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Old 05-28-2010, 05:26 PM   #1
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36 foot flex duct.


I have an older 2 ton AC system in FL. I would like to add a duct to supply a 12X15 room. Is it possible to run flex duct 36 feet from the plenum to a new room, if so what size??? Would it be worth trying to see if it works?? The option's are window AC, mini split or replace whole system.
JIm

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Old 05-28-2010, 05:44 PM   #2
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36 foot flex duct.


You can run it and would need at least 6" but you are robbing Peter to pay Paul. If your system is older and does not have any extra capacity the rest of the house will suffer. You may want to do a heat load calculation to find out what size system you need:Load Calculator
The new units are more efficient and in your climate that may save you a lot of $$. There may be some gov't rebates to help you.

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Last edited by yuri; 05-28-2010 at 05:46 PM.
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Old 05-28-2010, 06:53 PM   #3
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36 foot flex duct.


36', thats a long run. A 12X15 room could need an 8" depending on how straight the run is or isn't.
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Old 05-28-2010, 07:48 PM   #4
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36 foot flex duct.


Thanks for the responses. I should have added that the general opinion is that my current duct work is too small. The main duct off a 2 ton unit is duct board and has an outside dimension of 12X12 inch. I have no idea how thick the duct board is. It is feed by what appears to be 10" flex. This feeds 6 outlets in what is currently (without new room) a 780 sq ft house. the new room is 12x15 and will make the total area 960 sq ft.
According to their rule of thumb it takes 1 ton to cool 500 sq ft in this area and this type of house.
I truly appreciate your input, I know nothing about HVAC and cannot say I am overly impressed with the contractors I have met. One was pretty good but he does not want the job.
JIm
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Old 05-28-2010, 08:01 PM   #5
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36 foot flex duct.


Interesting tidbits on flex: http://rockwallcontrols.com/Resident...ag=air-ducting

http://www.mmmfg.com/pdfs/True_Cost_...x_10-27-06.pdf

Do not read this when tired: http://www.thefreelibrary.com/Pressu.....-a0201591046

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Old 05-28-2010, 08:11 PM   #6
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36 foot flex duct.


Your duct board is probably 1" thick. Making it 10X8 duct, which is too small.
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Old 05-28-2010, 08:56 PM   #7
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36 foot flex duct.


I live in Florida and I am in hvac. i recomend that you get the a new unit if your's is more than 12 yrs old. In the long run it will save you money. I would have to agree that you'r ducting is to small. But yes you can run flex that far but I would recomend that you also put a return in the room if possible. I would go with an eight inch supply in a room that big.

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