Breaking up Pipe Systems

I'm wondering what others are doing to break up their pipe systems, and how they are doing it. For example, I'll create a waste system for a stack of bathrooms. Then separately I'll create a waste system for another set of items, say an electric water cooler. At the bottom floor those 2 systems will have to connect together. As soon as I make that connection all the pipes, fittings, etc. from one system suddenly 'jump' to the other. The fixtures remain on their originally assigned system but everything else switches over. I've tried placing a union where I want the systems to diverge but nothing works. What exactly am I doing wrong here? Any suggestions would be great.

If you want to organize your systems that way you will need some sort of element at the tee where the two systems meet that has is on all three systems: the bathroom stack system, the water cooler system, and the combined "mains piping" system. You can't use an out-of the-box tee like this, you'll have to set up something custom if there is not convenient piece of equipment to use as the point where the individual branch systems end and the mains systems begins. The tee has 3 connectors, and each connector would be associated with the appropriate system. You can assign systems to specific connectors by right-clicking on the connector and selecting "add to system", then finding something else on that system and selecting it.

It is also very helpful to assign "equipment" to systems. Each system can have only one element modeled as it's "equipment". Therefore you need to pick the equipment that is central to all branches of the system. For example for supply air the equipment is the air handler, because all branches of the duct system eventually connect back to the air handler. For sanitary you might use an ejector pump as the equipment for your gravity system because that's where it all flows to. In the situation you described with multiple branches, you would want to use this custom tee as the equipment for each branch system, and something else downstream as the equipment for the mains system, so that the equipment is downstream of all other parts of the system.

The method of organizing systems I like to use avoids some of the confusion in what is described above by saying all sanitary piping is on the same system, regardless of what branch it is on. In this case, simply assign whatever is at the end as the equipment (an ejector pump if you have one, or if it drains by gravity from the building you could make a simple family that looks like an endcap with a connector to assign a system to at the end of the run leaving the building), and assign all your fixtures to that one sanitary system.

Thank you for the response, that makes sense. I've actually done the same thing to connect a supply air duct into a return air system. The issue I've had though with plumbing is getting the Fixture Units to propagate through that connection.

The only reason I've broken the system up this way is because I've heard and seen written advice to keep systems small for performance reasons. I'm assuming that you have not found that to be the case? I may just go that route and see what happens. Thanks again.

Well I definitely wouldn't say there aren't performance issues with the massive model I'm working on, but I've wasted enough time making changes based on what I've read "might" improve performance. Our systems are large and our model is slow, but nothing else that I've done based on what people say will speed things up has worked, so I've learned to accept its sluggishness.