New toilet venting

[Ryansteelers]

I recently added a half bath on the 1st floor of my home (septic system fyi)

To drain it, I tapped into the toilet pipe coming from my 2nd floor bathroom.

The existing drain pipe runs vertically from the 2nd floor straight down into the basement... I connected the new toilet drain pipe in the basement (below the new toilet) with about a 4 foot piece of ABS at a 45 from the new toilet to the existing vertical drain pipe.

Now, when I flush the 2nd floor toilet, the new 1st floor toilet gurgles and loses water down the pipe.

When the 2nd floor is flushed multiple times without the 1st floor being flushed, I'm getting some stink up through the new toilet.

I'm thinking an air admittance valve on the 4' piece connecting the new toilet to the existing pipe could solve my issue, but I don't want to go hacking away without asking first. My thought is, it will suck air as the 2nd floor toilet flushes rather than sucking water down from the 1st floor toilet

 

[Terry]

Maybe that would work, adding an AAV to the downstairs toilet. I have noticed the lower toilet being affected in some homes before, not ones that I plumbed. The home I'm in now, even though the plumbing is stacked does not do that. We typically vent the lower fixtures for just that reason.

To see if adding the AAV would help, first have someone flush the toilet above while you watch the lower one. If the water lowers only, then maybe.

If the water rises first, and then sloshes out a bit, then an AAV that only allows air to pull in won't do much for you. It may be air being pushed ahead of the water that is making water rise in the bowl, forcing water over the dam so to speak.

https://www.oatey.com/faqs-blog-vid...s-you-should-know-about-air-admittance-valves