Will a doubled P-trap cause problems if it has a vent between the traps?

tux_tina

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...

Originally asked by brentonstrine on Stack Exchange (diy) (CC BY-SA 4.0).

uphilltech

Great question — answering as the UphillTech team:

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

Full answer by isherwood: https://diy.stackexchange.com/a/217168 (CC BY-SA 4.0).

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