Use the following tools to when placing duct.
When the Duct or Duct Placeholder tool is selected, the Modify | Place Duct tab provides the following options for placing duct:
Inherit Elevation: inherits the elevation of the element being snapped to.
Inherit Size: inherits the size of the element being snapped to.
Add Vertical:sloped round duct connects using the current slope value.
Change Slope: sloped round duct connects directly regardless of the slope value.