Flex And Round Duct Sizing Charts Cfm 4 6 8 10 12 Inch Cfm 45 Of For instance, if the longest duct (from the blower to the farthest vent) is 30 ft, a 6″ duct carrying 200 cfm will result in a friction loss of 30 ÷ 100 x 0.3 = 0.09 in.wg. the friction loss due to the duct alone is one of the elements in determining the total friction loss in the duct system. The table on the right (or below if you’re viewing on a phone) shows the return duct size for hvac unit from 1.5 tons to 5 tons. based on 400 cfm per ton, the corresponding airflow is also provided. the next column is the diameter (in inch) for round rigid duct. the last column is the number of round rigid duct you need.
Duct Cfm Chart Round Flex Duct Calculation Guide 51 A rectangular 6×6 inch duct has an airflow of 110 cfm. bigger 6×12 inch ducts can handle 270 cfm airflow. the bigger the crosssection of ducts, the bigger cfm; that’s pretty obvious. the smallest 6×4 ducts can handle 60 cfm airflow. the biggest 42×12 ducts with 504 sq inch crosssection can handle 3000 cfm airflow. 30″ (750mm) 26″ x 30″ (650 x 750mm) 30″ (750mm) 28″ x 38″ (700 x 700mm) for the conversion between round ducts and rectangular ducts, the friction loss must maintain at 0.1 in.wg and below. then, the air velocity is also limited at 1500 fpm and below. next, the rectangular duct ratio must not exceed 1:4. Using the chart, you’ll see that for a 12” round flex duct, maximum cfm is 480 cfm or 500 cfm depending on the friction rate. duct size for 115 cfm. find the cfm closest to 115 in the table above. it will be found in different columns depending on the friction rate. if there isn’t an exact match, go up to the duct size. Now, multiply 3 x 400 cfm to get 1200. you would need a blower that can move 1200 cfm. pro tip: round up – if you’re more than halfway to the next larger size, round up to it. in our example, if you needed 33,500 btu, or 2.75 tons, round up to 3 tons and multiply by 400 cfm per ton for the same result – 1,200 cfm.
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