I have a 3 ton unit outside and a 3 1/2 ton in the attic. We recently replaced the attic unit from 3 ton to 3 1/2. Now the air return filter makes a whistling noise and the unit seems to be sucking so hard that the filter itself is bending. (We use pleated filters) The Return size we have now is 16X25 which is only 400 Sq in. I have been told that it should be larger but the area the return is in now can't go any bigger so we have to put an additional return in. Any suggestions on the sizes? Is the general rule of thumb 144 sq. in per ton? And is that per ton from Outside unit or inside unit. (The outside unit is only a 3 now but eventually will be a 3 1/2. Approx. 1500-1650 sq living area in home

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