choosing proper friction rate for flex duct

[acdude]

i am using a standard trane ductulator ,hand held slide type,and have been using an 0.1/0.2 friction rate to calculate duct size.

My issue is this,id like to find one rate to use as a standard for a rule of thumb type process and if its not the correct rate what is a good one to choose?

I am a Texas contractor and i am using r-6 flex duct in mostly residential applications..

can someone help?
TIA

 

[STYMIEDPIPER]

Hope this chart is what you are seeking

http://www.hartandcooley.com/flex/air_friction_chart.pdf

Good Luck

 

[imok2]

The generally recommended
design pressure drops for residential and light
commercial duct systems are 0.10 in. w.g. for the
supply side of the system and 0.05 in. w.g. for the
return side of the system. In some cases, particularly
in light commercial systems, 0.08 in. w.g. is used for
the return side. This reduces the overall pipe sizes
in the return duct system, but also results in higher
velocities and may increase noise levels. If system
noise levels are of importance, the design pressure
drop of 0.05 in. w.g. should be used.

 

[STYMIEDPIPER]

Referring to my response posted a few minutes ago

The older Hart & Cooley catalog (1995/1996)had the following title
"Flexible Duct Air friction Chart" which is now labeled on their web site as "Air Friction Chart"

Quick glance yields that the two charts are the same.

 

[acdude]

thnaks i guess the .1 i have been using is the correct rate..ill use the .05 for return and see how that works..

thanks

 

[RossABQ]

If you are hanging the flex duct over rafters, or otherwise routing them so that they are not straight, then a factor needs to be applied to the friction values shown on a Ductulator or the referenced chart above.

 

[trashcanman]

For flex duct, I would double the DP for straight duct. Thus, if a certain size straight duct produced 0.10"/100', flex duct same size will require 0.2"/100' for same CFM.