I have a pipe that is halfway under a door jamb and inside the concrete floor which when I run a the sink water is now coming out of it. This pipe is next to our AC blower unit inside the basement. I suspect this is somehow connected on the other end to our sewer septic tank and was never capped correctly when they made changes to this 40 year old house.

Does anyone know why there would be a line near the AC unit? (Could it have been an original condensation drip line into the septic tank?)

Any suggestions on how to uncover more of the line to cap it?

I am guessing this also means the septic is nearly full and needs to be pumped. Original owners didn't know where it was located to tell us at time of sale a year or so ago.

Any ideas on how to find it?

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  • The wooden box is part of what the air handler sits on and is connected to an air return vent. Feb 23, 2021 at 4:49
  • The pipe appears to be crushed near the end. I'd guess that was someone's attempt at "capping" the pipe. Obviously that crush has failed, so it's up to you to do it right.
    – FreeMan
    Feb 23, 2021 at 13:56
  • "septic is nearly full and needs to be pumped" - yes, post haste. I'd bust up the floor and expect to find a hole bashed with a hammed in the drain pipe that this just sticks into. ABS would need a barbed fitting, cast iron and clay don't have those. It might all be PVC and done 'right' but I doubt it. - Either way the concrete's gotta go, or all you can do is a stop gap (stuff it with something and let cement fill it when you patch the floor).
    – Mazura
    Feb 23, 2021 at 14:34
  • As far as finding the septic, when I bought a house with a septic system, knowing only that it was somewhere in the front yard, the septic company guy started stabbing down with a long spike in various areas of the grass until he hit it, then dug it up. If you don't know which part of the yard it's in, you can start with tracing where the drain pipe leaves your home.
    – Nate S.
    Feb 23, 2021 at 18:51
  • Do not listen to Mazura @Roberts2600. This house was built in 2007, as OP states. Cast Iron and clay are out of the question. Concrete doesn't "gotta go" until last case scenario... Does OP smell human waste constantly? Is slab wet from underneath in that area with no idea why? No, that was not stated. This is directed toward an AC unit, you don't say but It's likely there from new construction, and pipe could very likely be a defunct condensate line. As for barbed nonsense... ABS gets an ABS cap (under $10 at the box stores), and ABS glue. Feb 24, 2021 at 0:14

2 Answers 2


Poly is flexible, I haven’t seen it used as a drain like abs that is rigid.

With a septic system in winter it could be your field is flooded (standing water where the field is?).

Another cause is a partial plug up below this point it’s not always a failed system heavy rain fall and a flooded drainage field the water takes longer to dissipate into the ground.

That could have been a condensate drain for the AC system but not well done. I would cap it using a rubber cap. Fernco makes them.

Cut off pipe square slide cap on and tighten the hose clamp 5$ fix for now. There may be other ways but a fernco end cap would be my choice in this case.


It's tough to say what the cause is because it might just be that the end of that pipe intersects the sink drain line and the pressure alone siphons some liquid out, like maybe it's sub par plumbing and there was an old wash sink where the AC unit now lives... you could just cap it yourself.

Now incidentally, you might be right that your septic is due for replacement. You mention a pretty common indication that it could be full: the septic tank is at least one owner's tenancy old... maintenance should be around every 7 years. Now, if that pipe is flowing any time you use a drain, then yeah, that's a very strong sign but still could just be a bad plumbing problem like a low spot that some knucklehead tied into and if that were the case you could just cap it.

A good test would be, cap that mystery pipe, and run the sink to see if it backs up. It still may not mean your septic needs to be replaced, but it could tell you if that pipe is showing signs from some other blockage.

As for how to find it, the company you hire to replace it will snake/probe/cam the lateral with a probe and use a locator... basically same as locating a buried utility.

  • Thanks Jon. It seems this pipe is black ABS, black poly tubing or something similar. I have added another photo for clarity. I also think you are right on the sub-par work. I'll get it capped and hopefully I won't have any further issues. Again thanks! Feb 23, 2021 at 13:46

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