cfm in flex duct?

Old 06-12-05, 09:33 AM
nola mike
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cfm in flex duct?

looking to change my duct sizing to better regulate air flow. i've seen it before, but now can't find the rule of thumb for cfm through flex duct. looking for info on 5, 6, and 8". thanks.
Old 06-13-05, 10:40 AM
Join Date: Jun 2005
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Depends on the static pressure of the duct system.

Calculating proper duct size is one of the most overlooked areas of residential HVAC. Most systems are undersized.

There are many variables to determine what size of duct to run. The room sizes, heat loads, fenestration, distance from the air handler, etc. ...

Start here:

Manual J & Manual D calculations will get you where you need to be in sizing your system properly.

You might consider calling out a HVAC contractor to "consult" with you on the calculations.

Sorry there's just no magic rule of thumb!
Old 06-14-05, 10:24 AM
Ed Imeduc's Avatar
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Just a old rule is 5" 60cfm 6"100cfm 7"150 cfm and 8"is 200 cfm Like said a lot more to it than just that

Old 06-18-05, 09:34 PM
sandbagger's Avatar
Join Date: May 2005
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there is one "magic rule" worth mentioning: better flow in rigid ducts than flex.


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