OpenBuildings Energy Simulator offers a limitless number of possible HVAC configurations, consisting of subsystems, loops, connections and components.
In effect, OpenBuildings Energy Simulator's HVAC Manager is an interface into the Energy Plus simulation engine, which is generated by the US Department of Energy.
"EnergyPlus is a whole building energy simulation program that engineers, architects, and researchers use to model both energy consumption—for heating, cooling, ventilation, lighting, and plug and process loads—and water use in buildings. Its development is funded by the U.S. Department of Energy Building Technologies Office."
Accordingly, the systems defined in the HVAC Manager must comply with the rules of Energy Plus. You can find these prefabricated systems by selecting 'Application' in the top left corner of the HVAC manager menu. The relevant system(s) can then be copied to your Project and applied to rooms/zones as required.
An example of adding a PTAC air conditioning system, and assigning to rooms, can be found in the manual - here.
Please note, a comprehensive knowledge of Energy Plus is recommended should you wish to edit or create custom HVAC systems using the above interface. To create your own HVAC System, it is instead advised to use the Simplified System option, which can be found as below: