I have a house that was built in 1990. The kitchen sink has a normal trap under the sink. We do get a smelly drain at times - which initially was hought to be coming from the disposal.

The drain pipe then runs through the floor and enters a larger black sewer pipe and immediately into a trap. You can see a graphic of the layout here [up-ideas.com]

My questions are 1) is this a true double-trap and is this legal? 2) if, as I assume, it is incorrectly done, would the solution be to replace the lower trap with an elbow?


Many thanks
Roger

That's an "s" trap under the sink, and not to code. It is probably siphoning.And that's a double trap as well.



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As drawn you have a S-trap and double trap, both are code violations. To make it code compliant, remove the lower trap and replace it with a long sweep 90, remove the s-trap and replace it with a p-trap and sanitary tee, place a studor vent at least 4" atop the sanitary tee.

I see the double-trap no-no. But why/how is that S trap so bad? How is that so different than adding a P-trap that then empties into the outlet in the wall - which itself then 90s down into the floor completing the S? It's essentially what is depicted here except there's probably more horizontal run with the P-trap before turning down into the floor.

Seems the P-trap just elongates an S.

What my experience tells me is that at one time the S trap was and still is syphoning and losing it's water seal thus letting sewer gas up into the kitchen. The lower P trap was added to overcome the problem. What is needed is for the drain pipe to come up under the sink and then continue up the wall to a vent out the roof. The sink then connects to the drain pipe with a P trap. The installation of the vent is necessary to maintain the trap water seal to keep the sewer gas in the piping where it is supposed to be.

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Retired after 50 years of plumbing and heating.

RWP - great explanation - thanks

That is because you are a "homeowner" and not a plumber, besides trying to justify and rationalize what you have there. The sink drain that goes into the wall, if it is installed properly, does NOT go into an elbow going down to the drain. IT goes into a "T" which has a vent through the roof which PREVENTS it from becoming an "S" trap.

you have an indirect waste that has been modified.change the config to an un-trapped waste with air gap.3"riser to indirect waste within 18" vertical of the drain outlets..

Hi everyone, thanks for the comments. To add to the desctription, this sink is on a kitchen island, so a standard vent is not possible. I am thinking North Carolina Plumber has the best thought with the Studor vent.

Based on this discussion, I have another issue. The dishwasher drains directly to a vertical copper pipe that goes to its own trap below the floor - like the bottom of my original illustration of the sink. This drain has no vent and has not had any smells from the dishwasher. Would this be another case where a Studor vent needs to be added? Oh, the dishwasher is on the same island as the sink, so no place for a roof vent.

Thanks for everything folks!

Roger



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