The Disconnect Diffuser command is used to remove a connection between a duct and a diffuser on the drawing. A duct that is connected to a diffuser has its end point fixed to the diffuser insertion location. The duct cannot be moved away from the diffuser, and moving the diffuser moves the duct as well. This command will disassociate the diffuser and duct so they can be moved independently.
You do not need to use this command prior to erasing a duct. When a duct is erased, it is automatically disconnected from the diffuser.
To disconnect a diffuser, go to
Ribbon: DM HVAC→Diffuser Connections→ 
Disconnect Diffuser
Pulldown Menu: DM HVAC→Diffuser Connections→Disconnect Diffuser
You will be prompted to specify the diffuser to be disconnected.
Select diffuser to disconnect:
The selected diffuser will be disconnected from the duct. The duct and diffuser can be moved independently.