Question-and-Answer Resource for the Building Energy Modeling Community
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I have struggled with this as well. I don't think it has to do with the "Cooling Supply Air Flow Rate Per Unit Cooling Capacity". This value will affect the sizing of the system airflow rate, not the cooling capacity.
The best I can come up with is that the autosizing parameters that are used for sizing the dx coil are at the rated conditions, not the design day conditions. (rated conditions = the conditions used for calculating the performance curves) If you look at the EnergyPlus input/output reference (https://energyplus.net/assets/nrel_custom/pdfs/pdfs_v9.6.0/InputOutputReference.pdf) for any of the Coil:Cooling:DX objects, you will note that the Field: Gross Rated Total Cooling Capacity is calculated at the rated conditions. It does not calculate based on your weather file or design day file. The other issue is that autosize selects the maximum system airflow. Often the maximum airflow will be in heating, not cooling. The default rated conditions are:
So, here is an example calculation:
