Flex Duct Engineering Study Shows Quality of Installation Makes All the Difference
One of the primary reasons for the ever-increasing popularity of flex duct in new construction is ease of installation. Paradoxically, however, it is this very attribute that leads to its biggest drawback. Substandard flexible duct installation is a common and widespread problem that can have a detrimental effect on system performance.
When flex duct is properly installed according to local codes and manufacturer specifications, it can perform almost as well as a rigid metal duct system. But flex duct has higher friction losses than sheet metal duct due to its spiral wire construction, and turbulence due to sagging, kinks, runs that are too long or improperly supported, too many bends, and sharp turns can significantly increase resistance. In a study conducted several years ago at Texas A&M, researchers found virtually no difference in pressure drop between flex duct and rigid metal duct when properly stretched and installed correctly. But at just 4% compression, or “slack”, flex duct moved 37% less cfm than metal duct. At 15%, friction rates doubled, and quadrupled at 30%. Not surprisingly, duct performance levels were abysmal at 45% compression.
To ensure maximum efficiency, flex duct systems must be well designed and properly installed. The most basic and important requirement of flex duct installation is to use the minimum length of
duct for each connection and ensure that the inner liner is pulled tight to reduce air resistance. Sag should not exceed ½ inch per foot, and support straps should be at least 1.5 inches wide and spaced at short intervals no more than four feet apart. Bend radius should equal or exceed one duct diameter, but an even better practice is to use rigid metal for bends and flex duct for straight runs. Avoid too many ducts coming off the plenum, aka the dreaded and inefficient “ductopus”. And, finally, purposeful oversizing of HVAC equipment to compensate for expected air flow problems due to poor flex duct installation is an inadequate, not to mention inappropriate, fix.
