2 Traps at Kitchen Sink

[pdickerson]

I see this trap arrangment at least once a month. I am pretty sure it isn't right (the trap arm should be unobstructed between the trap and the vent), but I can't see that it would cause any real problems. I am curious to hear some more opinions.

[Jay Moge]

huh, not too sure. interested to hear too.

[pdickerson]

After looking around a bit, I may have found the answer. It appears that this is prohibited by the UPC (1001.1) but allowed by the IRC (3107.1).

I am still curious to hear some more opinions though.

[Ted Allen]

Need a "high loop" on the dishwasher.

As far as the p trap, I call it out as a problem. I can't give you the reason though.

I am just going by what I was taught in school.

[rmoore]

Paul,

Here's my theory (and it is only a theory)...

If you drained both sinks at the same time it is possible that the water still running from the second sink to drain completely would be effectively blocking the vent when the first finishes. That could result in the first trap being siphoned dry. While it's possible that enough water would "backflow" into the first trap when the second nears the end of draining, you probably cannot rely on it doing so.

It seems an unlikely scenario, but a single trap on a double sink would not have that remote possibility, and therefore be safer.

That make any sense?

As you said the UPC definitively prohibits this (one trap per trap arm). I'm not sure the IRC paragraph specifically allows it though. That is talking about sharing a vent, not a trap arm.

[Ted Allen]

Nice.

[David P. Valley]

IRC

P3201.6


Fixtures shall not be double trapped…


A single trap shall be permitted to serve two or three like fixtures limited to kitchen sinks, laundry tubs and lavatories. Such fixtures shall be adjacent to each other and located in the same room with a continuous waste arrangement. The trap shall be installed at the center fixture where three such fixtures are installed. Common trapped fixture outlets shall not be more than 30 inches (762 mm) apart.

[rmoore]

David,

I believe double trapping is another kettle of fish. It's when two traps are hooked up in series so that the waste water would have to travel through both. The issue there is not siphoning but that the second trap tends to clog up.

Paul's traps are in parallel and could probably best be described as twin (siamese twin?) traps.

[David P. Valley]

Originally Posted by rmoore

David,

I believe double trapping is another kettle of fish. It's when two traps are hooked up in series so that the waste water would have to travel through both. The issue there is not siphoning but that the second trap tends to clog up.
Paul's traps are in parallel and could probably best be described as twin (siamese twin?) traps.

It looks to me like the trap on the left is installed onto another sink....

[pdickerson]

Originally Posted by dvalley

It looks to me like the trap on the left is installed onto another sink....

That's right. There are two kitchen sink basins. The one on the left drains into a disposal, but that is not germane to the conversation. both sink basins have their own p-trap, after which the trap arms are joined at a Y. It then disappears into the wall where I assume there is a san-t with vent up and drain down.

As we all know, the correct way to plumb this is to join each basin pipe together before the p-trap. But occasionally I see it done like it is in the photo. UPC says this is a no-no, but the IRC allows it (though it may require a double Y so there is a clean-out). I am trying to figure out if it could ever actually cause a real problem. I can't see how it would.

[David P. Valley]

It should look like this...

[John Clayton]

The IRC does allow the use of two p-traps as mentioned earlier. However the wye where the two trap arms meet needs to be enlarged to 2" for it to meet code. The slip joints and union traps do serve as a cleanout if no others are present.

The end outlet and center outlet continuous waste assemblies as shown by David are a great way to connect two compartment sinks without food grinders. There is even a continuous waste assembly made to connect a food grinder with another sink compartment before a single trap. However most food grinders (ie. in-sink erator) require the use of seperate p-traps in their installation instructions.

[igompertz]

The trouble with a trap like that when draining all of the water from the 'dip' in the pipe can be 'sucked out' allowing sewer gasses to enter the home.

[Carla Horne]

I agree with David(as usual!)
and it looks like mold on the drywall

Carla

[Paul W. Abernathy]

It is perfectly fine to have (2) "P" traps , one on eash sink of a (2) sink setup. As long as the "P" traps are not Double Trapped ( more than one trap serving a fixture ) then it is fine as long as the weir is sized correctly.

The problem with a double trapped setup is if a large volume of water is released down the train it may carry with it neough pressure or velocity to pull the water right out of the traps...and we all know what that means.....Sewer Gases which may be allowed into the building.