That’s kinda what I figured. There is a clean out boxed out below the concrete floor. However, I was worried if I opened it everything would come out that spot because it’s lower than all the pipes. I’ll probably rent a larger diameter head snake and go from the line in the kitchen. Thanks!

Thanks for all the advice! Appreciate it.

All the original plumbing is cast iron. I’ve done some renovations to the upstairs and replaced what I could. But haven’t touched the basement stuff yet.

Do chemicals work well? I’ve found anytime I’ve used things like drano type chemicals it’s a short term solution. And if I snake it soon afterwards I’m always worried about getting a burn on any exposed skin.

No septic tank. It’s hooked up to the city. The drains on the main floor are working fine. How would a clogged vent pipe cause the problem?