How to Install a Branch on a Heating Duct Main Line

Hold a bell-mouth fitting against the side of the main trunk line in an open area. Ensure the fitting sits at least 24 inches away from any duct fittings that change the direction of or reduce the size of the trunk line. Pull a permanent marker along the inside edge of the fitting to transfer the opening size to the trunk line's sheet metal. Remove the fitting a set it aside.

Set the tip of a flat-head screwdriver in the center of the outline marked on the side of the trunk line. Hold the handle of the screwdriver at a 30-degree angle to the duct. Hit the tip of the screwdriver forcefully with a hammer to drive it into the metal. Move the screwdriver to place the handle at a 90-degree angle to the duct. Hit the screwdriver tip repeatedly with the hammer until you have a slit that reaches one of the marked lines. Push the screwdriver handle toward the floor to spread the metal apart.

Insert the tip of either a left or right cutting aviation snip into the slit. Cut along the marked line until the hole is complete. Discard the metal. Pull the protective paper away from the adhesive foam -- located on the back of the bell-mouth fitting mounting flange. Align the inside edges of the fitting with the sides of the opening. Push the fitting against the side of the trunk line.

Secure a 5/16-inch hex head adapter in a screw gun. Run one #10 self-tapping screw through each hole punched in the flange of the fitting flange with the screw gun.

Apply a 1/16-inch-thick coat of sealing mastic to the connecting point between the fitting and the trunk line with a paintbrush. Allow the sealant to dry for 24 hours before continuing.

Cut a piece of flex to the length needed to reach from the trunk line to the room with an insulation knife. Slide one end of the flex over the opening of the bell mouth fitting until it tightens to the taper of the fitting. Wrap a flex strap around the inside liner of the flex. Slide the jaws of a flex gun over the end of the strap. Squeeze the handles of the gun together until the flex is tight to the fitting.

Pull the outer flex insulation over the inner. Wrap a flex strap over the insulation. Pull it tight with the strap gun to hold the insulation in place.