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:
Create Riser: lets you create a vertical duct or pipe while working in the editor. Pick a start point and elevation, choose a new elevation in the Properties palette, then click Create Riser. You can also use it while laying out ducts or pipes by changing the elevation and clicking Create Riser to add a vertical segment.
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.