Air Admittance Valve

When my lift pump in my basement runs it sucks the water out of the trap for my upstairs bath tub. I was told on this forum that my venting is bad. To replace my venting would be a nightmare. I have been told to install a Air Admittance Valve but I’m not sure if I would install it at the upstairs bath tub before the trap or at the lift station in the basement. I’m hopeful that someone here can help me. Thanks, Jim

At the tub. Tub is not currently adequately vented.

I would add, at the highest point possible, preferably above the flood line of any fixture on the vent line…

Never assume

Make sure it is installed on the correct side of the P-trap!

So if I install it according to your sketch…when the lift pump runs in the basement and pushes the gray water or sewage out to the septic tank, the Air Admittance Valve will open and suck air and the water will stay in the trap? I’m not sure what the flood line is, could you explain? Thanks so much for helping me out. I hope this works!! Jim

Individual and branch-type air admittance valves shall be located a minimum of 4 inches above the the horizontal branch drain or fixture being vented. Stack-type air admittance valves shall be located no less than 6 inches above the flood level rim of the highest fixture being vented. If the bathroom contains a vanity that would be the ideal location.

So even though it does not suck the water out of the bath room sink trap it would still work to keep water in the trap in the tub? If so that would be so much easier as I wouldn’t have to get in behind the tub to make the installation. Please advise and I can’t thank you enough for educating me on this. Jim

The vent or aav should be downstream of the trap. The pump will suck air thru the valve instead of sucking water out of the trap.

The trap at the tub is the lowest water seal at the bathroom. When the sump is activated it is sucking the water out of the trap and then gains air or vent, by placing the AAV at the vanity you will now have introduced air or vent to the lateral.
I find it unusual that the waste system was installed with out a vent through the roof or an AAV. Before doing anything you should inspect for a failed AAV or that the through the roof vent has not become blocked i.e. bird nest or some other foreign matter.
If adding an AAV becomes necessary the vanity is where it should be located as that is the highest fixture in the system being vented