Sizing Design Day when "Sizing:Zone" object is not used

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Hi, I usually run my cooling sizing simulations without the Sizing:Zone object because I assume that EnergyPlus will automatically use the SizingPeriod:DesignDay that I entered, but my colleague argued that I have to create the Sizing:Zone object to properly assign the Design Days to the simulation and results in the correct sizing of the AC System.

My question is, if I do not create the Sizing:Zone object and I do not assign the DesignDay into the Cooling Design Air Flow Method, what day or which Design Day will EnergyPlus use as reference?

Thank you! Best regards, Hanif

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