Toilet vent confusion

Hey folks.

I'm confused about the vent requirements for a toilet, and it seems the more I read, the more confused I get.

Here is my current situation.
I want to move the toilet in our downstairs bathroom. It currently has no vent at all, but it's connected the main drain piping right at the point that the vent stack meets the piping. The vent stack and drain piping is 3" ABS
If I am reading, and understanding properly, this is allowed because the toilet and piping is within 72" of the vent stack. If this is in fact true and I move the toilet less than 72" does it need to vented?
To further confuse the issue for me, my upstairs bathroom toilet has a 1 1/2" vent. and the toilet has a 3" drain.
I have read that the vent sizing for a toilet needs to be 2", but in talking to a local plumber I was informed that a 1 1/2" vent is good because it's 50% of the vent stack size( I've not found this exception/explanation in any reading I've done). Is this a true statement?

If I need to install a 2" vent for my toilet downstairs, which will be on a 3" drain, where do I tie the 2" vent into the main 3" vent stack?

Thank You

Mike

Pictures would help, but I see your in CT, here we use the more lax plumbing code where different rules apply to venting and certain wet venting is allowed that other codes don't allow.

In terms of connecting a 2" vent for your first floor bathroom, that vent would have to come up to above the higest draining fixture. So basically it has to tie into the vent stack adove the 2nd floor drains sorta before it exits the roof. Depending on how your piping arragement is you may be able to tie into another vent that exists in your system if it is the right size.

For example in your first floor bathroom there is probably a sink and that sink should have a vent. If that vent is 2" and a properly plumbed vent then you may be able to tie your toilet vent into that same 2" line.

Again this all depends on if the venting is needing in your case. I'm not a plumber but have been around the board and my own plumbing projects to have a pretty good idea of what needs to be done in a regular situation.

Post some diagrams of what you have and what you propose.

I'll try to post a simple sketch when I get home. I have no 2" vents in my house. All vents are 1 1/2". The house plumbing was installed about the 1980 time frame when the house was renovated. I'm assuming it met code at that time.
I can run a 2" vent if necessary. It will require removing some sheetrock and running a vent up through the walls to the attic. I was trying to avoid tearing out a bunch of sheetrock if it wasn't necessary.
Bathroomm sink is a 1 1/2" vent, and so is the shower
I don't know if I can use an AAV. It appears that they may not be applicable for a toilet venting application?

The latest version of the IPC lists vent to toilet distance as "unlimited"