How to Decrease the Diameter of Ductwork

Ductwork is a round or rectangular enclosed channel designed to transport gas, such as air, at low pressures. It is often necessary to reduce the diameter of ductwork for reasons related to location or performance. Generally, you can decrease the diameter of ductwork by installing a reduced fitting.

Reducing fitting
Duct tape
Smaller diameter duct
Wrench

Instructions

- 1
  Determine the degree of duct diameter reduction based on the reason for the change. For instance, if a 12-inch diameter duct is not conveying at a sufficient pressure or velocity, a reduction to a 6-inch diameter duct would likely achieve the desired change.
- 2
  Locate a proper break point in the existing duct. This is generally a duct flange. Disconnect the duct flange using the wrench. Duct flanges generally have a lip and several bolt holes with nuts and bolts. Use the wrench to loosen the nuts and remove the duct.
- 3
  Install the reducing fitting at the duct flange. The reducing duct flange will match up with the existing duct flange on the larger diameter side.
- 4
  Secure the reducing fitting using the nuts and bolts from the original duct flange. Tighten with the wrench. There should be gaskets within the flange connection. If there are no gaskets, wrap the flange with duct tape to seal any gaps.
- 5
  Connect the smaller diameter duct to the smaller side of the reducing duct flange. This will have a similar connection flange as the larger side.
- 6
  Secure the reducing fitting using the nuts and bolts from the original duct flange. Tighten with the wrench. There should be gaskets within the flange connection. If there are no gaskets, wrap the flange with duct tape to seal any gaps.

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