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Old 07-23-2008, 01:06 PM   #1
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Flex Duct Advice


I want to replace flex duct in my attic that's old and deteriorating. I have 1600 sq ft. 3 bedroom 2 1/2 bath home with a pyramid style roof and the coil about centrally located in my attic. My question is this... what size diameter flex duct do I use to get the best deliver of cold air to the different rooms of the house....?

Currently in my living room there's 8" diameter duct going to 2 registers one is approx. 15 ft. long and the other about 4 ft long, the room is 336 sq ft. with a 9 ft. ceiling.
The duct going to my kitchen is 8" in diameter and approx. 17 ft. long, the room is 130 sq ft. with a 8 ft. ceiling.
The duct going to my master bedroom 9" in diameter and approx. 14 ft. long, the room is 182 sq ft. with a 9 ft ceiling.
The duct going to my secondary bedroom is 6" in diameter and approx. 19 ft. long, the room is 130 sq ft. with a 8 ft. ceiling.
The third bedroom has a 6" diameter duct and is approx. 15 ft. long, the room being the same dimensions as the secondary bedroom.
I'm a newbie so any help or insight will be greatly appreciated!

Last edited by ssporty21228; 07-23-2008 at 05:48 PM.
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Old 07-23-2008, 05:46 PM   #2
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Flex Duct Advice


unless there is a problem with airflow in one of the rooms use the same size. R-8
and for runs over 15 feet use hard pipe wraped with 3 inch insulation
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Old 07-23-2008, 07:15 PM   #3
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Flex Duct Advice


So, you say replace flex duct with hard pipe for runs over 15 ft...is this because of less resistance?
And what diameter of hard pipe should I use?
And the runs that are currently installed that are 6" in diameter, should I up those to 8" also?
Also, these 6" diameter runs branch off of the 8" runs...should I continue these branches or cut new holes in my plenum to run them from?
And...the register closest to the coil is 9 degrees colder than the other registers in my house...is this normal and what if any temp difference should there be?
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Old 07-23-2008, 07:45 PM   #4
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Flex Duct Advice


r8 is the flex insulation rating not the size , yes less resistance it's code here
the temp difference between registers is a product of inadequate insulation or leaky ducts ,pipe size only effects volume of air not temp. So to start from the beginning replace everything with the same size , excessively long runs should be done mostly with hard pipe of the same size with 3 inch insulation. If there is no problem with rooms being different temps don't bother redesigning the duct's just direct replacement with improved insulation rating
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Old 07-23-2008, 08:28 PM   #5
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For the info Geo...I'll definitely take your advice!
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