Beware of Optimum Start (CBECC)

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This is more of a word of caution than a question, although feel free to share insights if it helps others deal with this interesting aspect of CBECC / EnergyPlus modeling.

I noticed when working on a T24 compliance model using CBECC 2022 that the supply airflow and fan power were peaking at 4am for the Proposed model, and 7am for the Baseline. I decided to untick the "Optimum Start" box for the proposed VAV Reheat AHUs and voila! My compliance margin jumped from -7.3% (failing) to +3.5%.

I reviewed the IDF file and the EnergyPlus Engineering Reference (link) and the AvailabilityManager:OptimumStart was created and applied similarly in both models, using AdaptiveTemperatureGradient as the method. Full object descriptor below.

The moral of the story for me is: default to OFF for optimum start in this lovely compliance software. Also, never trust "optimum" or a checkbox that represents a complex sequence of operations.

AvailabilityManager:OptimumStart,
  Availability Manager Optimum Start 1,   !- Name
  Always On Discrete,                     !- Applicability Schedule Name
  AssemblyHVACAvail,                      !- Fan Schedule Name
  MaximumofZoneList,                      !- Control Type
  ,                                       !- Control Zone Name
  Availability Manager Optimum Start 1 Control Zones List, !- Zone List Name
  3,                                      !- Maximum Value for Optimum Start Time {hr}
  AdaptiveTemperatureGradient,            !- Control Algorithm
  3,                                      !- Constant Temperature Gradient during Cooling {deltaC/hr}
  3,                                      !- Constant Temperature Gradient during Heating {deltaC/hr}
  1.66666666666667,                       !- Initial Temperature Gradient during Cooling {deltaC/hr}
  1.66666666666667,                       !- Initial Temperature Gradient during Heating {deltaC/hr}
  2,                                      !- Constant Start Time {hr}
  2;                                      !- Number of Previous Days {days}