I came across a good post on how a 6 inch duct will not always deliver 100cfm. The writer brings up a good point with regards to how to size ducts and in particular the careful and correct use of a ductulator.
In an ideal world, a duct sized at say 0.1 inches per 100 feet should give you 100cfm if the duct size is 6″. However, there is a big difference between a theoretical calculation that you can do on a ductulator vs the real world application. The way the duct is applied has a lot to do with the delivered air flow. If its a residential application and the duct is a spiral that is laid down say in the attic, then there is great chance that kinks and other obstructions can impede the airflow reducing the airflow.
To make matters worse, most residential fans are typically fractional horsepower fans, meaning that their power or “oomph” to deliver air does not react well to these obstructions. You end up having quite low airflows. In addition, the inability to measure airflows on small jobs makes it harder to tell how well the duct is performing.
So what is the best way to deal with this. A couple of things should help,
- When sizing your ducts on a ductulator, try using a lower friction rate to allow for bigger duct size.
- Keep in mind that when installing that the less obstructions you have on your duct the better you will be able to achieve your design airflow.