The Plenum

• The plenum, sometimes called a bonnet, is the first part of the ductwork coming off the furnace. It is a box-shaped chamber that is made out of 30-gauge galvanized sheet metal. The installer attaches a plenum to the furnace outlet with metal screws. It acts as an air-mixing chamber. The surface area of the plenum has lots of space necessary for attaching the takeoffs and starting collars.
  
  

Takeoff Collars

• The installer will attach a takeoff collar to the plenum. Its job is to provide a way to connect the square or round duct pipes to the plenum. He fits the takeoff collar into a hole cut to size in the plenum. He bends over the end tabs of the takeoff collar inside the plenum and uses metal self-tapping screws to hold it in place. The open end is where the duct slides onto the takeoff collar.
  
  

Trunk Line

• The trunk line is a square duct pipe commonly made out of 30-gauge galvanized metal and is essentially a tunnel that is attached to the takeoff collar on the plenum. One end is crimped slightly smaller in order to fit inside the next section of duct pipe. The sections are held in place with self-tapping metal screws. The duct is supported approximately every 8 feet with hangers attached to the wood joists.
  
  

Branch Lines

• Branch lines are round pipes 6 inches in diameter that extend up from the top side of the trunk line to the register in the room. The installer will insert the starting collar for the round pipe into a hole cut into the top of the trunk line and secure it to the trunk line in the same way as the plenum's takeoff collar. The end of the trunk line, sometimes called the extended plenum, must be blocked with an end cap.
  
  

Elbows and Connectors

• There are times when the ductwork needs to bend around objects or a second line runs off it. In these instances elbows and Y-connectors are used. The installer will fit them into a hole cut in the line and connect them with screws in the tabs to hold them in place. She then runs the second line off from the connector. The last type of connector is the vent, which is another takeoff collar attached to a hole in the wall or floor. The installer connects it to the end of the branch line.