# Thread: Maximum air flow thru 12 flex duct?

1. Originally Posted by garyed
What you're forgetting is the static pressure is usually based on 100 ft. of duct length. If you try to move 800 CFM through 100 ft. of 12" flex then your static is going to be high like around 2.5 but if you're moving it through only 20ft. of duct its going to be around .05. Your velocity doesn't change so you're still going to be around 1000 fpm but the shorter length is what gives you less friction.
Help me understand this, you can move 1000 fpm velocity through 20ft of 12 duct at .05 static but at 100ft same size and same velocity I have .25 static?(you did mean .25 and not 2.5 correct?).

Say if i have 100 of 12 duct and i measure .05 static at the outlet of the duct with the grill removed am i moving 380cfm at less than 500fpm?

Or do i need to measure the static pressure drop between inlet and outlet, total equivalent length of duct, diameter of duct and FPM to determine CFM?

Ductulators and duct curve charts are not friction charts they are really FRICTION LOSS CHARTS.

After reading your post im a bit confused as to how to use this info for practical purposes and what measurements need to be taken to determine what.

Using TESP and a fan curve chart is easy enough and sizing a duct using a ductulator and a standard .01 friction loss is easy enough but i guess i lack the real intimate knowledge of measuring air flow through ducts.

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2. ^^^ that.

3. Originally Posted by Core_d
Help me understand this, you can move 1000 fpm velocity through 20ft of 12” duct at .05 static but at 100ft same size and same velocity I have .25” static?(you did mean .25 and not 2.5 correct?).

Say if i have 100’ of 12” duct and i measure .05” static at the outlet of the duct with the grill removed am i moving 380cfm at less than 500fpm?

Or do i need to measure the static pressure drop between inlet and outlet, total equivalent length of duct, diameter of duct and FPM to determine CFM?

Ductulators and duct curve charts are not “friction charts” they are really FRICTION LOSS CHARTS.

After reading your post im a bit confused as to how to use this info for practical purposes and what measurements need to be taken to determine what.

Using TESP and a fan curve chart is easy enough and sizing a duct using a ductulator and a standard .01 friction loss is easy enough but i guess i lack the real intimate knowledge of measuring air flow through ducts.

Sent from my iPhone using Tapatalk
You would measure the static at the beginning of the run, and at the end. That friction loss then compared in a FR slide rule, would tell you how much air you are moving. If you can calculate the TEL of that run.

4. Originally Posted by Core_d
Help me understand this, you can move 1000 fpm velocity through 20ft of 12” duct at .05 static but at 100ft same size and same velocity I have .25” static?(you did mean .25 and not 2.5 correct?).

Say if i have 100’ of 12” duct and i measure .05” static at the outlet of the duct with the grill removed am i moving 380cfm at less than 500fpm?

Or do i need to measure the static pressure drop between inlet and outlet, total equivalent length of duct, diameter of duct and FPM to determine CFM?

Ductulators and duct curve charts are not “friction charts” they are really FRICTION LOSS CHARTS.

After reading your post im a bit confused as to how to use this info for practical purposes and what measurements need to be taken to determine what.

Using TESP and a fan curve chart is easy enough and sizing a duct using a ductulator and a standard .01 friction loss is easy enough but i guess i lack the real intimate knowledge of measuring air flow through ducts.

Sent from my iPhone using Tapatalk

You are correct because at the same velocity your friction rate increases with the increased length & also about my decimal being in the wrong place, it should be .25. Now I have only measured static at the beginning of a run but I would assume the static would decrease proportionally as you get further to the end of the run. I only measure velocity at the outlet of duct & I would assume there would be very little static near that point. I was never schooled in duct design & whatever I've learned & think I know just comes from my own reading & research so I'm definitely no authority on this issue.

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ok system is brand new only 2 months old. manufactures specs say constant cfm ecm fan motor will deliver 700 cfm at all times so i am pretty sure its moving 700 cfm im just not completely understand how when its only 12" duct... keep in mind there is only 14 feet of return flex and 55 feet of supply flex

6. Originally Posted by freonfreak
ok system is brand new only 2 months old. manufactures specs say constant cfm ecm fan motor will deliver 700 cfm at all times so i am pretty sure its moving 700 cfm im just not completely understand how when its only 12" duct... keep in mind there is only 14 feet of return flex and 55 feet of supply flex
Usually they say will move set CFM at up to X amount of static/esp. Which is usually .8 or 1" of esp.

7. Max external static is often on the nomenclature tag these days.

A few years ago, had this heat pump tripping out on high head. TESP was 1.0" wc, but the tag said max was to be 0.6", new York RTU. So that means too much airflow restricted, on the order of 67%. Which is approaching twice the max rating.

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i think everyone is missing my point.the duct size is to small as per the duculator and recommended .05 column you should size ducts for but with 0.43 total static clearly the ducts are not to small soo what is going on is the duculator wrong or am i loosing my mind on this one!!

9. Originally Posted by freonfreak
i think everyone is missing my point.the duct size is to small as per the duculator and recommended .05 column you should size ducts for but with 0.43 total static clearly the ducts are not to small soo what is going on is the duculator wrong or am i loosing my mind on this one!!
Its more your understanding of duct work sizing that is wrong.

There is NO you should use .05 friction rate. Its just a incorrect rule of thumb.

10. Did you take your return static at the air handler between it and the filter. Where was your probe for the supply static. Does the air handler have a plenum, and then the 12" flex is connected to the plenum?

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return static between air handler and filter is 1.1.. probe was at beginning of supply flex witch is just a collar tapped onto air handler..

12. Originally Posted by freonfreak
return static between air handler and filter is 1.1.. probe was at beginning of supply flex witch is just a collar tapped onto air handler..
If return static alone is 1.1". Then you can't have a total static of only .43"

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