How to Install Metal Ducts

Measure the length of the ducting needed, using a tape measure. Add 10 percent to the measurement to account for possible waste or incorrect measurements.

Lay the metal ducting on the floor in the room where it is to be installed. Test fit the pieces that will make up the duct run to determine that you have an adequate amount and to reveal any flaws before beginning installation.

Insert a metal sleeve insert into the end of the first piece of ducting. This forms the piece that is inserted into the main heat vent from the furnace. Wrap the joint between the sleeve and the duct piece with foil-backed tape and smooth the seam for proper adhesion around the entire surface.

Add another insert to the other end of the ducting and fit the next piece into the open end of that sleeve. Continue inserting sleeves and duct pieces until the run is complete. To cut duct pieces to size, measure the amount needed and cut off the remainder with tin snips.

Place a piece of ducting against the side of the main furnace duct and outline its shape with a marker. Make a hole in the existing duct with a sharp hole punch and a light tap with a hammer. Insert the tin snips through the hole and cut the opening along the drawn outline.

Position a collar in the hole and tape it smoothly with strips of foil-backed tape. Lift the completed ductwork to the ceiling and have assistants hold it in place, making sure there are enough of them to support the ducting evenly so that the seams don't break at the sleeves and the ducts don't bend.

Attach metal support hangers every 2 feet along the run by holding one tightly against the bottom of the duct while hammering the hangers through the wooden joists using 1-inch framing nails. Fit hangers only over the metal ducts, not over the sleeves.

Tape the seam between the collar and the duct system, and check all seams for gaps or splits.

Connect angled or junction vent places by inserting them into the duct run and positioning the vent panel through the floor or down into the room for heating. Tape this seam in the same manner as the rest of the seams.