How to Replace Ducting

Determine the type of ductwork that you have in your house by sticking the duct knife into the side of the duct's foil covering. If the knife goes all the way into the ductwork, the duct is made from fiberglass. A knife blade that only goes in an inch represents metal duct. Place a piece of foil tape over the cut made by the knife to reseal the ducting.

Draw a rough sketch of your housing ducting. This may entail crawling through your attic or basement, but by thorough. Include each area that the duct changes size, branch lines that attach to the trunk line that comes off of the air handler unit, and flexible duct connections.

Measure the size of the duct work with the tape measure. Measure the outside dimensions of your ducting if it is made of fiberglass. If you have metal ductwork, cut the corners of the exterior insulation. Roll back the insulation to expose the metal ductwork. Measure the dimension of the metal ductwork. Measure the ductwork at each point that it changes size. Mark the dimensions of the ductwork on the rough sketch that you drew in step 2. Add the length of each section of ductwork. For example, 20 feet of 18 by 12 duct, 14 feet of 16 by 10, and so on.

Remove the ducting of your house after the new ducting arrives. With the hammer and screwdriver, break each connection between the lengths of metal duct. Insert the flathead screwdriver into the rolled flange, and hit the blunt end of the screwdriver with the hammer. Pry the screwdriver away from the duct to break the connection, repeat the process on the other side of the duct. For fiberglass duct, located the tape joints, and cut each joint apart with the duct knife.

Release the duct from its hangers, by matching a screw tip attachment for the drill with the fastener used to hang the duct. Hold the ductwork while you remove the screws from the hangers, as the ductwork will drop when the screws are removed.

Install the new ductwork as you remove the old, to keep the system balanced. When a round tap that feeds flexible duct or a branch line is needed, place the tap on the side of the ductwork. Trace around the inside of the tap with the permanent marker. Remove the tap. Place the tip of the flathead screwdriver in the marked area. Tilt the handle of the screwdriver toward the duct. Hit just above the tip of the screwdriver with the hammer. The screwdriver will penetrate the duct. Continue hitting the screwdriver with the hammer until you have a cut in the duct, 1/2 of the way across the tap. Cut along the traced line with the aviation snips. Fasten the tap to the duct with self-tapping sheet metal screws.

Attach the flexible duct to the tap after the duct work has been re-installed. Wrap a flexible duct strap around the flexible duct. Tighten the strap with the strap tool to secure the flexible duct to the tap.

Continue replacing the ducting one piece at a time until the ducting has been completely replaced.