Under my kitchen sink there are two PVC pipes coming out of the wall. They are 8 1/2" apart and appear to be similar, except one is capped. One is in use for the sink drain, and I'm hoping the other is a drain, too. The house is 22 years old; Seattle area. What are my chances that the second is a drain, too? New disposal interferes with the use of the original drain, and I'd like to use the 2nd drain for the new disposal, with a trap for each. What is the easiest way to verify that it's a drain?

One of them may be intended as a cleanout, but if you can hookup in a manner allowable by the code it should work just fine as a drain.

Another possibility is it's an AAV or Studavent.

Take a pic and post using Go advanced/Manage attachments

Thanks for the backup, KISS. AAV's are not allowed in Illinois except in mobile homes, so that did not even cross my mind.

Quote:

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AAV's are not allowed in Illinois

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But they are allowed else where.
Welcome to the Plumbing Page Chayn.
Here we post under all codes and we're real flexible about that.
Northern Illinois? Would that be my home town of Rockford by any chance? I took my apprenticeship tn my fathers shop in Beloit, (seems like a hundred years ago) Hope you like us and stick around. We can always use experienced plumbers around to keep guys like me straight.
Welcome again. Tom

Second pipe is capped, what kind of cap, glued on or removable. Stick a snake in a couple of feet, run water in sink, pull snake out and see if it is wet. If it is, stick a hose in it and see what happens. You could have had a considerate plumber. My disposal drain is stubbed out separately.