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    July 18, 2013
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Very Quiet Bathroom Fan, White Grille, 80 CFM.
 

Any issues using 4" ducting fan with 3" existing duct pipe

Our powder rooms' fan is 16 years old and loud (its the 696ND - says its 2.5 sones but it seems much louder!). I'd like to replace it with a modern quiet fan. The existing ducting is 3", and would be difficult to change as the room is in the middle of the house.

Most of the new quiet fans (< 1.5 sones) have 4" venting.
Is there any issue using a duct reducer (4" to 3") with these fans?

FYI, It a small powder room (< 50 sq ft).
What happens is your static pressure will increase (friction in the pipe) causing your 80 CFM fan to run at about 75 to 78CFM. All depends on the length of 3" pipe. The smaller pipe also increases the speed on how fast the air will travel down the pipe. Being a 80 CFM unit, shouldn't be a problem, but anything larger I would suggest going to a 4" to keep the noise down.
1 year, 1 month ago
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klarcher