Installation of Round Duct Fittings Onto Rectangular Ducts

Written by c.l. rease
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Installation of Round Duct Fittings Onto Rectangular Ducts
Round ductwork improves air flow through a heating-and-cooling system. (air conditioner, conditioning image by Greg Pickens from

Friction loss caused by inefficiently designed duct systems increases your utility bills by increasing the workload of the system's air handler. Installing round pipe branch lines in your ductwork decreases the surface area of the duct making contact with air flowing through the system, reducing friction loss. To install round branch lines, you must attach round duct fittings to the rectangular trunk lines moving air throughout your home. Two types of round fittings are common to duct systems. A third type of round fitting, the tap-in collar, is available but causes turbulence when connected to a rectangular duct.

Skill level:

Things you need

  • Bellmouth fitting
  • Tape measure
  • Dividers
  • Center punch
  • Hammer
  • Flathead screwdriver
  • Aviation snips
  • #10 self-tapping sheet metal screws
  • Screw gun with a 5/16 inch chuck adaptor
  • Duct sealer
  • 2-inch wide paintbrush
  • Permanent marker
  • Fibreglass duct spin collar
  • Duct board knife

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  1. 1

    Measure the diameter of the narrow end of a bellmouth fitting with a tape measure.

  2. 2

    Set a pair of dividers to one-half the diameter of the bellmouth fitting plus 1 1/2 inches. Make a depression on the duct with a centre punch and hammer to mark the centre of the bellmouth location.

  3. 3

    Place one leg of the dividers on the depression made by the centre punch. Swing the dividers in a circle to mark the cut line for the bellmouth fitting.

  4. 4

    Set the blade of a flathead screwdriver on the centre mark of the hole. Tilt the screwdriver at a 15-degree angle to the sheet metal duct. Hit the head of the screwdriver toward the duct with a hammer. Raise the screwdriver to a 45-degree angle when you cut through the metal. Continue hitting the head of the screwdriver until you have a cut extending from the middle of the hole to the draw line.

  5. 5

    Cut around the divider line with a pair of aviation snips. Discard the metal cut from the hole.

  6. 6

    Pull the paper backing from the foam gasket located on the back flange of the bellmouth fitting. Align the fitting with the cut hole. Press the fitting firmly against the side of the duct.

  7. 7

    Run one #10 self-tapping sheet metal screw into each prepunched hole in the mounting flange using a screw gun with a 5/16-inch chuck adaptor.

  8. 8

    Apply duct sealer around the installed bellmouth with a 2-inch-wide paintbrush. Allow the duct sealer to cure before turning on the air conditioning system.

  1. 1

    Mark the location for the spin collar on the side of the fibreglass duct with a permanent marker.

  2. 2

    Set the spin collar against the duct. Align the spin collar with the marks you placed on the duct with the permanent marker. Run the permanent marker along the inside edge of the spin collar. Set the collar aside.

  3. 3

    Cut along the marked line with a duct board knife. Remove the material from the hole. Cut a 1-inch slit starting from the cut hole toward an outside edge of the duct.

  4. 4

    Align the "V" notch located on the collar flange with the slit. Turn the spin collar clockwise to thread the collar into the duct.

  5. 5

    Apply duct sealer to the connection between the spin collar and the fibreglass duct with a 2-inch-wide paintbrush. Allow the duct sealer to cure before turning on the air conditioning system.

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