Should I give the dishwasher its own trap?

[Chris B.]

Hello, I'm helping a relative with her kitchen remodel. I have a question about how to rough in the drain for the dishwasher.

Some background: The kitchen previously had a double sink in front of a window and that was it for plumbing. It was all the original copper, 1-1/2" drain, 1-1/4" vent from when the house was built in the '60s. I don't have a great picture of the pipes before they were partially removed to frame a wall for insulation, but this blurry one shows where the vent was taken off (the sink was to the left, out of frame):

 

[Chris B.]

At one point, I added a dishwasher by wyeing into the horizontal connection between the two sinks. The trap arm into the wall was at a higher elevation than the horizontal connection between the sinks so when I cut the pipe to insert the wye, lots of nasty water gushed out (which, fortunately I figured out was going to happen).

I talked to her a few weeks ago and she told me that the dishwasher sometimes backed up into the sink. I'm not sure how much or how often. So I suggested that she address this issue while she has the walls open (she bought a new dishwasher, the old one is gone).

My question is, is it likely that the reason for the backup was the extra water that had to be pumped because of going 'uphill'? This will be addressed with a new rough in, of course. Or is part of the problem that the drain was only 1-1/2", going about 8' back to the 3" stack? Will changing the drain (after the vent) to 2" address this even if the trap is still only 1-1/2"?

What I'm considering doing is providing a seperate 2" trap inside the wall for the dishwasher, which will wye into the 2" sink drain as it runs horizontally. Or am I nuts and this is total overkill?

 

[Sponsor]

 

[hj]

It is a "bad connection". Do it the normal way and drain the dishwasher into a disposer or "branch tailpiece" so you avoid water backing up into the dishwasher, or odors coming out of it.

 

[Ontario Plumber]

First of all, sort out the issue with the backslope. This could be the reason the sink was backing up, or the drain could need snaking to get rid of the grease build up.

There is no need to add an additional trap for the dishwasher. 1.5" Drain for the kitchen is sufficient providing the drain is clear. Connect the dishwasher to the drain underneath the kitchen sink with either a dishwasher wye or a tailpiece with dishwasher adaptor on it. Obviously make sure you connect the dishwasher upstream of the trap to prevent smells. Loop and clip the dishwasher drain hose as high as possible underneath the sink so that if there ever is a back-up, then you would see it in the sink before it backed up into the dishwasher.

 

[Chris B.]

First of all, sort out the issue with the backslope. This could be the reason the sink was backing up, or the drain could need snaking to get rid of the grease build up.

There is no need to add an additional trap for the dishwasher. 1.5" Drain for the kitchen is sufficient providing the drain is clear. Connect the dishwasher to the drain underneath the kitchen sink with either a dishwasher wye or a tailpiece with dishwasher adaptor on it. Obviously make sure you connect the dishwasher upstream of the trap to prevent smells. Loop and clip the dishwasher drain hose as high as possible underneath the sink so that if there ever is a back-up, then you would see it in the sink before it backed up into the dishwasher.

Thanks guys, sorry I didn't reply sooner. I'm running a new drain (and vent) in ABS. There will be a single sink and I'll install a dishwasher wye vertically on the fixture outlet.

I do have another question: I'm running 2" back to the TY and bringing 1-1/2" through the wall (at 16" O.C. AFF). Is taking apart the 1-1/2" trap adequate as a cleanout for the drain or should I provide a seperate 2" cleanout?

 

[hj]

If you do not, or cannot, install a cleanout in the vertical line, then the trap can function as the cleanout, although a dedicated one would be a handy feature.