In Revit, air density, air viscosity, and air flow are used to calculate duct sizing. The values for air density and viscosity are specified under Duct Settings in the Mechanical Settings dialog.
You specify air flow for each duct system component by editing the component family and specifying the value of its HVAC connector’s Flow Configuration parameter as either System, Preset, or Calculated. For diffusers, the default Flow Configuration is specified as Preset, and for air handling equipment, the default Flow Configuration is specified as Calculated.
As a result, the flow for the air handling equipment is calculated as the aggregate air flow of the flow for downstream components (diffusers) connected in the system. When Flow Configuration is specified as Preset, a user-defined value is specified for flow. When Flow Configuration is specified as System, a percentage of the system air flow is allocated to each of the downstream components, according to the Flow Factor parameter. The Flow Factor parameter is specified as a value between 0 and 1, with the total for all downstream components equal to 1.
When sizing duct, you can apply constraints to branch ductwork to limit the maximum height and width for the sections being sized. Both height and width apply to the diameter of round ductwork. When both the size constraints and the flow constraints cannot be matched, the size constraints take priority, and an alert is displayed, indicating that not all of the sizing parameter could be satisfied.