# Thread: equal friction vs velocity

1. ## equal friction vs velocity

Ok so I think I'm right but I'm around so much that is wrong I might be hurting my confidence. Everyone here is screaming that I should be creating ducting at .1 friction per 100'.....which is fine...but on a 4 ton system thats putting me at 1100 fpm....From everything I know thats awfully high, isn't it?
I design it for 700 fpm(especially bc the client complained about noise)....granted that takes me down to.03....but i dont see that as a bad thing...just means i have a 20x18 duct. wont i lose a LOT at 1100 fpm?

Am I an idiot (for this reason specificially) or are they right?

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invest in ear plugs. 0.1 is to high try 0.06 for your supply

3. the difference you need to look at is Trunk versus Branch ductwork.... I have always designed my ductwork by velocity and have never gotten in trouble yet but I also designed my TRUNK Ductwork at different velocities then my BRANCH ductwork.... my Ext. Statics have always been great at no noise issues.... I also stop using DUCTULATORS they sux. try this formula---- duct sq.ft. x fpm or (radius sq. x pi)/144 x fpm example - 10 inch flex = 78.53982 sq.in./144 = 0.545415 sq.ft. x 500 fpm = 272.7077 cfm

4. Originally Posted by tranedog
the difference you need to look at is Trunk versus Branch ductwork.... I have always designed my ductwork by velocity and have never gotten in trouble yet but I also designed my TRUNK Ductwork at different velocities then my BRANCH ductwork.... my Ext. Statics have always been great at no noise issues.... I also stop using DUCTULATORS they sux. try this formula---- duct sq.ft. x fpm or (radius sq. x pi)/144 x fpm example - 10 inch flex = 78.53982 sq.in./144 = 0.545415 sq.ft. x 500 fpm = 272.7077 cfm
According to my ductulator, it would take a 9 inch flex to move 270 cfm at a friction of .08". Since flex doesn't come in 9" dia. I would move up to a 10" and arrive at the same size. What sux about a ductulator?

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Not knowing how to use one!!!

6. flex duct does come in 9" it is the last odd diameter it does come in but I like even numbers too... lol

7. no I have mastered using ductulators it is just much easier not having to run to another device when I can just do the math in my head...

8. It's funny, the different opinions on friction. I was taught to use .08" for supply and .07 for return. I have spoken with some who use .1" for supply and .08" for return. Now, I see someone using .06". I guess as long as velocity is kept below 1000 fpm and a suitable amount of static pressure is generated, a system will work reasonably well?

I've also known people, when using nothing but 6" pipe for the branches, will count the number of runs, multiply by 2 and add 2 to get the width of an 8" rectangular duct. For example: a house with 12 runs 12X2=24 24+2=26, so an 8X26 duct would be needed.

9. arghhhh...i really need to take that nci course on air balancing...

so .08 at 1600 cfm still puts me at 1000 fpm....so again...whats wrong with 700fpm? shouldnt it be whisper quiet??

10. 700 fpm would be quiet. However, with that low of friction, I would be concerned about the systems ability to generate enough pressure to give you good airflow out your registers.

11. Originally Posted by doc havoc
700 fpm would be quiet. However, with that low of friction, I would be concerned about the systems ability to generate enough pressure to give you good airflow out your registers.
hm. right outta manual d and on the back of the ductulator :recommended velocty for residential main ducts 700-900 fpm.,...maybe we've all been doing it wrong and didnt know it.

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700 for the supply is fine.

13. side note: how long does it take them to get back to you on a professional membership?

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