Slow flow basement toilet with Air Admittance Valve on vent. A few years back we finished the basement with a full bathroom and laundry room that all drain to an ejector pump. The laundry sink, washer drain, ejector pump and shower share a vent stack to a AAV. The bath sink and toilet share a vent stack to a AAV. A few weeks ago I noticed a sewer smell in the basement and then discovered that the cover on the holding tank was loose. Upon tightening the cover, the toilet then began draining slowly. I was certain that there may be blockage in the vent stack or the AAV had failed. I removed the AAV from the vent stack and snaked the vent pipe all the way into the holding tank. As a test, I flushed the toilet and all was good. I then replaced the AAV to the vent stack and again the toilet drained slowly. So I then concluded that the AAV had failed. I replaced the AAV with a new Studor AAV; same result of slow draining toilet. So in summary, with the AAV the toilet drains slowly; remove the AAV from that vent stack and the toilet flushes as you would expect.
Pictures attached showing rough plumbing drain and respective vent stack.
There are 3 other vent stacks in the plumbing system throughout the house that vent directly to the roof; however none are connected to the basement.