OpenStudio model gives different results when running multiple times

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I would reckon it is the Energyplus simulation itself that gives different results. If you want to make sure, go to the run/ directory inside your companion directory: * From the Run Tab, you can click the "Show Simulation" button and get right to it. * If your model is /path/to/mymodel.osm, it's at /path/to/mymodel/run/

And run that multiple times with EnergyPlus directly.

Anyways, I would start by looking at the eplusout.err and looking at the warnings carefully, especially the ones that indicate convergence issues or temperature out of bounds.

Edit: After extracting the IDF and running 200 simulations in EnergyPlus directly, I was getting always the same answer. I did the same with openstudio, and I found some discrepancy.

After investigating, the issue is that the Controller:MechanicalVentilation can be different, with respect to the DesignSpecificationOutdoorAir it lists for the "Thermal Zone 2".

Thermal Zone 2 has 5 spaces, all of which are classrom but one which is corridor. And sometimes, the Corridor DSOA is picked, most times it's the classroom one.

['90.1-2010 - PriSchl - Classroom Ventilation 1',
 '90.1-2010 - PriSchl - Classroom Ventilation 1',
 '90.1-2010 - PriSchl - Classroom Ventilation 1',
 'PrimarySchool Corridor Ventilation',
 '90.1-2010 - PriSchl - Classroom Ventilation 1']

This is the same issue as NREL/OpenStudio#4503

To work around it, you should use a specific thermal zone for the corridor.