I understand the basics of toilet operation and venting, but not sure how to solve this problem. I have a brand new house and toilet and there are septic odors coming from the overfill tube. It is my understanding that when the tank fills, the fill valve fills up the toilet bowl by the small hose that goes into the overflow tube and this water goes into the bowl. So, the overfill tube should only have odors if the bowl has odors or if the s-trap in the toilet is not full of water. There is a vent pipe placed about 3 feet from the toilet off the main 3" line to the septic tank. Even if it was blocked by something, the s-trap should still block odors. Looking for answers. Thanks.

  • Very odd. I would take off the top bowl disasemble it and smell all the part. and look in the holes.
    – plumbum
    Feb 25, 2019 at 14:30
  • 1
    I would start off by cleaning the toilet by placing bleach into the tank and bowl and cleaning the overflow tube with bleach as well... just in case it is bacterial. I suppose there is a small chance that the toilet has a crack that is letting gas circumvent the trap. Do you see any damage to the toilet? Feb 25, 2019 at 14:41
  • It is a brand new toilet and I did clean the bowl. I do actually smell the odor coming from the overflow tube, so I have narrowed down the problem somewhat. Maybe the toilet ring is cracked or got damaged on installation. But I don't see the connection of that with the overflow tube.
    – JM Benoit
    Feb 25, 2019 at 16:57
  • I agree, the question is: how does septic gas get from the waste pipe to the overflow tube? The only mechanism, if the trap is full, would be a crack in the toilet past the trap that is allowing gas to seep up to where the water enters the base (which is then venting up the fill tube. This is presuming your diagnosis is correct that the smell is septic.; sometimes new plastic can have a smell, that to me, is like excrement. Feb 25, 2019 at 17:47


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