Thread: Improper Venting of Drain Trap

Am I correct in that this sink drain trap is not properly vented because the vent inlet (within the wall) is below the trap weir? My thought process is along the same lines as if there were too great of a slope, or too long of a distance in the trap arm putting the vent inlet over 1 pipe diameter below the trap weir... I would have expected to see a longer tail piece on the sink to eliminate the need for the last two 90 degree elbows.

Originally Posted by Andrew Kolar

Am I correct in that this sink drain trap is not properly vented because the vent inlet (within the wall) is below the trap weir? My thought process is along the same lines as if there were too great of a slope, or too long of a distance in the trap arm putting the vent inlet over 1 pipe diameter below the trap weir... I would have expected to see a longer tail piece on the sink to eliminate the need for the last two 90 degree elbows.

You are correct.
It is called an "S Trap", and is not allowed.

Thank you for the reply. Just because I am curious, I was under the impression that as long at that upper most horizontal section of pipe was long enough, this wouldn't technically be an "S" trap, it would however still not provide for proper venting of a P trap. Correct?

Here is another example that better illustrates what I am trying to explain...

Also, the house is less than 5 years old.

I would call those "effectively S-traps" as they are not S-traps in the common sense of the word but they effectively create an S-trap in the strict sense of what S-traps lack --- they lack proper venting.

The weir of the trap (the bottom of the inside of the pipe at the trap outlet which keep water in the trap) and where the drain (trap arm) is connected to the drain should allow a level air space from the trap weir to the vent.

Those photos show the drain piping going down more than enough to close off that air space, with the air space closed off, the drain does not vent properly.

Exactly what I wanted to confirm. Thank you both for your replies!

That's an "S" trap and should definitely be mentioned in the report as improper plumbing drain, and it probably wasn't done by a professional plumber.

Originally Posted by Jerry Peck

I would call those "effectively S-traps" as they are not S-traps in the common sense of the word but they effectively create an S-trap in the strict sense of what S-traps lack --- they lack proper venting.

The weir of the trap (the bottom of the inside of the pipe at the trap outlet which keep water in the trap) and where the drain (trap arm) is connected to the drain should allow a level air space from the trap weir to the vent.

Those photos show the drain piping going down more than enough to close off that air space, with the air space closed off, the drain does not vent properly.

Remember that a tail piece can be too long. If it is, the momentum of water falling to the trap can flush out enough of the trap as to render the seal not existent. as shown in Jerry's drawing where sewer gas can enter through the trap into the room. So do not allow the tail piece to be too long by plumbing the outlet at the correct height for the specific fixture.

Good info. We've run into that a couple of times ourselves.