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raz 02-28-2009 10:56 AM

4" to 3" reducer in bath exhaust fan - impact?
I know that it is not recommended but what would be the real impacts of using a 4" to 3" reducer in a bathroom exhaust fan duct? The exhaust duct run is straight horizontal, approx 16' in length.

I have the opportunity to greatly reduce installation complexity if I use the existing 3" exhaust ducting and connect it to new 4" exhaust fan via the reducer. I just have to decide if I can live with the impacts. Any/all inputs appreciated.

yuri 02-28-2009 01:49 PM

Reduced capacity of the fan for one. After chili night that can be a big problem.:laughing: LOL. It may also overload the fan and if it overheats then there is an insurance/fire risk problem/situation.

Yoyizit 03-01-2009 11:18 AM

it's in here somewhere

I knew it was here somewhere. . .
Your spreadsheet can handle the fractional exponents in the formula.

At 50 CFM, 3" pipe has 0.74 resistance and 4" pipe has 0.18, but the CFM will drop with 3" pipe. At 25 CFM the resistance of 3" pipe is 0.2.

BTW, I think those kinds of motors can stand an overload indefinitely.
"Impedance Protection is when the windings of the motors are designed so that they limit the amount of heat generated even, in abnormal situations such as a locked-rotor condition. Impedance Protected motors are normally small motors such as the shaded pole type shown. Impedance Protected Motors are in category XEIT2."

raz 03-01-2009 03:00 PM

re:4" to 3" reducer in bath exhaust fan - impact?
Excellent comments folks - thanx so much. And thanx for the referral to the Engineering Toolbox - another cool site.

yuri 03-01-2009 03:28 PM

Make sure you keep it clean. We had a private church school burn half way to the ground because of an overheated bathroom fan and lint. Firewall saved the rest of the complex. Why not buy a fan with a 3" exhaust?

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