supply duct size?


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Old 03-15-14, 08:04 PM
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supply duct size?

I am installing a 2 ton split ac system. Vertical airhandler, with 2 registers right off the supply plenum, and 2 supply registers 5' away from the plenum, and 1 supply register 10' from the plenum.
I understand the rule of thumb is 400 cfm/ton. So, I assume that the cfm of my supply ducts, should add up to 800 cfm. Is this correct?
 
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Old 03-17-14, 01:54 PM
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Are you putting in new duct? Or using existing? Are you installing new condenser and furnace?

I
 
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Old 03-17-14, 02:05 PM
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The "400" rule is at the fan. You will loose air flow as you increase duct run and duct material type. Some interesting reading is the benefits to using the old duct. Sheet metal ducting has lower friction loss per unit length which equates to more air flow to the grills which typically means for an entire home you may be able to down size the unit. It also equates to shorter run times because more air into a room at the desired temperature will heat/cool said room much quicker. You can find tables online that tell you the air loss per unit length for the duct size and type. Don't fall victim to the discussion that "if you install the flex duct right" it is the same. I have never seen flext duct installed correctly in a real application, only a manufacturer display and there it was still higher friction loss over sheet metal ducting. There are some applications out in the real world that utilize both sheet metal with a little flex duct at the ends of the run. This might work. Look into it.

By the way, it appears that all of your runs are fairly short. I would suggest either increasing the size of your grills, both supply and return, or slowing your fan down. What will happen if you are not careful is you will have high velocities (Q=VA) which will result in noise. Nobody enjoys hearing a rattling air grill!

Hope this helps!
 
 

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