Almost two years ago we had a contractor remodel our circa 1900 house's kitchen and bath. New plumbing throughout. A year and a half later, for no apparent reason, the commode would not flush. Pulled commode, snaked line, no clog found. After much experimentation, removed AAV under kitchen sink, and commode flushed fine. Figured it was the AAV, so replaced that, but no improvement. Then thought it was the main vent stack, so snaked it multiple times - finding no clog, but commode started flushing fine again. That was 3 months ago. Now the commode is not flushing unless the kitchen sink AAV is off. Snaked every line in the house, but no clogs found (including main vent). With everything in place, running water down the shower will cause it to try to pull air through the commode. The main vent stack connects to the sewer line downstream of the commode and bathroom vanity, but before the shower. With eveything in place, all drains in the house seem to work fine, except the commode. Also had the septic tank pumped and flow from the system to the tank is normal. Any ideas? Thanks for your time. PS - the contractor re-routed the vent stack during remodel; It now comes down from the roof, takes a 90 degree turn, over the shower ceiling, another 90 degree turn and then down to the crawl space where it turns yet again to connect to the system. Still, it worked great for 18 months.

With everything in place, running water down the shower will cause it to try to pull air through the commode. I don't have to tell you that you have issues with your vent system when the shower's attempting to vent through the toilet. I can't tell you what's wrong without being there myself but you should get another plumbing contractor in to advise you on your options.

the contractor re-routed the vent stack during remodel; It now comes down from the roof, takes a 90 degree turn, over the shower ceiling, another 90 degree turn and then down to the crawl space where it turns yet again to connect to the system. The vent line can have as many bends as the installer wants providing proper slope's maintained from the terminal end back to the fixture it's venting.
Still, it worked great for 18 months. And I might just have a answer to that also.
If, with all those twists and bends the contractor left a dip or bell in the vent line the system would work just peachy UNTIL rainwater built up in the pipe enough to block the flow of air. This would take quite a while to build up but once built up it would act as if there were no vent at all. There's one explanation and about the only one I can come up with. Get someone in to check the last contractors work. Good luck, Tom

Thanks for your answer. I went back up on the roof and ran a snake down the vent. After the first turn, there is, indeed, water in the vent pipe. It all makes perfect sense: not blocked enough to keep the small drains from draining, but too much to allow the commode to drain. Now my question is: how to get the water out. I think the pipe runs slightly uphill after that first bend, allowing water to build up. Short of tearing out the shower ceiling, I'd love to be able to push the water through. Is that possible? Can I use compressed air? And, once the water is out, can I put a "cap" or "U-bend" fixture on the vent pipe at the roof to keep water out in the future. As we looked back, both of these problems occurred after particularly heavy rains. Thanks again for your help.

You'll have to go back, cut out the pipes with backfall and reinstall them with proper slope. Simply removing the water will set it up for the next heavy rain. The Vent MUST be run with no traps. Good luck, Tom

Thanks again, Tom. I pushed the water through using my snake, so the commode is working now. I'll need to get somebody in to tear out the ceiling and re-run the pipe at proper slope. And hope we don't get a heavy rain in the meantime.

Good luck and let me know if we can be of any farther assistance, Tom