When creating plumbing systems, Revit does not have a default way of showing where the piping changes from sanitary system to a vent system. This was easy to do in AutoCAD since we just used a different line type or a layer with a different line type to differentiate the vent portion. Revit wants to show connected pipes as one system so we will create an easy solution to this issue.
It is important to show the different appearances for documentation purposes, so this article will show how to easily show a transition between the two systems which can then be shown as separate colors, line types, etc.
The following is the final appearance of the process.
The following simple steps are an example of the process:
- In a project file, place a toilet and then connect piping to the toilet, including having the piping going above the toilet connection representing the vent portion.
- Highlight the pipe above the tee. (This works with other fittings as well.) Pick on the connector at the bottom of the pipe and drag the end of the pipe up away from the tee.
- With the upper pipe still highlighted, go to the Properties palette and change the System Type parameter from Sanitary to Vent.
- With the upper pipe still highlighted, drag the grip back down to the top of the transition.
- Pick the Modify button.
The piping system has now been separated into Sanitary and Vent systems for display and calculation purposes.
Applying Technology to Architecture 
Hi Doug. I tried this to solve the age old MEP problem. It works to separate the piping from sanitary to vent until you reconnect the modified vent piping to the fitting. When you do this it changes the sanitary piping to vent piping.
Jim, that process usually works for me, but I just added a 2nd method that I have not had fail on me. It is creating a transition family to place in the pipe just above the tee. The article is at https://aectechtalk.wordpress.com/2015/07/17/create-a-sanitary-to-vent-system-transition-in-revit-2nd-method/.