Re: How do you determine the proper duct size for a fan duct?
The only thing I know about ducting I found out when installing my kitchen fan and wanted to discharge outside. The ventilation company told me that when using corrugated ducting the actual flow is half of the duct diameter. In other words 6" corrugated ducting has the same flow as 3" smooth ducting. This has to do with turbulence created by the irregular surface in the corrugated ducting.
I also changed my bathroom duct to smooth ducting and noticed a significant change in air movement.
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