It is not unusual for multiple chillers piped in primary secondary configuration to be unable to fully load. The reason is that the primary loop flow rate and secondary loop flow rate are very seldom if ever the same. This is because the primary pumps are typically constant speed and the secondary pumps are typically variable speed. That means there is almost always flow in one direction or the other in the de-coupler/crossover bridge/bypass line. I would check the Delta T across my primary loop (chillers) and compare that to Delta T across the secondary loop (coils/load). If there is no difference then you have no flow in the de-coupler line. If there is a difference in the Delta T between the two, then there is flow in the de-coupler line in one direction or the other. You indicate that you are unable to reach setpoint when the flow in the de-coupler goes down. Typically that would indicate that you are pumping more water in the secondary than in the primary and are blending the nice cold chilled water you just made with return water. When this is happening, compare chiller discharge temperature with secondary supply chilled water (downstream of the de-coupler). If the supply to the secondary is warmer than the chiller discharge temperature, then you are blending return water with your chiller supply water. There are steps you can take to remedy that problem also. A simple sketch would be very helpful.

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