How to cut duct work for a reducer

Written by daniel holloway
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How to cut duct work for a reducer
(ventilation ducts image by Ragne Kabanova from

HVAC technicians often use a reducer on the main trunk line of a duct to create a smooth transition from a larger size duct to a smaller one. A ductwork reducer helps the airflow maintain a higher velocity, meaning that the air will reach farther parts of the home. Many common duct sizes are available for purchase through warehouses, along with reducers and other fittings. However, there is a simple method for cutting ductwork to accommodate a reducer for any size rectangular ductwork.

Skill level:

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Things you need

  • Left and right tin snips
  • Sheet metal screws
  • 28-gauge sheet metal
  • Permanent marker
  • Tape measure
  • Two sizes of duct
  • S-straps
  • Drive cleats
  • Duct hangers
  • Sheet metal hand bender

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  1. 1

    Lay out and measure the two pieces of ductwork you wish to combine with a reducer. At least one dimension should be the same to keep the ductwork on a level plane. For instance, you can reduce an 8- by 16-inch duct to 8- by 12-inch. The larger of the two ducts will remain whole, so you can set it aside or secure it in place on the main trunk line.

  2. 2

    Mark the smaller duct 8 inches from the end on one side that is equal in length to the larger duct. For instance, if you are reducing an 8- by 16-inch duct to 8 by 12, cut one 8-inch side to accommodate the reducer.

  3. 3

    Use tin snips to cut out one side of the duct from the end to the 8-inch mark you made.

  4. 4

    Secure the smaller duct in place by placing the cut end into the larger duct. Secure the uncut side of the duct with a drive cleat. Level and plumb the duct and hold in place with screws and duct hangers.

  5. 5

    Measure the angle from open end of the larger duct to end of the cut opening on the smaller duct. Write down this measurement.

  6. 6

    Create a pattern out of sheet metal using the measurement of the angle between the ducts and the outside measurement of the ducts. Cut out and fold the pattern using a sheet metal break or bending tools. The finished pattern will have three planes and will be triangular.

  7. 7

    Screw the sheet metal pattern in place on the duct. Secure to the larger duct with a drive cleat.

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