Quote:
Originally Posted by Paulbstockton6@hotmail.co
17x20 is good for about 1600cfm and 12" is good for about 500cfm..... that checks out
Grille size 17x20 = 340sqin x 2 = 680cfm... problem
So the grille for the 12" round duct (capacitu = 500cfm) vs grille (680cfm) good.
Grille size for the 17x20 square duct (capacity = 1600cfm) vs grille capacity (680 cfm)..... not so good and this return is noises than the other.one.
So you have. .500+680cfm= 1180cfm. (Choked) for a 5 ton system which likely set to 1500 to 1700cfm....
What just happened...so now you will have excessive pressure (big deal, I'm in the same boat) This will make the blower motor work hard...drawing more amps.
I predict that if you tested static pressure you will be exceeding the .50wc spec...
So, since we are deficient in return air, look at the return plenum, or that 17x20 duct....can you tap into that and run an additional return somewhere where it might be benificial....?
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I took a closer look, and the return is even worse than calculated, and it is way more noisy as you predicted.
The knuckleheads have the plenum placed well below the grill, and didn't seal anything. The foam is my attempt to seal it up. So the excess capacity of the return duct is made even worse by the grill being misaligned with it. Maybe I can bend the plenum up to the grill? Don't know how these are normally joined/sealed to it.
Could I simply add a 2nd 17x20 grill next to the existing one and feed it into the return?
Here's the other side of the wall, showing the return plenum sitting on a 2x4. Don't know why the forum put the image upside down.
My furnace control board schematic if that's useful at all. I can't find specs on the motor anywhere. Strangely, the more restriction there is, the less power it draws. I turned the fan only on, and it draws 750 watts. When I remove the filter and open the grill, it draws 800 watts.
