How are CO2 concentration values initialised? [closed]

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I added a ZoneAirContaminantBalance object to my model and noticed that the zones with occupancy but without natural nor mechanical ventilation systems have a very high CO2 concentration value since the first timestep (between 6000 and 10000 ppm). Where come those values from? It looks a bit as if energyplus sizing initialized those values with the accumulation of CO2 for about a year instead of using the outdoor concentration. Is that the case?