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I have a range hood in my kitchen that moves air from a 6 in pipe reduced down to a 4 in pipe. This causes air flow turbulence which makes the operation noisy and inefficient. I'm thinking that if i can reduce the speed of the blower motor the hood would work more efficiently and quietly. The blower motor is 190 watts, has 3 preset speeds and is 120v/60hz. Is there a way to lower the preset speeds?

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Simply put replace the 4" pipe with 6" pipe. 4" pipe is just too small for a range hood. This is going to be the only realistic way to make your range hood more efficient.

To put this into perspective....

  1. The air movement volume is based upon cross sectional area.
  2. The 4" pipe cross sectional area is 12.56 square inches.
  3. The 6" pipe cross sectional area is 28.27 square inches.
  4. The 6" pipe can thus handle more than twice as much air flow
  5. Bends, corners and outside vent covers for 4" pipe are going to be much restrictive than corresponding 6" components.

So do the job right rather than making a science project out of modifying your range hood motor speeds!!

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I think your only safe bet is to find another switch with 5-speeds. This should start at a lower wattage to slow the fan further. You'll need to match the existing switches specifications, this should be embossed on the switch.

Otherwise, replace the 4" duct with a 6" or install a cone shaped reducer if it's not already. You're quite correct going to a smaller diameter can cause a rumble, but really the low setting shouldn't be causing anything noticeable.

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