Use the Duct tools to draw ductwork in a project to connect air terminals and mechanical equipment. Ducts can be drawn horizontally and vertically.
The first time you draw ducts in a project, specify the routing preferences for the type of duct being placed. See Specifying Routing Preferences for Duct.
You can draw horizontal and vertical ducts using the Duct commands on the Systems tab 
HVAC panel, or by using the Duct options from the context menu when right-clicking the connector on duct ends, duct fittings, air terminals, mechanical equipment, and duct accessories.
Placeholder duct
Draw placeholder duct in the early stages of your design to indicate the approximate location of a duct run, or to show a layout that has not been fully sized. Placeholder duct shows as single-line geometry without fittings. Using placeholder duct can allow for a well-connected system while the design is still unknown, that can be refined at a later stage of the design. You can convert placeholder duct to duct with fittings.