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I am modeling a simple district cooling "rig" on the supply side and several chilled water coils on the demand side of a chilled water loop (cooling). There is also a variable speed pump on the supply side, just before the chilled water coils, because the model won't run otherwise. I have set the chilled water loop temperature difference to 10 oF. In contrast to a constant speed pump, where the chilled water delta T may fluctuate wildly, I would expect the variable speed pump to ramp up or down to maintain the 10 oF pretty closely at almost all times. However, what I observe is a similarly wild fluctuation, and it is essentially never at 10 oF, but a few oF above. Any clues as to what I might be missing?

I am modeling a simple district cooling "rig" on the supply side and several chilled water coils on the demand side of a chilled water loop (cooling). There is also a variable speed pump on the supply side, just before the chilled water coils, because the model won't run otherwise. I have set the chilled water loop temperature difference to 10 oF. In contrast to a constant speed pump, where the chilled water delta T may fluctuate wildly, I would expect the variable speed pump to ramp up or down to maintain the 10 oF pretty closely at almost all times. However, what I observe is a similarly wild fluctuation, and it is essentially never at 10 oF, but a few oF above. Any clues as to what I might be missing?