I didn't realize how little I knew about pluming until I started reading Tom's answers here. An ''s'' trap is where it turns and goes back down again without a vent, just as sinks all did a long time ago? A few years ago, I traced a foul odor in the kitchen of my church to a drain without a trap. I bought the pieces to a ''P'' trap, added an elbow, and fixed the problem. No more odor, and no other observed problems.

The drain feeds into the top of a horizontal pipe between 2 other drains. They have traps at the bottom feeding into the pipe horizontally. After the third drain, the pipe turns and goes down into the concrete floor, all without any vent. Before I added the trap, the one drain would have worked as a vent. The kitchen was built in the 50's, with a couple of remodelings since. Are all 3 of them ''S'' traps, and if so, any reasonable fix? They all drain fine.

speedball1

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I never said "S" traps wouldn't work. They've been in service for years. The framers of the plumbing code always take a worst case scenario and assume it's going to happen and then try to mitigate it. Just look at the wacky scenario they set up to validate backflow preventers.

Checkout The Standard Plumbing Code Book, Chapter 14, (Vents and Venting) Section 1402.1 (Siphonage and Back Pressure) where it states in part, "The protection of trap seals from siphonage or back pressure shall be accomplished by the appropriate use of vents, revents, back vents, loop vents, circuit or continuous vents, or combinations thereof."

It is possible that with a unvented trap to lower the water seal by the suction of the draining water. Lowering the seal will allow sewer gas to escape past the trap. Likewise it is also possible for lint to build up in a unvented washer trap and siphon water out of the trap by capillary action.
That's why "S" traps are outlawed. Not because they won't work but for the protection of the trap seal itself. Just because it COULD happen doesn't mean that it will. The framers tried to cover all bases and that's why "S" traps are outlawed in every state and Canada. Hope you had a great Forth. Tom