To display a shaded vertical duct, use the “DM HVAC->Fittings->Shaded Vertical” command. Select the vertical duct that you want shaded. To remove the shaded vertical duct, use the use the “DM HVAC->Fittings->Reset to Automatic Fitting” command. You might to need to run it a couple times to find the specific vertical duct that has the shade attached to it.
The shaded vertical fitting replaces the typical elbow or branch on the vertical duct. If you need that fitting for the 3D representation of the ductwork, it might be necessary to insert an additional duct to insert the shaded vertical fitting on.