Does Duct Length Affect Cfm?

For straight duct sections, the effective length is the same as the length. ... In my article on this research, I showed from their results that a 6″ duct moving 110 cfm when pulled tight will move only about 70 cfm with 4% linear (longitudinal) compression and about 40 cfm or less at 15% compression.

Does reducing duct size increase cfm?

The key takeaway here is that air moves from a larger to a smaller duct, the velocity increases. When it moves from a smaller to a larger duct, the velocity decreases. In both cases, the flow rate — the amount of air moving through the duct, in cubic feet per minute — stays the same.

Does length of duct matter?

The size of your heating and cooling equipment dictates the size of your duct work. When your home was initially built, that was the easiest time to ensure the ducts were sized and installed correctly. However, just because your duct work is “out of sight” doesn't mean it should also be “out of mind.”