Air bubbles in basement toilet when upstairs flushed

[Nickcgoodall]

I live an a split level home. You can definitely tell that the basement was finish after the fact and that the basement bathroom was added on after the fact.
My issue: When you flush the upstairs toilet, the basement toilet bubble and shoots water 6-12 inches out of the toilet. The bathroom is not vented through the roof. It has an air admittance valve on it, 48 inches off the ground. I thought that maybe the valve was bad, so I replaced it. Did not help. However I did take off the valve to see what it would do. If there is no valve in it, no bubbles. Another thing is that this only happens during the months that I run my air conditioner. There is a floor drain that the water runs into from the furnace. Or however that works. During the non hot months, there is no water draining, so the drain is empty. From what I can tell, the bathroom plumbing and this drain meet up bc they are in the same room, but under cement, so only an educated type guess. So I know it has to do with the air flow, but how do I fix it. I’ve tried two different valves, neither work. Help!

 

[Twowaxhack]

Septic or city sewer ?

You may have a partially clogged drain making this problem show itself in an unusual manner because it doesn’t have an atmospheric vent.

You have an AAV, aka air admittance valve.

 

[Nickcgoodall]

Septic or city sewer ?

You may have a partially clogged drain making this problem show itself in an unusual manner because it doesn’t have an atmospheric vent.

You have an AAV, aka air admittance valve.

City. I had the main line snaked and cleaned last year but didn’t do anything. So it could be a partial clog in the sink or toilet?

 

[MicEd69]

Based on your description, what could be happening is that the upstairs toilet is vented before the connection of the basement toilet. So, when the top toilet is flushed, air is being pushed through the main drain line and up the toilet branch. With the AAV installed, which is a one-way valve allowing air into the pipe but not out of the pipe, the air bubbles up through the basement toilet bowl. When you remove the AAV, the air goes out the AAV connection.

But that means that you have something preventing the air from going towards the sewer like an obstruction or a partial obstruction or perhaps a whole house trap, aka running trap.

As this happens only when you run the air conditioner, one theory to explain that would be that the P-trap in the floor drain dries out and the level is lower and the air bubbles up through that P-trap as it is less restrictive. But that would also mean that you would have a sewer smell coming from the drain in the cooler months.

So, do you have a whole house trap, and do you smell sewer gas in the cooler months?

 

[Twowaxhack]

City. I had the main line snaked and cleaned last year but didn’t do anything. So it could be a partial clog in the sink or toilet?

It wouldn’t be a problem with the fixtures. It’s a pipe issue.

The best way might be to vent that bathroom to open atmosphere and wait and see what happens. Problem might go away and never return.