Flex ducts and CFMs

[Cli-Max Indoor Air]

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

[Irishmist]

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.

[suemarkp]

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.

[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.

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.

[jerrygr]

There is a ductulator for flex.

[tinmantu]

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"

[coolguysfl]

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

[tinmantu]

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

[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.

[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.

[coolguysfl]

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

[tinmantu]

Thank you for the link coolguysfl!

[dash]

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.

[steve3871m]

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

Nice reply Dash. I see those charts (attached image from OP) in Home Depot etc all the time and hard to supress myslef when I see a home owner looking at one.:D

My ACCA ductlator is nice, has galvanized, helical, ductboard and liner built in. On avg flex about 25% higher friction rate. I compared with J/B and Atco owns friction rates and was right on. The off brands have a much higher friction rate.

Most the charts I see are based on .1"wc which we know doesn't work in the real world depending on P/D and TEL for the total ASP. The one posted here was from NCI at least.

[steve3871m]

2 weekends ago I was in Home Depot (not in uniform) and trolled down the HVAC isle for kicks, it was packed with home owners buying static killing pleat filters and restrictive grilles and some older lady with s/a filters.

This one couple was discussing changing the r/a grille with a new one (in hand) that was stamped faced and el cheapo brand probably 30% restrictive (what no bar spacing ) and also checking out the wood s/a registers...(did I say spacing)
Ugh!!!
Hard to resist speaking up.... Oh well future service call!:D

[coolguysfl]

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.

The 20' term sounds hinky... not my rules.... it's on the back of the manufacturer's ductolator

[mcair]

You can trust the air flow chart you have from NCI.
It's based on actual air flow measurements taken with TSI flow hoods and VelociCalc hot wire anemometers at hundreds of homes across the country.

The ACCA duct calculator is pretty close to what's actually installed in the real world if it's installed properly.

[dash]

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.

You can get the best one at www.acca.org in the bookstore.

[m-cooling]

I dont see how this member got to the professional staus.Come on 6 in flex my in generally will carry 100 cfm not exceding lenghts of 20 ft or so. worked 15 yrs for me so far

[steve3871m]

You can get the best one at www.acca.org (http://www.acca.org) in the bookstore.
Yep best one out there, even better than SMACNA's.

[steve3871m]

I dont see how this member got to the professional staus.Come on 6 in flex my in generally will carry 100 cfm not exceding lenghts of 20 ft or so. worked 15 yrs for me so far Have you ever measured your results with an AF hood or HWA?

If the ASP was figured and correct FR was used (assuming TEL method) then you would be good. A 20' 6" with max FPM of 700 then your looking around .03" FR.

[partroy]

at .10 static 6'' is usually 110 cfm's per 100' of duct as per smacna and 175 cfm's for 7'' and 235 cfm's for 8'' i know this the basic in NJ.

[jrbenny]

I highly doubt your 'static' is .10 IWC.

Manual D. I highly recommend a read through it.

[partroy]

My back .1 iwc

[steve3871m]

at .10 static 6'' is usually 110 cfm's per 100' of duct as per smacna and 175 cfm's for 7'' and 235 cfm's for 8'' i know this the basic in NJ.
STOP!! Do not get caught up in using .1 arbitrarly on the ductaltor. Your new to the industry right? Get a manual D book and learn it. With exception to what we installed I have not seen a correclty desinged duct system in the last year, my area. Reverse engineering is easy and I can see many guy's try using .1" for the whole job, with reality (after reverse engineering) they avg about .2 to .25!!!

Example .1" would only be relavent if the TEL was 200' and there was .20 ASP

[partroy]

Yes your right some times I use .1and a half and .2

[steve3871m]

I can see many guy's try using .1" for the whole job, with reality (after reverse engineering) they avg about .2 to .25!!!

In the above I'm meaning this is bad news .2-.25. for the average situation.

[partroy]

good now that we got that out of our system.We all ways use .1 and some times .1 5 (half) depending on duct work runs fathest from system and cfm's house manual j.

[jrbenny]

Uncle. :rolleyes:

[davidr]

You mean a 6" duct doesn't always put out 100 CFM? :confused:




































:D

[m-cooling]

I dont see how this member got to the professional staus.Come on 6 in flex my in generally will carry 100 cfm not exceding lenghts of 20 ft or so. worked 15 yrs for me so far

I might have jumped the gun here if this poster is an arrchitect making plans ror hospitals,etc. Otherwisemore friction on flex lines slows cfm . especially lines 30 ft or longer. which is illegal here Thanks Brian

[dash]

Yes your right some times I use .1and a half and .2


Sad but honest,and seldom correct.

