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Old 05-02-2011, 02:49 AM   #1
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Duct size Check.


Im installing central air in my small ranch home, the main floor is just under 900 sq feet, and the same for the basement which is getting finished. Did a Manual J load calc and picked up a 2.5 TON luxaire split unit.

Getting ready to peice together my duct work, I've already picked up 8" flex for all my rooms , and a 6" flex for the bathroom. ( along with all the nessasary collars, ceiling boxes, etc. I've also picked up a 16"x25" return grill, and 16" flex for my main return.

My problem is trying to figure out what size supply trunk I need for my air handler, and also the return air duct trunk

Also, the actual connections are throwing me a curve. It looks like it'd be easy enough to connect to the supply end, as i have a flat surface to screw into, but the return side is all open and doesn't give any hint on what to actually connect the duct to.

my air handler dimensions are 21.5x17.5.

Here is some info. on the units, as well as some pictures as what im talking about.

Return side:



supply side:


I have the " 30B " unit.






CFM chart produced by "HVAC calc " assuming total cfm is 1000 ( 400 per ton @ 2.5 ton =1,000 CFM ???)


Manual J



Thanks in advanced.


Last edited by alecmcmahon; 05-03-2011 at 03:44 AM.
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Old 05-02-2011, 09:46 AM   #2
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Duct size Check.


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Last edited by alecmcmahon; 05-03-2011 at 03:39 AM.
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Old 05-03-2011, 07:31 PM   #3
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Duct size Check.


bump, is a 12"x18" 10 foot long to small/big?
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Old 05-03-2011, 08:18 PM   #4
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Duct size Check.


What's your TEL (total effective length) of your longest run? Not enough information to tell you any definate sizes. But I am sure someone will be along with a ductulator and pick an arbitrary figure to tell you.

Will the system you chose handle the sensible and latent? Just because a load calc says 2.5 tons doesn't mean it will handle the load for your climate. I am a Carrier/Bryant dealer and their 2.5 Ton won't cover your load calc.
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Old 05-03-2011, 09:08 PM   #5
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Duct size Check.


A 12X18 should be ok. Hard to tell not knowing the length of the longest supply or return.

On the return of that unit. A small sheet metal collar for duct board to go over works good. Or you can use a sheet metal return plenum, and wrap it.
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Old 05-03-2011, 10:01 PM   #6
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Duct size Check.


Longest run of 8" flex will be under 15-20 feet or so on main level. The chase to the basements might be something like 40' from supply off ac till the farthest duct in basement


Any material I can read on proper duct sizing ? At 10 foot long it won't drop the static too low ?

I want to achive .50 WC correct ?

Last edited by alecmcmahon; 05-03-2011 at 11:40 PM.
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Old 05-04-2011, 04:17 AM   #7
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Duct size Check.


Manual D by ACCA is what you would want to read for duct sizing.

Its not just the linear length that needs to be accounted for. But also all bends and turns have an equivalent length.

.5" or less is what you want to shot for.

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