# Thread: Flex ducts and CFMs

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## Flex ducts and CFMs

do these sound right for cfm rating of flex duct?

for instance I have been told 6'' flex is 110 cfm

this says 6'' flex is 75 cfm

2. the cfm depends on the static the system is based upon. Usually a 6" round is about 120cfm using galvanized pipe. With flex pipe, there is a greater resistance to airflow so the cfm may be less with a higher pressure. I would anticipate that you would get somewhere between 100 and 120 cfm through the 6" flex. It's not that 6" flex or round pipe "is" or "isn't" a certain cfm. The size pipe is 'capable' of a certain airflow at a certain static pressure.

Can you expand on the context of your question?

All the best, John.

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There is also a FPM limit for each type of pipe. I'm not sure why (noise, turbulence, duct abrasion...), but Manual D lists these limits. Smooth metal duct has the highest limit, and flex the lowest.

So there are two limits: velocity (feet per minute), and static pressure loss. You can't exceed either one.

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Cfms for 6" duct are what they are .depending on the duct system.

First a Manul D to determine the actual Friction Rate (FR)when designing the duct system.

6" round metal
@.04FR=67 cfms

@.06FR=82cfms

@.08FR=96cfms

@.10FR=107cfms

If flex it wil be less cfms in each case.

The more resistance,turns,length,etc., that the duct system has the Lower the FR will be.

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There is a ductulator for flex.

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Originally Posted by onearmsteve
do these sound right for cfm rating of flex duct?

for instance I have been told 6'' flex is 110 cfm

this says 6'' flex is 75 cfm
doesn't sound too far off to me...I deduct 25% airflow when going from solid to flex and run 7" runs instead of 6"

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Originally Posted by dash
Cfms for 6" duct are what they are .depending on the duct system.

First a Manul D to determine the actual Friction Rate (FR)when designing the duct system.

If flex it wil be less cfms in each case.

The more resistance,turns,length,etc., that the duct system has the Lower the FR will be.
To the right I added in the Flex CFM @ the same Static

6" METAL /////// FLEX
@.04FR=67 cfms 52 cfm

@.06FR=82cfms 64 cfm

@.08FR=96cfms 74 cfm

@.10FR=107cfms 82 cfm

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Originally Posted by coolguysfl
To the right I added in the Flex CFM @ the same Static

6" METAL /////// FLEX
@.04FR=67 cfms 52 cfm

@.06FR=82cfms 64 cfm

@.08FR=96cfms 74 cfm

@.10FR=107cfms 82 cfm
Nice specs....how many feet are those based on?.....like a ductulator is based on per 100 feet after calculating in turns and elbows to arrive at a distance. The reason I ask is that I couldn't imagine running 100 feet of running flex on a run and figuring 15 feet per turn and hoping for even achieve 82 cfm
Last edited by tinmantu; 01-26-2007 at 07:03 PM.

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The FLEX #'s I posted are right off the FLEX ductulator. Flex has different rules:

Flex Sizing is based on 20' of EQUIVILENT duct

EQUIVILENT means each 90 = 10' , when 40' EQUIVILENT is met, you have to increase duct size X 1 size... 2' or less = decrease X 1 size

Most of the installations I find with radically excessive duct statics are cause by misinformed contractors using a regulator DUCTALATOR instead of a FLEX ductulator.

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I wish I had access to a flex ductulator so that I could prove to my boss why I want to over size runs instead of going with tin standards ....all I can try to do is use logic. Many times logic loses to profit.

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Originally Posted by tinmantu
I wish I had access to a flex ductulator so that I could prove to my boss why I want to over size runs instead of going with tin standards ....all I can try to do is use logic. Many times logic loses to profit.
Disturbing but true ----

The data is available @ www.atcoflex.com
or call them @ 800-877-3828 and buy a ductulator

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Thank you for the link coolguysfl!

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Originally Posted by coolguysfl
The FLEX #'s I posted are right off the FLEX ductulator. Flex has different rules:

Flex Sizing is based on 20' of EQUIVILENT duct

EQUIVILENT means each 90 = 10' , when 40' EQUIVILENT is met, you have to increase duct size X 1 size... 2' or less = decrease X 1 size

Most of the installations I find with radically excessive duct statics are cause by misinformed contractors using a regulator DUCTALATOR instead of a FLEX ductulator.

Though your flex cfms posted is about the same as an ACCA flex ductolator,I cna't agree about being based on 20' of equivilent feet,it's based on 100' with theirs.

Now atco may have a method ,it's not Manual D,may work fine but so does Manual D.

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