- 1). Locate the manufacturer's estimation of the Cubic Feet per Minute (CFM) that the air conditioning unit will produce and use this as an estimate of the total air flow requirement of the intake. Assume an 800 CFM flow rate for this example.
- 2). Determine the volume of the ductwork you have installed by multiplying the area of the ductwork by its length. The area can be calculated by multiplying the radius squared by pi. For example, for eight inch ductwork, the formula would be: (4 inches x 4 inches) x 3.14 = 50.24. Now if the length of the ductwork to the air conditioner is eight feet, simply multiply the area by the distance to get: 50.24 x 96=4823.04 or about 400 cubic feet.
- 3). Divide the CFM estimate from the air conditioner by the number of cubic feet provided by the ductwork. In our example, 800/400=2 feet, so the diameter of the intake for the air conditioner should be two feet.
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