First, read through this pdf file on plumbing.
The lav will siphon dry without an AAV where the cleanout is. That is a no-brainer.
The toilet is being vented by the lav, which is also not vented. When the toilet flushes, it pulls water from the trap of the lav.
Installing a Studor vent there allows the toilet to pull air instead.
The tub drain pipe to the trap is too long. That is also a problem with siphoning. I would shorten that distance by half at least, and add a Studor there too.
The washer should have a p-trap above the floor, not below. And that trap should also be vented to prevent being siphoned.
Terry, Thank you so much for responding to my questions. What is the best way to re-plumb this to ensure no sewer gas enters the house? We do not want to use any AAV or studor vents.
Here is our situation. Our house was not properly vented. We hired a master plumber to put in proper venting, but these pictures show how he redesigned the system, which still allows sewer gas to enter the house as you stated. He says that is not possible, but is willing to come and fix/change it for an additional charge.
One of the other master plumbers who initially viewed our venting problem, said that we needed a 2 inch vent pipe run behind the vanity and to connect it to the toilet as a wet vent. He said the bathtub needed its own vent. This entails cutting the wall in two places, which is okay. We can repair the wall. We want the most fail-safe way to plumb it so that sewer gas does not enter the home.
What do you feel is the best way to vent this bathroom?
Thank you for mentioning in your response that the trap to the washing machine needs to be above the floor. He has it below the floor and it has odor emitting from it as well. How far up off the floor should the trap be in the wall? The washer has a dedicated 2 inch vent pipe on the downstream side. The vent pipe was originally plumbed on the upstream side, but he changed it to the downstream side.
I also have a question about underneath the kitchen sink (see new photo). He put in a 2 inch vent pipe dedicated to the kitchen sink but the tail of the setup underneath the sink may be too long. I am not sure what the problem is but there is a horrible noise coming from the drain every time I run water in one of them, so something may not be quite right there as well.
We greatly appreciate your help.