To display a shaded vertical duct, use the Shaded Vertical command.

Ribbon: DM HVAC->Ducts->  Shaded Vertical

Pulldown Menu: DM HVAC->Fittings->Shaded Vertical

Select the vertical duct that you want shaded. The shaded vertical fitting will replace the typical elbow or branch on the vertical duct, as shown below.

If you need the elbow or branch fitting for the 3D representation of the ductwork, it might be necessary to insert an additional duct to insert the shaded vertical fitting on, as shown below.

To remove the shaded vertical duct, use the Reset to Automatic Fitting command.

Ribbon: DM HVAC->Ducts->  Reset to Automatic Fitting

Pulldown Menu: DM HVAC->Fittings->Reset to Automatic Fitting

You may need to run the command more than once to find the specific vertical duct that has the shade attached to it.