How to Reduce Ducting in a Basement

As ducting runs from your furnace to each successive vent, it must be reduced in size. This reduction in size is necessary because as air leaves each vent, the velocity in the ducting downstream of that vent will decrease unless you reduce the size. If the velocity falls too low, the air will remain in the ducting for a time longer than what was intended. This leads to heat loss through the ductwork into the surrounding air, and as a result, your system will not be as efficient.

Gloves
Tape measure
Pipe crimper
Sheet metal screws
Cordless drill
Duct sealant (optional)
Caulk gun (optional)

Instructions

- 1
  Measure the diameter of the existing ducting that you need to reduce.
- 2
  Calculate what size you need to reduce your ducting to. Use an online duct calculator (see Resources) to determine your new duct size by the amount of CFM, or cubic feet per minute, of air that will flow through it. For example, if your ducting will be flowing 200 CFM after the reduction point, you will need to reduce to 8 inches.
- 3
  Select the correct reducer. For example, if you need to reduce from 10 inches to 8 inches, you need a 10-inch-to-8-inch reducer.
- 4
  Examine the end of the existing ducting and ensure it is crimped. If not, crimp the end with your pipe crimper. Repeat for the smaller end of the reducer.
- 5
  Slide the large end of the reducer over the crimped end of the ducting about 1 1/2 inches. Fasten the reducer to the ducting with sheet metal screws.
- 6
  Slide the uncrimped end of the smaller size ducting over the small end of the reducer by about 1 1/2 inches. Fasten the ducting to the reducer with sheet metal screws.
- 7
  Install further ducting as needed to finish the system. Further reductions may be necessary as your CFM volume decreases.

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