The manual for a certain (100CFM/4") bathroom exhaust fan has a restriction on using 90-degree elbows close to the unit.

Why is this? Is it okay to use 90-degree elbows further down the exhaust path? (The image would suggest so, but I guess it couldn't hurt to check) Is there any good reason for this restriction?

Fan Link: https://www.homedepot.com/p/Delta-Breez-GreenBuilder-Series-100-CFM-Wall-or-Ceiling-Bathroom-Exhaust-Fan-ENERGY-STAR-GBR100/204347760

Image from Manual: Image from Manual

1 Answer 1


The squirrel cage fan relies on velocity, not pressure, to move air. A near elbow stops the momentum and reduces airflow rate. If you need 90 degrees close to the fan, put 2 90s together and swivel the joints to make it into 2 45s.
Down the path a 90 elbow is fine. If the run is long or has many bends, use a double 45 or try to keep the path as straight as you can. At the outside, find a vent cap that seems less restrictive. The flapper valve in the fan should open and close freely. You can often hear it drop closed when you turn off the fan. Wait a couple of seconds til you hear it.

  • That sounds right to me. I'll wait a bit before accepting your answer to see if anyone else wants to respond as well. Thanks! Dec 16, 2018 at 19:16

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