Difference between EnergyPlus Unitary Heat/Cool System and Ideal load Air System

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@Thit if the unitary heat/cool system uses hydronic (hot water or chilled water) coils, then you will need a Plant Loop (central water system) serving those coils, and the Sizing:Plant object to do design day load calculations for each Plant Loop.

If the unitary heat/cool system is placed on the supply side of a central air system (Air Loop HVAC), then you will need the Sizing:System object to do design day load calculations for each air system.

The Ideal Loads Air System is a "magic box" that connects directly to each zone to serve loads -- it doesn't require central air or water systems.

All situations would require a Sizing:Zone object for each zone in order to calculate zone loads, whether they are conditioned by the Ideal Loads Air System or anything else.