equal friction vs velocity

[Gross]

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?

[second opinion]

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

[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

[doc havoc]

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?

[second opinion]

Not knowing how to use one!!!

[tranedog]

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

[tranedog]

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...

[doc havoc]

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.

[Gross]

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??

[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.

[Gross]

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.