Question-and-Answer Resource for the Building Energy Modeling Community
 | 1 | initial version |
In summary, if a zone air temperature changes by > 0.3 $^0C$ or 0.5 $^0F$, EnergyPlus uses a smaller timestep than what's defined by the Timestep object type to iterate over how the HVAC system serving that zone should respond and avoid instability in the resulting zone air temperature. How small that timestep becomes is dynamic, and you can read more information in the Variable Timestep section of the Engineering Reference.
The "Detailed" reporting frequency option of Output:Variable objects would normally report every model timestep, but also add rows to show those portions of the simulation where this smaller timestep is used. Note that this "Detailed" reporting frequency can ONLY be applied to output variables that have an HVAC timestep instead of a Zone timestep. From the Output Details and Examples section for the eplusout.rdd file:
“Zone” variables are calculated and can be reported after each Zone/Heat Balance timestep (ref: TimeSteps in Hour command). “HVAC” variables are calculated and can be reported with each variable HVAC timestep. “Average” variables will be averaged over the time interval being reported whereas “sum” variables are summed over that time interval.
"Zone" vs. "HVAC" meaning the first word you see in the comment for a given output variable in the RDD output file. Example excerpts from the RDD output file, with ** around the key comment words I'm referring to:
Output:Variable,*,Zone Mean Air Humidity Ratio,hourly; !- **Zone** Average [kgWater/kgDryAir]
"Detailed" reporting frequency would always match the model timestep (Zone).
Output:Variable,*,Zone Air Heat Balance Internal Convective Heat Gain Rate,hourly; !- **HVAC** Average [W]
"Detailed" reporting frequency would show the smaller timesteps, if they are used (HVAC).
You can read this post for another reference, as well as see a diagram for this regarding EMS program calling points.
