The Erase Duct command is used to erase ducts from the drawing. All entities from the selected area are filtered out except for duct centerlines, making it easier to select the ducts to erase. Other than the filter, it works exactly the same as the standard CAD ERASE command. Whether you use this command or the ERASE command is strictly user preference.
To erase ducts, go to
Ribbon: DM HVAC->Ducts-> 
Erase Ducts
Pulldown Menu: DM HVAC->Duct Centerlines->Erase Ducts
You will be prompted to select the ducts to be erased.
Select objects:
The selected ducts will be erased from the drawing and database.