How to properly route drain under kitchen sink

[Ivan Drobnjak]

Hello all,

Long time lurker and learned a lot here but finally had to create an account in order to ask a question. I am remodeling my kitchen and just got the countertops and sink installed. The new sink is deeper than the old one and now I need to install the food disposal and drains. Please see attached picture for the setup, I have two drain entries (I think) in vertical line so I assume they are both connected to the same stack. Previously the top one was used for the sink and bottom one with the built in p-trap was unused. I assume this second one is meant as a separate drain for a dishwasher and just not used. It is not capped off either which makes me wonder about gasses.

With the new sink being deeper, the measurements are now good to use the bottom drain line (12 inches below bottom of the sink basins) so I was just thinking of capping off the top drain line? I don't need two I think because I was going to connect the dishwasher directly to the food disposal.

What makes this slightly challenging is that I need the food disposal on the left sink which is larger and the one to be used more. It is a large disposal (vertically and after installing it I will only have 2-3 inches to the bottom of the cabinet, so I want to measure and cut everything first because once I install the disposal it may be hard to get back there. Questions for using the bottom drain line:

1. There is a built in ABS p-trap, glued in right before exit there as you can see. Can I just cut the vertical pipe to where I need to connect it to the right hand sink drain opening? I plan to go food disposal on the left to right sink drain to back of the cabinet, turn left behind the water supply lines and connect to the built in p-trap after cutting the vertical piece to fit. I realize the largest disadvantage of this is that the p-trap is a bit further from the sink and I cannot disassemble it for cleaning. I thought about having a p-trap under the right sink basin on top of the built in p-trap, but I read on these forums not to have 2 p-traps in the same line due to trapped air between them and potential for clogging.

or

2. Cut off that built in p-trap on the bottom drain line close to the wall and turn the drain line immediately 90 degrees right to go behind the water supply lines and along the wall, then turn it another 90 degrees to reach the right sink basin drain and put a p-trap right under the sink drain and connect?

The second option is harder because I don't have much pipe there to work with against the wall and get that 90 degree elbow glued on properly. There may be just enough pipe, but not sure.

Any direction here is appreciated!

 

[Jadnashua]

I think that you're probably right about their thoughts on using the lower one for a dishwasher. I'd pour water down it and make sure it works. If so, then I'd cap the upper one and use the lower one. You may want to cut off the trap and reorient it with maybe a longer trap arm to get it in a good position for your new install. That would require enough leftover length to attach a trap adapter for tubular. So, you might want a coupler, enough new pipe to get things where you want it, then the trap adapter (sometimes called a Desanko), then a new trap and riser to the outlets.

 

[Sponsor]

 

[Terry]

You will want to use the top one for a cleanout. How big of a disposer do you have?

 

[Ivan Drobnjak]

As there were no replies first 2 days, I just went with my gut and kept the p-trap where it was, using the bottom drain outlet.

Here is the routing I dude in the first pic and second pic is finished product. Everything drains well and I don't see any leaks. The dishwasher is not installed all the way yet, which is why I didn't loop the drain hose yet under the sink surface, but I installed a clamp to do just that once dishwasher is all the way in. The disposer is huge as you can see (Costco purchase), there is less than 3" clearance to the bottom of the cabinet.



Now, the only thing I didn't do is cap the top outlet, but I was going to permanently cap it with glue. I'm glad I didn't since you say I should get a screw cap to use as clean out should I need it in the future. I didn't see a screw cap with interior threads at the Home Depot, only clean out caps with exterior threads. I may need to look elsewhere though as it would make sense something like that should exist.

 

[Terry]

Can you go straight from the p-trap to the sink on the right? You've added a 90 bend that could have been eliminated.

 

[Reach4]

Can you go straight from the p-trap to the sink on the right? You've added a 90 bend that could have been eliminated.

Or if not, maybe he could use a slip 45 instead of the slip 90. It is hard to tell if the cold supply would interfere with that.

 

[Ivan Drobnjak]

Can you go straight from the p-trap to the sink on the right? You've added a 90 bend that could have been eliminated.

Yes, I suppose I could have. I just thought it would be cleaner this way, routing it along the wall and not crossing over my water supply lines and valves. I did make sure that the straight pieces of the drain assembly are all sloping down just a bit, to help the water flow.

Which, speaking of water valves, one of them ended up leaking so I went ahead and replaced both of the old ones I had left under there. Would have been easier to replace them before setting the cabinets, but my handy man only did one out of three for some reason. Now they are all done.

Thanks for help and advice!