Help with kitchen sink drain and vent replumb

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K H

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Hi all,

I'm hoping to get some help with my kitchen sink re-do. I'm replacing the old sink, old disposal, and old dw. The old double bowl sink did not drain well during the dishwasher cycle and sometimes at other times. I've checked the vent at the roof but haven't snaked or camera'd it. New sink is a single bowl. New garbage disposal wants a flange elbow not a lateral drain arm.

In the process of preparing for the new installs, I pulled the sink cabinet (reasons) and found that the existing p trap was actually lower than the height where the drain enters the wall. I ended up opening the drywall behind the dishwasher and then pulling both sink base cabinet and the cabinet next to the dw. In doing so, I've revealed the full extent of the sink drain. I'd guess at least some of this run is the reason for my previous slow drain.

Let's see if I can make a quick drawing. Ok, please excuse the sloppiness. The line for the drain vs the cylinder represents parts I'm not particularly altering vs what I would be altering. I've omitted the standard under the sink cabinet plumbing. This post is just about my rough plumbing alterations.

So basically, the existing drain runs 5' to the vent and slopes a bit too much (1/2" in 12"), is too high for modern sinks, and the old p-trap was too low. I'm thinking of capping this long run, adding a 2" drain directly behind the kitchen cabinet, hooking up to same 2" drain run in the unfinished basement (pretty easy access), and hooking up to the same 1.5" vent run above the existing drain. Existing drain would likely be capped. Not on the drawing: currently no cleanout. I'll find a way to add one somewhere under the sink cabinet.

So here are my questions:
- Is this a reasonable plan? Any problems or gotchas?
- Will it potentially help my dw/disposal/sink draining better when the dishwasher is draining?
- Looks like 12" would be a good height for my sani tee behind the sink cabinet? Agreed?
- Can I reduce to 1-1/2" at the vent/drain connection (2x1.5 tee) or should that reducer be above the sani-tee? If so, how far?
- What's the best way to add a cleanout under the sink assuming I'm adding this new 2" drain?

FYI I would love to hire my pro plumbing contacts but the industry here is overbooked around here. Even the guys I know personally can't schedule me. So I'm DIYing. I live in an area where that is allowed. While I'm not thrilled with drain work, I can certainly glue up and pitch the pipe.

I appreciate any advice in advance.
 

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Reach4

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Is the new sink draining thru the floor to the basement or crawlspace?

"New sink is a single bowl. New garbage disposal wants a flange elbow not a lateral drain arm."
Disposals often come with curved discharge tubes, but you can change that out to a straight discharge tube. I expect that still won't really help, since your new disposal discharge flange is lower than the existing santee side inlet.

Note that transitions from vertical to horizontal needs to be long sweep, 45s, wyes, or combos. The place where kitchen waste transitions from vertical to horizontal is a common clog point.
 

K H

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New drain for kitchen sink: wall is doable and makes the vent connection cleaner. As I understand it, going through the floor is frowned on?

Yeah, if I could just lateral the disposal and call it a day, I would. But there is a height problem.

So I can’t use a sani tee vertically to enter the back of the cabinet? Needs to be a wye? I guess that’s easy enough

See anything wrong with the new vent idea?
 

Reach4

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Your idea is probably fine. But it may be easier to insert a second santee below the existing one, or to cut out the existing, and put in a new plastic santee.

If you are putting the new santee from your sketch into the wall, how will you make the drain connection below?

Note that if you split a vent, the connection needs to be at least 6 inches above the flood level of the sink.
 

K H

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Connection below: Wye on its side into the existing 2" horizontal run that meets the 4" main drain stack. Easiest part.

Split vent: I did not know that rule. Thanks for mentioning it. That will make splitting the vent harder. I'll have to think about that. I do see the logic in the rule though.

Replacing the sani tee instead: I suppose that works. Avoids the vent split. I've just always believed the long run to and fro for the sink was contributing to the slow drain. But that's not a believe based in science or anything, just a feeling.

Thanks for looking over my revision ideas! Appreciated.
 

Reach4

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If you have that access below, and you swap out the santee, I would also replace the 90 below with a long sweep, if it is not already that.

You also have the potential to add a cleanout, although you can use the trap adapter as a cleanout. The dedicated cleanout could make for easier rodding.
 
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