Note: I got the user's model privately.

So your problem is that all of your heating coils that are on the demand side of the Hot Water PlantLoop have an availability schedule that turns it off from April 1st to September 30. off. Your plant loop runaway temperature temp happens when the availability schedule goes to zero (with a bit of a time lag so the plant heat builds up).zero.

You should turn off your plant loop according to the same schedule by adding an AvailabilityManager:ScheduledOff with this same schedule assigned. Then your file runs to completion.