I have a sewer-cleanout near the back porch of my house, which is below ground level so it's in a round box, and also within this box I have found a smaller piece of pvc pipe. The square 'handle' part of the sewer-cleanout has been cut to allow this smaller pipe to point into the larger drain.

enter image description here (Please see attached picture, the smaller pipe terminates right at the point where it meets the larger pipe. The sewer pipe is effectively open at this point, meaning nasty smell in my sun-porch!)

To try to figure out where this drain comes from, I have run every faucet and drain the house and also used the sprinklers and I still haven't seen water come from the small drain. (I also verified that this is not the Air Conditioner drain line, that goes another place.)

Any suggestions on finding where this drain comes from, so I can be certain that it's safe to cap it?

  • Do you have a sump pump anywhere? If you're really brave, try pumping air back into the mystery line & see if something "pops" elsewhere. In any case, you certainly want to install some sort of adapter so the system is sealed. Jun 16, 2016 at 12:37
  • thanks! bonus points for thinking of an adapter to make this a sealed connection. even better though, i confirmed with the previous owner that this pipe is now unused. Jun 16, 2016 at 14:51

1 Answer 1


Here is a suggestion for investigation. Start by removing the perforated sewer-cleanout and temporarily capping the suspect drain with a size reducing fitting. Then try one or more of the methods in this answer to a similar question.

  • 1
    those are good methods for troubleshooting so i will accept your answer! fortunately I was able to communicate with the previous homeowner and found that this pipe is now unused. it went to a septic tank but now the house is connected to city sewer. Jun 16, 2016 at 14:50

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