How to Build a Furnace Duct

Decide on a location for building a new duct line off of the furnace’s main supply duct. The main supply duct appears as a rectangular tunnel made of galvanized steel. The duct runs to a main supply trunk where the home’s existing supply air ducts branch out. Make sure the space on the main supply duct can fit an additional duct line.

Locate an area on the main supply trunk to install the new duct line. Standard duct sizes are 6 inches in diameter, so cut a 6-inch diameter hole into the supply trunk wall unless a different-sized duct is needed. Use the duct collar to draw an outline of the hole, then use the hole saw to start the cut. Finish cutting out the hole using tin snips to cut along the outline markings.

Slip the duct collar onto the new duct so that the collar tabs insert inside the duct. Bend the tabs into the duct to form a tight fit.

Connect additional duct sections as needed to reach the room register vent. Join sections together with self-drilling zip screws, using three screws per section. Seal off all seams and gaps along the duct line by wrapping with foil tape.

Construct the room register outlet for the furnace duct. Based on the size of the register vent, cut out a rectangular hole in the floor of the room.

Attach the register boot to the end of the furnace duct. Using zip screws, secure the boot along the edge of the register outlet and into the subflooring material.