Many reducing trunk systems really don't need to be reduced at all, since they are short, less then 24 linear foot.
As one poster stated, there are calculations done based on how many CFM will be moved through the next duct. These calcs are from manual D. Manual D is what you would use to determine what friction rate to use to size the trunk and supply lines. Using an arbitrary friction rate is asking for a bad duct design, and air flow problems.
You might want to get a copy of Manual D, and go through it. You will be surprised when you find out that there is virtually no such thing as a less then 100 foot total equivalent length duct system. And .1" friction rate is virtually never the correct friction rate to use. You'll also learn that friction rate and static pressure are not the same thing. Even though hordes of installers think it is.
As one poster stated, there are calculations done based on how many CFM will be moved through the next duct. These calcs are from manual D. Manual D is what you would use to determine what friction rate to use to size the trunk and supply lines. Using an arbitrary friction rate is asking for a bad duct design, and air flow problems.
You might want to get a copy of Manual D, and go through it. You will be surprised when you find out that there is virtually no such thing as a less then 100 foot total equivalent length duct system. And .1" friction rate is virtually never the correct friction rate to use. You'll also learn that friction rate and static pressure are not the same thing. Even though hordes of installers think it is.
