Yes with Revit 2015 is possible to assign a Piping System's (and Duct) Calculations parameter to either: All, Flow Only, None, or Performance (as of Update Release 3). Using a specific setting helps Revit pay attention to connected elements that we are concerned about calculations for and ignore any that we are not, or that Revit can't actually calculate meaningful flow or sizing data for yet, such as Sanitary piping...it only calculates flow, not sizing yet.
The new Performance setting is intended to improve Revit's performance while you are editing large connected MEP networks. It will also disable Duct/Pipe sizing for the system it is assigned for... READ MORE at Autodesk's Help
My other older self here: http://forums.autodesk.com/t5/user/viewprofilepage/user-id/46056
