In the realm of second homeowner woes, I had a P-Trap assembly under the bathroom sink. It was a straight connection to the pipe connected to the main sewer line. (least I think that was the case). It was not vented and it worked just fine. But after dealing with a clog the corroded bottom of the P-Trap (which was made of chrome plated metal) broke and left a 1/2 inch long hole in it.
Suffice it to say, I had to replace the entire assembly. As with most projects, more pieces needs replacing. I replaced the sink flange and tail pipe and the P-Trap now made of plastic.
As you can see in the image above that I used the original back plastic pipe leading down the wall.
When water drains now it backs up and eventually gurgles. Also when it backs up it leaks where the tail pipe and the P-Trap meet. My research tells me this is because this assembly is not properly vented. Is this true?
I ask only because the former assembly did not have venting problems. Is it possible that the former parts were venting through bad seals?
When does an under the sink P-Trap assembly need venting?
Best Answer
All traps need venting in all cases. Without a vent, the water seal in the trap gets sucked out and sewer gases can enter the room. There's also all sorts of examples where someone thought they were providing venting, but done in such a way that clearing the water seal was still a possibility. Some of these methods even used to be accepted practice, such as S traps. There's still several appropriate ways to achieve proper venting. For example, in island sink situations, a loop or foot vent is acceptable.
It's hard to say why the previous installation worked without venting. For sure, one way or another, the siphon action was disturbed by the entrance of air somehow, either from oversized pipe or leaking fittings. Regardless, where do you go from here? The downward facing ell in wall needs to be replaced with a sanitary tee for starters. The top of the new tee needs to tie into a vent somewhere, without travelling downwards. Is there a vent pipe nearby, such as from a toilet or shower? Or a pipe could perhaps be extended up into the attic, where it should be much easier to make a run to a vent pipe?
If all that sounds like too much, get a mechanical vent such as jberger suggested. They are not accepted by many codes, but they do work. Just be sure it's attached to a long enough pipe so that the valve is above the flood level of the sink. No doubt this all sounds like a real PITA right now. Once it's all done, you can take pride and sleep well knowing you have a properly functioning and completely sanitary sink drainage.