Sewer smell from drain but only when water is running?!

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MRubenzahl

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Kenoza Lake, NY
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When the shower is on, sewer smell comes up from the drain. I can't figure it out! Clues:

  • It only happens when the water is running.
  • No other drains (bathtub, sinks) in same room or elsewhere do this.
  • It's a rural home with septic system, about ten years old.
  • For years, the shower was very low flow but when I got here, I fixed the flow (a defective washer was plugging the shower head supply hose) and I think that's when the smell started.
  • The shower drain pipe goes straight down for about 7 feet, then there is a trap, then a horizontal run where it joins other drains on the way out.
Any ideas? I am stumped!
 
It grows stinky stuff in the vertical drop.

You could move the trap higher and use a studor vent to protect its trap seal.
 
Thanks, @Twowaxhack for your expertise and insight. Next step is clear: Call a plumber with your level of expertise. Hope I can find one (I am in a very rural area).

I think I can rule out gunk in the pipe above the trap, since the smell is only while water runs. I appreciate your guidance about the long run above the trap, which explains why the problem started when I fixed the flow problem. I am thinking it also might be a venting issue. Studor vent might be the remedy or maybe vent is wrong; and raising the trap would be great but I don't know if the house construction makes that area accessible.

Regardless, now above my pay grade and I will summon the pros!
 
Yes the longer drop can pull a siphon on the trap. I am currently dealing with the same issue on a large project for Amazon. The inspector is letting us exceed the 24" max only on the floor drains for the Air Handling Units.
 

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