New kitchen sink drain

[Marzec]

I will be installing a new kitchen drain in one of my apartments units. The 2" drain pipe in the photo connects to a 90 where the cabinet is cut out. Right after the 90 it is connected to the existing drain with a wet vent that is behind the wall and goes through the roof.

My plan is to simply cut down the 2" pipe and add a 2" 90. Then I will either add a 2" to 1 1/2" reducer or a flush bushing and add a 1 1/2" trap adapter. From there it's just installing the double sink drain, trap and trap arm to the trap adapter.

Another option is to use a 2"x2" x1 1/2" tee instead of a 90 and then install a clean out like in the second photo.

Please let me know if this is how a pro would do it.

**For clarification in case there was confusion, the second photo is not my install. I included it just to illustrate the possibility of adding a similar clean out.

 

[Reach4]

Another option is to use a 2"x2" x1 1/2" tee instead of a 90 and then install a clean out like in the second photo.

You need an AAV at least 4 inches above the trap arm. You can unscrew that AAV, and use the hole as a cleanout.

The trap arm has to be at least 3 inches long.

 

[Sponsor]

 

[Marzec]

You need an AAV at least 4 inches above the trap arm. You can unscrew that AAV, and use the hole as a cleanout.

The trap arm has to be at least 3 inches long.

As I mentioned, there is already a wet vent behind the wall for the kitchen sink drain.

 

[wwhitney]

As I mentioned, there is already a wet vent behind the wall for the kitchen sink drain.

The vent connection for a trap must be taken off within one pipe diameter of fall from the trap outlet. So with the trap raised up relative to what's in the wall, nothing in the wall can provide the required venting for the trap.

Cheers, Wayne

 

[Reach4]

As I mentioned, there is already a wet vent behind the wall for the kitchen sink drain.

It would be a dry vent.

How far above the floor does that drain pipe enter the wall? If your photo shows a shelf, and there is enough space between the shelf and the floor, you might be able to cut away some of the shelf, and put in a trap that uses that vent in the wall.

A plumbing pro may have added a new sanitary tee higher in the wall before installing the sink cabinet.

 

[Marzec]

The vent connection for a trap must be taken off within one pipe diameter of fall from the trap outlet. So with the trap raised up relative to what's in the wall, nothing in the wall can provide the required venting for the trap.

Cheers, Wayne

For clarification in case there was confusion, the second photo is not my install. I included it just to illustrate the possibility of adding a similar clean out.

The 90 in the first photo is basically directly connected to the 2" drain/vent stack in the wall. The vent runs up the wall and out of the roof. The 2" pipe in the photo will be cut down almost to the 90 so the trap outlet will be about where the word " type" is printed on that pipe.

For some reason in this unit the connection to the drain/vent stack in the wall seems a little lower than the other units. That's why the cabinet had to be cut out like that in the back.

 

[Marzec]

It would be a dry vent.

How far above the floor does that drain pipe enter the wall? If your photo shows a shelf, and there is enough space between the shelf and the floor, you might be able to cut away some of the shelf, and put in a trap that uses that vent in the wall.

A plumbing pro may have added a new sanitary tee higher in the wall before installing the sink cabinet.

For clarification in case there was confusion, the second photo is not my install. I included it just to illustrate the possibility of adding a similar clean out.

I'm referring to it as a wet vent because the drain/vent stack in the wall serves as both the drain and the vent for the kitchen sink. It is my understanding that is referred to as a "wet" vent.

The fitting at the bottom of the first photo is a 90 and is basically directly connected to the 2" drain/vent stack in the wall. So the drain enters the stack in the wall exactly right there where you see that fitting. The vent runs up the wall and out of the roof. The 2" pipe in the photo will be cut down almost to the 90 so the trap outlet will be about where the word " type" is printed on that pipe. Also, that is not a shelf, it is the bottom of the cabinet.

The drain/vent stack in the wall is not PVC. It is cast iron (drain section) and galvanized steel (vent section). It didn't really make sense at this time to get rid of all that in the wall and replace with PVC.

 

[wwhitney]

I'm referring to it as a wet vent because the drain/vent stack in the wall serves as both the drain and the vent for the kitchen sink. It is my understanding that is referred to as a "wet" vent.

No, a wet vent is a section of pipe that simultaneously serves as a drain for one fixture and a vent for another fixture. If you have a stack in the wall behind the sink, with a san-tee, and the stack above the san-tee serves as a dry vent, and the stack below serves as a drain, that's the normal way to do dry venting, and is not a wet vent. [Now if another kitchen sink drained into that stack at a slightly higher san-tee on the same stack, then the portion of the stack between the san-tees could be a vertical wet vent.]

