If you have a large drawing, we have long recommended that you use System Boundaries to break up the drawing for processing. But what if the system to be calculated stretches across other systems? You can still use boundaries, just use more of them, and make them isolate the main (more or less). See below:
In this building, I drew three boundaries, one around the system at left, and two around the main and underground. They can overlap, and they can contain extra pipes that do not feed the deisred system. When you calculate, or list or go to 3D, just select all three and it will work just fine. Do not worry if there are some extra pipes inside those windows, they wil get filtered out by the software.
PS – One little known thing about boundaries is that the name given to them means nothing to the program. When it comes time to select boundaries, the programs refer to them not by name, but by the fact that they were selected.
The H-Factor 
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