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Our plumber discovered that the drain pipe under our bathroom sink (which is done properly with a P-trap and vent) has a second P-trap down in the crawl space, which is incorrect.

I've been researching this and understand that having two in-line P-traps can cause drainage problems because air gets trapped between the two traps and water can't push it out.

However, in this case, there is a vent between the two traps, so I think that particular problem may be ameliorated. It's still done incorrectly though, and I'm trying to assess the risks.

What other problems could this second P-trap cause?

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  • It could be a pain to get a snake down that line. Is there something else on that line or any reason for the trap?
    – Ed Beal
    Feb 23, 2021 at 15:18
  • settling of crud causing a clog Feb 23, 2021 at 15:18
  • I had this in a previous house. It was 1941 construction. I could only guess that the underfloor trap was the only trap when originally installed. When an under-sink trap was later installed, given the vent, the effort to remove a (now) very old cast iron trap inside the basement ceiling was "deprioritized". Just guessing.
    – jay613
    Feb 23, 2021 at 17:03

1 Answer 1

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  • Debris is more likely to settle in the second trap due to lower drain velocity at that point
  • It's more difficult to clear debris there because you can't conveniently disassemble the trap
  • System cleanout with an auger is impeded by the extra trap bends, which tend to be sharper than other underground bends
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  • 1
    So the result of those issues would be slow draining or a completely clogged drain that is potentially impossible to clear with an auger? Feb 23, 2021 at 17:15
  • Yes, though the actual likelihood of a problem has more to do with type and condition of plumbing.
    – isherwood
    Feb 23, 2021 at 19:06

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