The 2" pipe in the photo will be cut down almost to the 90 so the trap outlet will be about where the word " type" is printed on that pipe.

Still too much drop to use the dry vent in the wall, you are only allowed 1-1/2" of drop between the outlet of a 1-1/2" trap and the vent connection. If you leave the 90 in place, rather than having the trap arm go straight into the wall, you would need to add an AAV within the cabinet, before the trap arm turns down. So your options are to lower the p-trap and modify the cabinet as required, or use an AAV, or open up the wall and move the san-tee up.

Cheers, Wayne

 

[Reach4]

The drain/vent stack in the wall is not PVC. It is cast iron (drain section) and galvanized steel (vent section). It didn't really make sense at this time to get rid of all that in the wall and replace with PVC.

If you ever do cut into steel or cast iron, make sure the pipe above is clamped securely before cutting. A riser clamp and some carpentry is often appropriate to do the pipe clamping.

When cutting with a sawsall blade, you have to be really careful to not poke thru the other side of the wall as I did ... also for future reference.

Also, that is not a shelf, it is the bottom of the cabinet.

If that bottom-of-cabinet is just a few inches above floor, that is a weird place for a santee. Did you elevate the floor?

 

[Marzec]

No, a wet vent is a section of pipe that simultaneously serves as a drain for one fixture and a vent for another fixture. If you have a stack in the wall behind the sink, with a san-tee, and the stack above the san-tee serves as a dry vent, and the stack below serves as a drain, that's the normal way to do dry venting, and is not a wet vent. [Now if another kitchen sink drained into that stack at a slightly higher san-tee on the same stack, then the portion of the stack between the san-tees could be a vertical wet vent.]


Still too much drop to use the dry vent in the wall, you are only allowed 1-1/2" of drop between the outlet of a 1-1/2" trap and the vent connection. If you leave the 90 in place, rather than having the trap arm go straight into the wall, you would need to add an AAV within the cabinet, before the trap arm turns down. So your options are to lower the p-trap and modify the cabinet as required, or use an AAV, or open up the wall and move the san-tee up.

Cheers, Wayne

Interesting points regarding a wet vs dry vent. And thank you for the advice. I will go ahead and use an AAV for this install. Any advice specific to using an AAV? My understanding is that it should be up as high as possible in the cabinet. But is say 6" above the trap outlet sufficient?

 

[Marzec]

If you ever do cut into steel or cast iron, make sure the pipe above is clamped securely before cutting. A riser clamp and some carpentry is often appropriate to do the pipe clamping.

When cutting with a sawsall blade, you have to be really careful to not poke thru the other side of the wall as I did ... also for future reference.


If that bottom-of-cabinet is just a few inches above floor, that is a weird place for a santee. Did you elevate the floor?

Yes, I have had to secure both cast iron and galvanized vent stacks in some other units I have rehabbed. I have used blocking and some combination of metal strapping and clamping to secure them.

For galvanized pipe a sawzall blade works well and you definitely have to be careful. For cast iron I rent a pipe snapping/cutting tool for around $20. I've tried diamond sawzall blades and they don't work well. I haven't found a blade that works on cast iron. And it really wouldn't be any cheaper anyway. I have used a grinder with a cutting wheel in some situations.

I agree as far as it being an inconvenient place for the Santee (it's actually a double Santee because the adjacent unit also drains into it). My other similar units I have rehabbed are not like this. The Santee is a little higher up in the wall. The old cabinets might have sat a little lower. I honestly don't recall at this point as it's been quite a while since I did the demo. I didn't elevate the floor, but the old cabinets from the 60s may have sat lower in the back. The new cabinets may sit a little higher?...not exactly sure since this project has been going on a while. When I eventually do the adjacent unit I'll definitely be paying closer attention to this. If I could do it over I would have corrected it in the wall while I had it open. Didn't think of it.

 

[Reach4]

Any advice specific to using an AAV? My understanding is that it should be up as high as possible in the cabinet. But is say 6" above the trap outlet sufficient?

4 inches above the top of the trap arm is sufficient.

https://pdf.lowes.com/productdocuments/5ba8fc66-fcf7-47a9-8165-fa48230b197c/48106355.pdf

Also see Figure 4 on page 2 of 3 of https://www.iccsafe.org/wp-content/uploads/19-17449_CodeNotes_Air-admittanceValve_FINAL_HIRES.pdf

That may let you unscrew the AAV, and run a snake in from above.

 

[Reach4]

I've just bought a new sink

FAKE