01-30-2009, 11:04 AM
If you're using a 10" fan, then the larger duct is practical. My duct run is only 4', so the 4" duct works fine, but you're right: if the run is too long or includes multiple elbows, the smaller duct will restrict the airflow. There's a trade-off between duct size and fan capacity, too: a small capacity fan will have as much trouble moving air through a relatively short, large-diameter duct as it will with moving it through a long 4" one.
Wayne
Wayne