Plumbing – How to move utility sink and extend trap / drain pipe under it

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.

No, there should not ever be two traps on a single drain line.

The point of the trap is to create a water barrier which prevents sewer gasses from coming up the empty pipe and into the house.

When you have two traps on the same line, you end up creating a kind of vapor block in the line which prevents it from draining properly.

What do you do?

Get yourself some 1 1/2" PVC (not CPVC, which is yellow, PVC which is white) with a 90 deg elbow, and also get two Fernco couplings (black plastic couplings with straps on each end that can be tightened) that will fit the pvc and iron drain pipe.

Use a metal blade on a sawz-all and cut out the p-trap in your basement. (Have a bucket handy!) Leave room to connect the Fernco coupling on both sides. I'd cut the right hand side out just past the coupling.

Measure and cut the PVC so that with the 90 elbow the ends line up with the newly cut drain lines. Glue the PVC together and confirm the fit, then connect to the old drains with the Fernco.

Viola - a proper drain line that will drain smoothly w/ no vapor lock.