You have existing HVAC equipment. You've documented the existing system and connected everything. Once you demolish it in a later phase however it becomes disconnected in the earlier existing phase.
This comes up every now and then during discussions and a recent thread at AUGI brought it up there too. Jason Martin, with Autodesk, wrote a nice response worth echoing:
The basic problem is that things like ducts and pipes are only allowed to be connected to one other thing "per connector". If I have a duct that is connected to an elbow the connection between those two are "properties" of those two elements (i.e. the duct knows it is connected to the elbow and the elbow knows it is connected to the duct).
The secondary problem is that the "temporal" concept in Revit (phasing) is primarily a "display only" concept (there are some special cases where other things are done, but primarily it is limited to display). I'm not arguing that adding temporal intelligence to these elements isn't a good thing, as there are a number of other cases that it would solve (i.e. like the ability to move something that exists in "this" phase to a different location in a "future" phase phase), but it isn't there today.
From a practical standpoint this means the mantra of assign stuff to systems can't be met completely with existing demolished stuff. Something I'm sure the designers at Autodesk are wrestling with internally.
With regard to the AUGI thread and getting the appearance of the documents they wanted, Jason went on to suggest that they use Filters based on a different parameter than System Type since that parameter is lost when the elements are demolished.