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CFM estimate based on grill/filter sizes?
I have a split return & filters that are 14x20 and 14x25 (take off one inch in each direction for measuring the grill size). I need my returns to support 1000cfm (250cfm x 4 ton) to meet the spec my my AC. I realize there are many factors that go in to cfm and I need to find a competent AC company (if mine is not) to actually check the cfm, but I am curious if all else is perfect, if these grills are sized large enough where that is possible. I read somewhere that I could take the sq footage (14x20)+(14x25)=630 and divide by 200 (not sure if this is 250 in my case?). If I use / 200 calculation, I only get 3.15ton. Does this mean that my returns are undersized? Is there some other calculation I should use to determine if these two returns are large enough to support 1000cfm assuming other conditions are not problematic?