# Flex Duct CFM chart?



## user_12345a (Nov 23, 2014)

Depends on the friction rate - i like http://efficientcomfort.net/asp/ResDuct_Web/ResDuct_Web.asp


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## digitalplumber (Jul 8, 2011)

used .1 on the sliderule, thought that was pretty much the standard.


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## beenthere (Oct 11, 2008)

digitalplumber said:


> In the following chart: http://buildingincalifornia.com/wp-content/uploads/2014/03/duct-sizing-chart.pdf, it states that a 9 inch flex duct has a CFM rating of 225.
> 
> On a slide rule duct calculator I have, it states that a 9 inch flex duct is rated at about 205.
> 
> Which is it? where can I get accurate CFM flex duct values?


Depends on which compression the chart maker used, as to what CFM it will flow. And what FR the they used.



digitalplumber said:


> used .1 on the sliderule, thought that was pretty much the standard.


It is. It is also usually the wrong value.


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## beenthere (Oct 11, 2008)

A typical 15' linear run of flex, is 80' equivalent length. Add in the trunk line and return equivalent lengths, and you have well over 200 foot of total equivalent length. So using .1" FR for all of the duct work means you will loose over .2" of static pressure.

Using an air handler with a .4" ESP rating. Subtracting .03" for supply register PD, .03" for return grille, .03" for supply damper, .07" for fiberglass air filter. That leaves you with an ASP(available static pressure) of .24". Using a total equivalent duct length of 285'. 

.24/(285/100)=.08" FR


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## digitalplumber (Jul 8, 2011)

beenthere said:


> A typical 15' linear run of flex, is 80' equivalent length. Add in the trunk line and return equivalent lengths, and you have well over 200 foot of total equivalent length. So using .1" FR for all of the duct work means you will loose over .2" of static pressure.
> 
> Using an air handler with a .4" ESP rating. Subtracting .03" for supply register PD, .03" for return grille, .03" for supply damper, .07" for fiberglass air filter. That leaves you with an ASP(available static pressure) of .24". Using a total equivalent duct length of 285'.
> 
> .24/(285/100)=.08" FR


 Well that confuses me more, using the .08" on the slide rule brings 180 CFM, for the example above. 

So based on that, .08", a room needing 200 CFM (according to the slide rule) needs a 9.25" duct. So a ten would be used, is that correct?


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## beenthere (Oct 11, 2008)

digitalplumber said:


> Well that confuses me more, using the .08" on the slide rule brings 180 CFM, for the example above.
> 
> So based on that, .08", a room needing 200 CFM (according to the slide rule) needs a 9.25" duct. So a ten would be used, is that correct?


Or 2-7" to maintain velocity, or to get proper throw.


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## digitalplumber (Jul 8, 2011)

So where do you start with determining friction rate? Does each blower/furnace have a rating?


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## beenthere (Oct 11, 2008)

All furnaces and air handlers have a ESP rating, some list it a test rating. You subtract all external device pressure drops from that rating. That gives you the ASP(available static pressure). Then you use the total equivalent length of the duct to determine what FR to use. By using the formula I posted earlier.


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