[m-cooling]

Have you ever measured your results with an AF hood or HWA?

If the ASP was figured and correct FR was used (assuming TEL method) then you would be good. A 20' 6" with max FPM of 700 then your looking around .03" FR.

Ive ussed velometers with stacks to check exact air flow. Im just saying (especially one commercial jobs I cant even get away with much more tan 6 ft. of flex anywhere. I dont believe this would slow any air down. I dont service commercial units daily. Thanks some of your abbreviations were over my head.

[AIR PRO]

.1 and a half and .2?

Are you just guessing at what these #'s mean or are you really serious? :rolleyes:

[partroy]

we are a residential,commercial,and industrial company so we use different static pressures.

[partroy]

Just installed 2 trane 2-stage furnaces 1-3 ton and 1 3.5 ton with coils and condensers at a TD north bank .This was a bid job so the engineer made the specs and designed all duct work at .05 static.the duct is big 24 X 14 returns and 24 X 12 supplies.This is going to be a very slow flow of air .I guess he missed the class on pressurizing the duct.:cool:

[dash]

Just installed 2 trane 2-stage furnaces 1-3 ton and 1 3.5 ton with coils and condensers at a TD north bank .This was a bid job so the engineer made the specs and designed all duct work at .05 static.the duct is big 24 X 14 returns and 24 X 12 supplies.This is going to be a very slow flow of air .I guess he missed the class on pressurizing the duct.:cool:

How did you determine he used .05 ,or could it be .5 static?

Is the main supply trunk 24X12 or are you meaning the supply grilles?

Did you measure the static?

[davidr]

How did you determine he used .05 ,or could it be .5 static?

Is the main supply trunk 24X12 or are you meaning the supply grilles?

Did you measure the static?

He looked at the ductulator. :D

[dash]

He looked at the ductulator. :D

No hints!!lol

[partroy]

The ductulator says .05 less than the .1 what we usually use.The duct is 24 x 14 on the return side and the supply is 24 x 12 this is not the register size the will vary to each room size but they are 2' x 2' ceiling lay in diffusers.:cool:

[dash]

The ductulator says .05 less than the .1 what we usually use.The duct is 24 x 14 on the return side and the supply is 24 x 12 this is not the register size the will vary to each room size but they are 2' x 2' ceiling lay in diffusers.:cool:



What thwe ductolator shows is the FR(Friction Rate) ,not the actual Static Pressure.The FR is the static per 100' of equivalent duct,so if the equivalent feet on the job was 100',which it isn't then it would be the static for the duct,but still not the true static,which includes other PD(pressure drops).


The ESP(external static pressure) of the duct system is likely close to .50 ESP.

[steve3871m]

we are a residential,commercial,and industrial company so we use different static pressures.
Huh? .....

[rsmith46]

Originally Posted by partroy
Just installed 2 trane 2-stage furnaces 1-3 ton and 1 3.5 ton with coils and condensers at a TD north bank .This was a bid job so the engineer made the specs and designed all duct work at .05 static.the duct is big 24 X 14 returns and 24 X 12 supplies.This is going to be a very slow flow of air .I guess he missed the class on pressurizing the duct.



How did you determine he used .05 ,or could it be .5 static?

Is the main supply trunk 24X12 or are you meaning the supply grilles?

Did you measure the static?

He had to mean the ductwork itself was designed at .05 instead of .10 and that the FPM will be low because the ductwork is over sized.

Dash, do you consider oversizing much of a problem?

[dash]

He had to mean the ductwork itself was designed at .05 instead of .10 and that the FPM will be low because the ductwork is over sized.

Dash, do you consider oversizing much of a problem?


Yes if it results in higher cfms then required.Adding a damper or baffle in the main supply and/or return trunk can increase the static ,if needed to get the required cfms.

[djk4502]

Wow some of these posts are all over the place. He used .05 friction rate per 100 foot of duct not .5 static. Like dash said. you cannot use rules of thumbs for sizing ductwork as you should not use them for sizing heating and cooling. Acca manual d is a excellent read and good to size ductwork. Its not there to oversize duct its there to size it properly. Its said to say to many people have been drinking the koolaid for to long and convienced themselves that there guessing in ductwork sizing is correct.