Cooling load of a DOE prototype Building In EP

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I think it depends on the Airloop in your model. I don't know about OpenStudio, but using IDF file you can get ROOM load by (just an example!):

Output:Variable,*,Zone Air Terminal Sensible Heating Energy,hourly; !- HVAC Sum [J]

If you subtract it from the sensible cooling load on the cooling coils you can obtain ventilation load. Look at this for more informantion. You should use EMS if you want this variable reported every time step (here)