Twice in the last 5 years ALL of my drains have stopped draining at once. Plunging, snaking, etc do nothing. Water bubbles with the pressure of a plunger then painfully slowly drains. I'm 99% sure this is a venting issue.

I drew a crude diagram of my plumbing system and the system vent shares the drain pipe from the upstairs toilet. I think what happens is someone flushes enough "stuff" and it creates an air lock on the way down (its a galvanized steel pipe.) I don't think its a true clog, I think its because the air lock it creates.

Both times I've taken off the toilet, snaked the drain and after light contact feel the "stuff" release and fall--problem solved for that time. The drain pipe from the toilet down is 3", the vent from there up is 1.5".

Are there any clear solutions to the problem? Installing an AAV? A Positive Air Pressure Attenuator? Or do I need to actually install a vent that doesn't share a pipe with a toilet?

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  • First check if opening of that port marked on diagram as "clean off" is helping with problem. If yes then can add some kind of vent there. Or if helps and adding vent there is not possible add vent somewhere on the exit pipe. If opening do not help then You have bigger problem and need to hire specialist. Adding active devices can make overpresure in instalation and cause leaks.
    – k_z
    Commented Feb 1 at 12:46
  • This last time I opened the clean out. The water was filled almost to the top. I walked away for a few minutes, then it was empty and back to flowing.
    – mike
    Commented Feb 1 at 13:11
  • are you sure you stack isn't cast iron? galvanized was used for water supply. is it the bathroom on your diagram without a vent? you need a vent on a bathroom group. Commented Feb 1 at 17:22
  • What do you mean by "air lock"? What's the difference between that and a clog? (To get a true air lock, your drainage system would necessarily have an uphill section somewhere, where I expect you mean something different than my intuition for air lock.) And have you discovered any "stuff"-like residue in the fixture with the lowest drain in the house? By all drains, you mean all? Or just the upstairs ones, for instance? You drew a dogleg in the upstairs pipe coming down. Can you see down to the depth of the blockages after clearing them? If so, what's the condition of the pipe down there?
    – popham
    Commented Feb 1 at 19:35


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