I have a fairly small, old house with a similarly small, mostly old (and probably incorrect) plumbing system. Upstairs is a toilet that had a mechanical vent in the wall behind it. In the downstairs bathroom is a washer, shower, sink and toilet, all going into a septic tank. The downstairs appliances were built without any vents, but the toilet did have a 3" pipe extending from below the closet flange into a wall cavity, and traveling about 7' up and dead ending with a cap on it. There are no through the roof vents.This system worked without any apparent problems for 8-10 years.
In the past few months, the upstairs toilet has started to bubble (furiously at times) when the downstairs toilet is flushed. Not every time, but enough to make the user of the upstairs toilet nervous, and occasionally unhappy. My first thought was that the mechanical vent had failed, so I replaced it with an AAV (Magic Vent by Rectorseal). Still the problem persisted, but again, not every time. I cut open the wall behind the downstairs toilet, shortened the 7' pipe to 3', and installed another AAV. Still, the problem occurs, but not every time.
What is going on? I assumed the bubbling upstairs toilet was due to the downstairs toilet not being vented properly, but it is still happening. The fact that it worked for years with only a mechanical vent upstairs keeps haunting me.
Any ideas? Could it be an intermittent clog somewhere? The septic tank isn't full. It's only been a few years since it was emptied.
Here's a drawing of the current setup:
"Argue for your limitations, and sure enough they're yours."