HVAC Systems Modeling
Core concepts
Air system modeling rests on understanding system hierarchy: an air terminal connects to duct, duct connects to equipment, and the System Browser reveals whether the network is actually connected. Candidates must know supply, return, exhaust, and outside air system types and how a system is created (select equipment, then Duct, or use the Air Terminal's system connector). Understand duct types and routing preferences (fitting selection by shape and size), duct sizing methods (equal friction, static regain, velocity), and the difference between a duct segment and a duct run when editing.
Space and zone logic
Spaces capture room volume and airflow; zones aggregate spaces for equipment sizing. A space that is not placed, or is placed but not enclosed, breaks the gbXML and load workflow. Mechanical equipment families carry connectors whose system classification drives connectivity - a connector set to "Global" will not join a supply system.
Traps
Common question traps: the Space Naming Utility synchronizes room names/numbers from architecture and must be rerun after architectural changes; duct insulation and lining are type or instance parameters and affect appearance, not connectivity; and "Check Systems" warnings usually mean disconnected or mismatched connectors rather than geometry errors. Another trap is confusing the vertical justification of a duct with its elevation offset.
Worked example
A supply system shows fewer terminals than expected. Rule of thumb: open the System Browser, expand the air system, and count terminals; then select each missing terminal and verify its connector classification and that it physically touches the duct. Connectivity, not proximity, defines membership.