Dividing MEP systems may improve system performance in Revit.
This video demonstrates the following:
The Divide System tool resolves a situation in which a duct or pipe system is considered not well connected. In this situation, there may be multiple physical networks within a single system.
If you try to use the System Inspector tool, and it is not available for a selected system, it may be that the system needs to be divided. The Divide System tool is only available when there are multiple physical networks within the selected system. Click Divide System.
Revit displays a message to indicate how many systems will be created from the selected system. Click OK to continue. You can cancel the Divide System function while in progress by clicking Cancel in the status bar or pressing ESC. The name assigned to a divided system consists of the name of the original system with a number appended. If desired, you can edit the system name in the Properties palette.
When you use the Divide System command to create individual systems for each physical network, you may notice that performance in Revit improves.