I am trying to improve air flow for a 5 ton horizontal unit located in the attic.
The present return grille is a 20x36 which I now understand is an odd size and I should use a 24x30 (the same 720 sq ins). The distance from the return grille, in the ceiling, to the evap coil opening of 22x26, is approx 10 ft.
If my math is correct, when I use round flex duct, I should use 24"? However, I am told that the common size is 18" or maybe 20". I would like to understand.
Sam

I am trying to improve air flow for a 5 ton horizontal unit located in the attic.
The present return grille is a 20x36 which I now understand is an odd size and I should use a 24x30 (the same 720 sq ins). The distance from the return grille, in the ceiling, to the evap coil opening of 22x26, is approx 10 ft.
If my math is correct, when I use round flex duct, I should use 24"? However, I am told that the common size is 18" or maybe 20". I would like to understand.
Sam


See file