How is the grill size determined?
Our wood return air grills are sized by your unit air requirements. One hundred square inches of clear area per ton of air conditioning seems to provide a good air flow to the unit. Our wood return air grills average approximately 52% clear area and metal grills average approximately 64% clear area, a difference of 12%. If you use the premise that each ton of air conditioning (12,000 BTU) requires 400 cubic feet of air per minute with a return side velocity of approximately 350 feet per minute (a comfortable velocity in the home) then a 3 ½ ton unit requires 350-400 clear square inches. A filter 24″ x 30″ = 720 sq. in. x 52% = 374 clear square inches, which should be adequate for this 3 ½ ton unit. 11. How is the rough opening for the wood return air grill determined? The rough opening of our wood return air grills is determined by taking the grill size (which is also the filter size) and adding ¾” to the width and ½” to the height. EXAMPLE: 20″ x 30″ grill requires a rough opening of
