Vent issue-help me understand this

coolmom42 wrote:
Your air admittance valves in the drain lines have failed. There is one under both the kitchen and bathroom sinks somewhere. They are supposed to allow air to move in one direction-- into the drain lines when you dump the tanks, to avoid sucking the P-traps dry. But they are letting stinky gas from the gray tank in, when the gray tank space is being filled by your shower water.

When you open the tank drain, that lets the displaced air go on down the drain. Right now it's burping back up through the air admittance valves.

Air admittance valve

It's also likely your tank vent is plugged--either with bird or insect nest. The tank vent could also have slipped down (at the factory) and be sitting right on the bottom of the tank.

Air admittance valves; that was my first thought. How do you know they have failed, other than a smell? I took both off and rinsed them out etc.; the diaphragm moves freely so I assumed they were working, but I guess not.

Also, the smell is strongest in the upper kitchen cabinet, not under the sinks where the valves are...

As far as the tank vent, I took the cover off and things looked clean all the way to the tank.

Makes sense that the smells are going down the drain hose with the tank drain open. I guess I'll replace the air valves and see what happens.

At this point I am just grateful that leaving the drain valve open gets rid of the smell! :)

Thanks much for your input.