own p-trap or can I put a wye/tee above the existing p-trap for the new drain to share the existing p trap? Would the existing venting behind the walls be sufficient? The house was built in '94 - assumedly to code back then. Thanks.

 

[WorthFlorida]

Cut out the 45* elbow and replace it with a WYE. It might be prudent to install an AAV at the new sink since the location of the vent may not be known. Since there are two separate sinks a p-trap is needed for each. This trap shown is for a kitchen sink, 1.5" but it might be larger. Bathroom sink traps are 1.25". Use p traps that are all slip nut with the adapter glued on at the drain. Using slip nuts it can easily be taken apart with a blockage ever had to be snaked.

To get the sink p trap to go more toward the rear you'll need the wye on the right with the long sweep or add a 1/8 elbow for the one on the left. You must use wye's since it is a horizontal drain. At the new sink use a long sweep elbow to the drain.

 

[Sponsor]

 

[Kmpcfp]

Cut out the 45* elbow and replace it with a WYE. It might be prudent to install an AAV at the new sink since the location of the vent may not be known. Since there are two separate sinks a p-trap is needed for each. This trap shown is for a kitchen sink, 1.5" but it might be larger. Bathroom sink traps are 1.25". Use p traps that are all slip nut with the adapter glued on at the drain. Using slip nuts it can easily be taken apart with a blockage ever had to be snaked.



To get the sink p trap to go more toward the rear you'll need the wye on the right with the long sweep or add a 1/8 elbow for the one on the left. You must use wye's since it is a horizontal drain. At the new sink use a long sweep elbow to the drain.

Thank you, photos were very helpful. The main vent stack is in the attic near where the new sink will be. I could potentially run a vertical vent line between the studs in the wall behind the sink and connect it to the main vent stack without much of a hassle if that would be better than using an AAV.

 

[wwhitney]

The two sink drains are spec'ed to be 29 1/8" apart.

That means you do have the option to use a single trap (as it's under 30"), but doing so would make it harder to meet your other goal of keeping the plumbing against the back wall.

Do you know what happens to the pipe after it exits your cabinets? If it immediately hits a san-tee in the sidewall, that would be good. When you cut the pipe, you should be able to shine a light down the open end to see if the first hidden fitting is an elbow or a san-tee.

IPC (Maryland's plumbing code) allows two fixtures drains to join and then be vented downstream via a common vent. And it allows up to 6' for a 1-1/2" trap arm. So if you cut the existing pipe to the right of the 45, and verify there's an immediate san-tee in the wall, you can add a coupling, a pipe segment, a flat combo + trap adapter for the right sink, another pipe segment, and a long turn 90 + trap adapter for the left hand sink. Then the sink tail pieces, traps, and first trap arm segments would be tubular. All the horizontal pipe needs to be at 1/4" per foot slope, and the distance from the left hand trap outlet to the inferred san-tee in the wall has to be under 6'.

So if you can't verify an immediate san-tee in the wall, you need to figure out what the piping geometry is to the existing vent take off to determine if the above plan would comply with the 6' trap arm limitation on the left hand sink trap. If not, you'll need to add a vent, either a san-tee directly in the wall, with a dry vent running to your existing vent piping, or an AAV in the cabinet.

Cheers, Wayne

 

[Kmpcfp]

Thanks! I have a sneaking suspicion that the existing vent is somehow shared already with the sink in the bathroom on the other side of that back wall. Are 3 sinks within 6' permissable to share a 1-1/2" line? There are also 2 shower/tub drains in that same corner as well. I am not sure how they are tied into the vent either. The 1/2 bath is also right beneath this bathroom. This is the only venting that is in the attic. The wall that these both go down into is the wall that is to the right of the existing sink.

That means you do have the option to use a single trap (as it's under 30"), but doing so would make it harder to meet your other goal of keeping the plumbing against the back wall.

Do you know what happens to the pipe after it exits your cabinets? If it immediately hits a san-tee in the sidewall, that would be good. When you cut the pipe, you should be able to shine a light down the open end to see if the first hidden fitting is an elbow or a san-tee.

IPC (Maryland's plumbing code) allows two fixtures drains to join and then be vented downstream via a common vent. And it allows up to 6' for a 1-1/2" trap arm. So if you cut the existing pipe to the right of the 45, and verify there's an immediate san-tee in the wall, you can add a coupling, a pipe segment, a flat combo + trap adapter for the right sink, another pipe segment, and a long turn 90 + trap adapter for the left hand sink. Then the sink tail pieces, traps, and first trap arm segments would be tubular. All the horizontal pipe needs to be at 1/4" per foot slope, and the distance from the left hand trap outlet to the inferred san-tee in the wall has to be under 6'.

So if you can't verify an immediate san-tee in the wall, you need to figure out what the piping geometry is to the existing vent take off to determine if the above plan would comply with the 6' trap arm limitation on the left hand sink trap. If not, you'll need to add a vent, either a san-tee directly in the wall, with a dry vent running to your existing vent piping, or an AAV in the cabinet.

Cheers, Wayne

 

[wwhitney]

So two things:

1) I believe under the IPC you could vent your whole house with a single 1-1/2" vent (although if you have too many WCs, then the minimum pipe size may bump up to 4", which would make the minimum vent size 2". I forget the details in the IPC on number of WCs). Your picture shows a 2" vent and 1-1/2" vent tying into it. So presumably some of the fixtures are on the 1-1/2" vent, and some are on the 2" vent.

2) There is one venting wrinkle to your plan to change one lav to two lavs. Namely, if the current lav is wet venting one or two showers/tubs and/or one or two WCs, then the current minimum allowable drain size from the lav san-tee to the point where the lav drain meets the first wet vented fixture drain is 1-1/2"; and beyond that point the minimum drain/wet vent size is 2", even if the first fixture is just a 1-1/2" bathtub drain. If you add a second lav with a second trap, then the drain for the two lavs from the san-tee to the first downstream fixture would have to be 2". Basically a wet vent with 2 or more fixtures has to be at least 2".

So if the lav is currently wet venting another fixture, and if the drain serving it is 1-1/2" and not 2", then you are supposed to enlarge the drain to 2" if you add a second lav with a second trap.

Cheers, Wayne