Duct systems are logical entities that facilitate calculations for flow and sizing of ductwork. Once you have air terminals and mechanical equipment placed in a project, you can create supply, return, and exhaust air systems to connect the components of the duct system.
Use the System Browser to verify that all of the components are assigned to the correct duct system.
By default, there are 3 system types for duct: supply air, return air, and exhaust air. You can create custom system types to handle other types of components and systems. For example, you can create a high pressure supply air system. You can also modify the type parameters for a system type, including graphical overrides, materials, calculations, abbreviation, and rise/drop symbols.
Discipline-specific views are essential when designing mechanical systems in a project. They make it possible to place and view the components in the system. Because the components are placed at a specific height in the spaces in a project, the views that you create should specify an appropriate view range and discipline.
Revit provides several templates that specify many of the view properties needed to define discipline-specific views. See Applying a View Template.