System Inspector not available when selecting pipes in Revit

Issue:

When selecting pipes in Revit, the System Inspector tool does not appear on Modify | Duct/Pipe tab.

Causes:

• Calculations have been turned off for the piping system or set to performance.
• The system is not well connected.
  Note: A pipe system may be considered not well connected if there are multiple physical networks within a single system.

Solution:

1. First troubleshooting is to verify that ‘System tab: Piping tools’ is active in the user interface tab in Revit options. If not, turn it on.
   For Ducts, turn on ‘System tab: mechanical tools.’
   
   
   
2. Verify that mechanical calculations are turned on. Mechanical calculations should be set to 'All' or 'Flow only'.
   
   Note: the system inspector shows information about pressure loss, static pressure and flow for sections of mechanical systems, and it requires that flow calculations be turned on in order to process information.
   
   
3. Multiple physical networks within a single system may result in a situation where a system is considered not well connected. To resolve this, use the Divide System tool.
   See more details here - Help | Video: Divide MEP Systems | Autodesk.