Paying particular attention to keeping fins per inch low has both immediate and long-term energy benefits. Fewer fins per inch results in more free area for the air to pass through, leading to lower pressure drop per row of coil. To avoid the need for an excessive number of coil rows, design for 8 FPI – and, as a worst case, never exceed 10 FPI – to optimize energy performance and reduce internal static pressure (ISP). While it is common in the HVAC industry to size ductwork for a pressure drop of 0.1 inches of water column (in. w.c. per 100 ft of duct, thereby reducing the associated fan AHP through the ductwork by 28%, according to Equation 3.