How to Replace Plastic With Metal Ductwork

Measure the plastic duct you want to replace. Purchase a flexible metal duct that has the same diameter and is at least 4 inches longer than the plastic duct.

Measure the metal duct and puncture a small hole with the tip of a manual screwdriver to indicate the 4 inches more than the length of the plastic duct. Insert metal snips into this hole and cut the duct to length, making short snips until you've severed the metal duct all the way around.

Remove the plastic duct by unscrewing the clamps that hold it in place or cutting away tape with a utility knife.

Position the metal duct in the gap created by removing the plastic duct. Press the edges inward to crimp, or fold, them and insert the ends in existing ducts. Press inward until the metal duct has around 2 inches in each of the existing ducts or adapters it connects.

Drill four small holes equally spaced around the perimeter of the lip of the old ducts with an electric drill. Press through so you drill through both the outer duct and the new metal duct inside.

Screw metal screws into each of the eight holes you drilled.

Apply mastic all the way around each of the two joints with a paintbrush. Liberally cover both joints, including the screws you just installed. While the screws give the joints structural support, the mastic keeps the duct air tight.