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My home A/C unit is 4 ton. Is a 30 x 20 inch air supply grille large enough to supply proper amount of air?

You mean your return grill right?

Your supply might leave the furnace in a 30x20 duct, but it would typically terminate at individual 4"x10" registers around the house.

Trying to remember the formulas from my installing days. At 400 cfm per ton for air volume you'd need to move 1600 cfm for a 4-ton unit. At 100 cfm out of each register you should have around 16 registers in your home. Fewer than this many (like <10?) might slow the air down too much (high static pressure) risking an evaporator freeze-up.

More vents won't hurt efficiency or capacity, but the air might not feel as cold or as fast coming out of the individual vents to elicit that "aaaahhhhh" response.

So, BOTH your supply and return ducts have to have the capacity to move 1600 cfm easily and I believe 20x30 would.

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