Question-and-Answer Resource for the Building Energy Modeling Community
 | 1 | initial version |
I'm interested in the issue and tested with an ExampleFile (V23-1-0) with weather data of Chicago, but the Zone Air CO2 Concentration output did not vary by season. I don't think your issue is a bug in EnergyPlus.
Your system outdoor air is constant, so economizer or nightcycle is not the cause.
Another likely cause is that your model is a typical VAV reheat system. If your system supplies air to multiple zones, your issue could happen. You said the zone is a core zone without any exterior surfaces and no infiltration. The load of the zone is supposed to be low and almost constant. If other zones have large cooling loads in summer, those zones have much supply air. Much outdoor air is supplied to other zones and less outdoor air is supplied to this core zone in summer, which results in higher Zone Air CO2 Concentration in this core zone. For VAV system, the amount of outdoor air to each zone varies even thouth the amount of system outdoor air is constant. Please check the Zone Air Terminal Outdoor Air Volume Flow Rate and System Node Mass Flow Rate of Air Outlet Node for all zones.
