How to Add a Sheet Metal Duct to an Existing Furnace Plenum

Installing an additional sheet metal duct onto an existing furnace plenum is a modification to your heating system that can be accomplished in just a few minutes' time. This is necessary when installing HVAC ductwork in a home addition, as well as any time you open a new heating register in a room of your home. Begin the process by selecting a duct to install that is slightly smaller than the size of the plenum itself.

Magic marker
Electric drill
Metal shears
Pliers
Duct tape

Instructions

- 1
  Hold the offshoot duct against the side of the furnace plenum where you intend to install it. You can use round or square duct, whichever works better for your application. Make certain that the offshoot duct is positioned after the plenum's pusher fan, if it's fitted with one.
- 2
  Scribe a line around the circumference of the offshoot duct, with the magic marker. This will provide a guide line for you to cut away the access port for the duct.
- 3
  Drill a hole in the middle of the scribed line to provide access for the sheet metal shears. To be effective, drill at least a 1/2-inch hole.
- 4
  Cut from the center of the marked outline with the shears, until you have cut within 1 inch of the scribed line. From this point, cut 1 inch inside the line to remove the center of the access hole.
- 5
  Cut the remaining 1 inch of material into flanges that will be used to secure the duct to the plenum. Each flange should be approximately 1 inch square.
- 6
  Bend the flanges out at a 90-degree angle from the side wall of the plenum, and then slide the offshoot duct into the access hole. Use the pliers to press the flanges against the wall of the offshoot duct.
- 7
  Wrap the flanges with duct tape to hold the heating duct in place, and complete the offshoot installation.

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