CO2 concentration in room with SpawnOfEnergyPlus

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I ventilate the model with outside air with a CO2 concentration of 400 PPM. However, the CO2 concentration in the room settles at 261 PPM. What could be the cause of this problem?

Here is how I add CO2 to the fluid mix: package Medium=Buildings.Media.Air(extraPropertiesNames={"CO2"})

Here is how I define the outside CO2 concentration in the source block: fill(400/1000000, 1)