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This is another question in my series of understanding my drain/sewer.

I have a bathroom sink with a p-trap that enters the bathroom wall. Underneath the bathroom is a basement where I see the pipe going straight up (from a wye in a horizontal sewer line).

Is there any way to test/confirm whether this is properly trapped and vented without opening the wall?

divB
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  • Waterproof endoscope. I see one on Amazon for 40 USD. It could be a fun toy to have laying around after this one use. – popham Nov 03 '23 at 18:14
  • Look in the attic for the plumbing vent. It could have been tied together with other vents in the wall and run off horizontally inside the walls, so seeing the vent in the attic would be a sufficient condition for verifying proper venting, but not a necessary condition. – popham Nov 03 '23 at 18:16
  • @popham I actually have one but the wire is way too stiff. No way to get it through the p trap – divB Nov 03 '23 at 18:17
  • I assume the p-trap is plumbed with glue. You can't remove the p-trap? Cutting it out doesn't entail opening the wall. – popham Nov 03 '23 at 18:20
  • You could cut the pipe in the basement, cap it, and then blow air through the drain line. Technically that's only a necessary condition, though. Who knows how that air through the vent line would be escaping. – popham Nov 03 '23 at 18:25
  • Is there a toilet in the room? Usually flushing the toilet will cause a bit of "bounce" in the trap water level, enough to see with a flashlight. If the vent is clogged, there will be no/little bounce as it's airlocked in. – dandavis Nov 03 '23 at 19:24
  • Ok, I found a vent pipe roughly at the same spot in the attic. Having said that, there is a bathroom in the 2nd floor to which that pipe could belong too. But I hope that it's indeed properly vented! – divB Nov 03 '23 at 20:40

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I actually learned another trick that worked: Flush a nearby toilet while listening into the standpipe. Is the flushing audible from inside the pipe? If yes, there is no trap.

divB
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