Mentor, you are asking the right questions. This shows my ignorance in heating systems. The pressure differential is what tells the VFDs to slow down or speed up, and that is the pressure I am trying to understand. That pressure increased causing the VFDs to slow down. Do you know why?
Couple of answers. Water in the supply loop either goes through the uni-vent or continues to the end, where the return loop collects all uni-vent discharge and the supply loop remnants. Thus, the supply loop simply dumps into the return loop if the uni-vent valves all shut, which is the same...
Did an experiment in a school's heat system. Turned off the unit ventilator valves all at once. The pipe pressure raised significantly causing the circulation pumps to back off. Some notes, this is a reverse return loop, hot water flowing (not steam), unit ventilators are 2-pipe.
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