Vents on short toilet branches

[Bens]

We're relocating the toilet even closer to the stack than it was before.
In the picture the new flange will be coming down just next to where the joist has been butchered.
This means we're talking about a branch to the stack which is maximum 12 inches.
The only place to add a vent is in the middle of that short run - pretty tight quarters.

I'm curious why a vent is needed in such a configuration? Is a siphon going to be broken by a vent so close to the stack anyway?

 

[Terry]

What do you have coming in from above?

 

[Sponsor]

 

[Bens]

Second story bathroom...so shower, sink and toilet.

 

[Terry]

Normally you would vent the toilet on the lower floor before it enters the stack from above. The toilet flushing above can affect the water level in the bowl below otherwise.
The lower toilet could enter the stack lower down with a wye and then that vent would tie into a vent six inches above flood level of fixtures served.
How was it vented before?

 

[Bens]

It was vented with this nifty thing. The galvanized vent seems to go to the right place. Of course we found rust in that near horizontal vent run!