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On a 3 ton attic job some of the guys I know in the business will use 7 (or 6) flex duct to the rooms in the house. So, using the duct calculator using .10 friction per 100 ft. a 7 inch duct will supply about 160 CFM to a room.

I was wondering how many Btu's per hour will be removed from the room under those conditions?

2. Originally posted by 42yrs exp
So, using the duct calculator using .10 friction per 100 ft. a 7 inch duct will supply about 160 CFM to a room.
There is a good possibility that the airflow is nowhere close to 160 CFM.
These guys are creating a tremendous business opportunity for you by installing duct systems this way.

3. Depending on the length of run, that 7" could be delivering 180 cfm, or 120.

They make a duculator for flex, you should order one.

As I'm sure some one may have told you already, if you do a room by room manual J, that will tell you how many cfm, each room requires.

4. Originally posted by beenthere
Depending on the length of run, that 7" could be delivering 180 cfm, or 120.

They make a duculator for flex, you should order one.

As I'm sure some one may have told you already, if you do a room by room manual J, that will tell you how many cfm, each room requires.

A manual d will tell you what friction to use, which will tell you what size flex to run.

That is, if you want to go by the book.

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It's not the length of the 7" run that determines the cfms delivered.

Check out Man. D @ http://www.acca.org

It's the total equivalent length of the longest supply and return path,which determines the Friction Rate to use on the ductolator,which tells you how many cfms a 7" flex can deliver at that FR.

7" flex,is likely somewhere beteen 80 to 120 cfms,per a flex ductolator.

6. Originally posted by dash
It's not the length of the 7" run that determines the cfms delivered.

Check out Man. D @ http://www.acca.org

It's the total equivalent length of the longest supply and return path,which determines the Friction Rate to use on the ductolator,which tells you how many cfms a 7" flex can deliver at that FR.

7" flex,is likely somewhere beteen 80 to 120 cfms,per a flex ductolator.
LOL... Ok, I'll sit corrected.

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