Frodo, your interpretation makes sense to me. I could see why IPC would logically find either OK. I am not a plumber.
What was the "i would sleeve that duct..." part? Accidental paste?
Terry... I know what you have always done, same as me, drain back to the fixture it serves
What I am looking for is the precise interpretation to the IPC vent grade definition.
the way it is worded, You can install a vent any way you want, as long as it has fall.
The IPC plumbers i am running into are saying that "AIR DONT CARE"
which is technically true, it just makes the work look like total ass