Flex duct CFM chart
How much air each size of flexible duct carries, fully stretched or compressed, and the size you need for a given airflow. Flex is not sheet metal, and the chart shows by how much.
Inputs
Result
Flex duct CFM chart, fully stretched
Airflow each size carries at three common friction rates. Change the installation above to see what compression does to every size.
| Size | cfm at 0.05 | cfm at 0.08 | cfm at 0.10 | fpm at 0.08 |
|---|
Flex is not sheet metal
The inside of a flexible duct is a fabric liner stretched over a wire helix, and air moving through it rubs against a surface far rougher than galvanized steel. ASHRAE puts the roughness of fully extended flex at about 3 mm, against 0.09 mm for galvanized. Run through the same physics, that gives flex roughly twice the friction of metal duct at the same size and airflow.
That is why a size that works in metal does not automatically work in flex. Most of the time the step between sizes absorbs it; near the boundary it will not, and a room comes up short of air.
Compression is what really kills flex runs
Researchers at Lawrence Berkeley National Laboratory measured what happens when flex is installed the way it usually is: a little short, sagging between supports, bunched at the ends. Their results are the compression formula above. An 8 inch duct at 15% compression carries about half what it does pulled tight, and at 30% a 6 inch duct has more than nine times the stretched loss.
The correction weighs heaviest on small ducts, which are exactly the 5 and 6 inch branch runs to bedrooms that most often come up short.
Installing it so the chart holds
Pull it to its full length and cut off the excess rather than coiling it in the attic. Support it every 4 to 5 feet with straps at least 1½ inches wide so it does not sag. Keep bends gentle — a radius of at least one duct diameter — and never kink it over a joist. Keep runs short: flex is for the last stretch to the register, not for trunks.
Questions
How many cfm can a 6 inch flex duct carry?
About 70 cfm at a friction rate of 0.08 and 78 at 0.10, fully stretched. The common rule of thumb of 100 cfm only holds at around 0.16 in. w.c. per 100 ft, roughly double a typical design rate, and at 15% compression a 6 inch run manages around 30 cfm at 0.08.
How many cfm can an 8 inch flex duct carry?
About 150 cfm at 0.08 and 170 at 0.10, fully stretched. Sag it to 15% compression and the same duct carries about 73 cfm at 0.08, which is why installation matters as much as size.
Is flex duct worse than metal duct?
For airflow, yes. Fully stretched flex has roughly twice the friction of galvanized metal of the same size, and compressed flex is far worse. It is cheaper and quicker to install, which is why it is used, but it belongs on short final runs to registers rather than on trunks.
Does flex duct compression really matter?
More than anything else about it. LBNL testing found that compression of 15% can more than quadruple the pressure loss in a 6 to 8 inch duct, and 30% can multiply it nine times in a 6 inch. A badly hung run can carry half the air the same duct carries pulled tight.
What friction rate should I use for flex duct?
The same design friction rate as the rest of the system, usually 0.05 to 0.10 in. w.c. per 100 ft, worked out from the available static pressure and the longest run. Do not use a higher rate to make flex fit; size the flex up instead.
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