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I have seen this problem before. Both possible causes I can recall were related to the Controller:WaterCoil objects attached to the hot and chilled water coils. Not sure if either of these are the cause, but at least they're easy to test.

1. The minimum actuated flow was slightly above zero in the IDF because OpenStudio has a default of 0.0000001 (taken from E+) instead of zero.

   • For a constant-speed pump, if the flow request > 0, it runs at full speed.
   • Check this field in the IDF. If it is not zero (exactly), change it to zero and see what happens.
   • Might be related to this E+ issue?

2. The controller convergence tolerance was “Autosize,” which for some reason didn’t appear to be working.

   • I wrote code to change this field to 0.0001 for hot water coils (where controller action = Normal).
3. I don’t have an explanation for this one, but at one point I suspected this was an un-reported E+ bug.