Thanks rraustad. What is the zonecrossmixing airflow you used in this exercise, and how much higher or lower is it compared to the airflow supplied by mechanical conditioning airflow? I assume in your case, there is only natural ventilation and no mechanical ventilation? In my model, I notice that because I have high zonecrossmixing airflow rates in the zones, e.g. 5-15ACH, the hourly outdoor air intake of the AHU is increased compared to a similar model without the zonecrossmixing objects. So I was wondering if the high zonecrossmixing airflow affects how the zone supply airflow is sized.

This is easy to test with your file. Just remove the ZoneCrossMixing objects and compare the eio results with the original file. As for my results above, you can look at the example file 5ZoneNightVent2.idf to see what values the ZoneCrossMixing object used.

Thanks rraustad. I will try look into the example file. I also want to know how this is calculated in the engine, something similar to the steps outlined in the Engineering Reference. Is there any documentation for that?

If you search the Engineering Reference for "ZoneCrossMixing" you will see the equations.