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IES VE - VSD fans in cooling tower give huge saving

Hi Everyone, we are doing an energy model using IES for a typical office which uses cooling tower. The task is to compared the following two cases:

  1. Reference Case: cooling tower fan set to "constant speed"
  2. Proposed Case: cooling tower fan set to "VSD fan"

We found the the energy consumption by the cooling tower dropped significantly for the 2nd case, which is beyond reasonable amount. What's annoying is that we cannot see the fan curve that IES used for the cooling tower fan and how it affects the condenser water outlet temperature and the control logic behind it. Has someone noticed this problem before? Thank you!!