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Ductwork length is critical in HVAC systems because ductwork that is too long can significantly reduce the efficiency of the HVAC unit. Think of it this way: Ducts are full of air. When a HVAC unit turns on, it first has to blow all this air out of the way in order to get hot or cold air into the room. Knowing how much air is in any length of duct can help determine if the blower will be able to efficiently keep moving that volume of air. The volume of the duct is best expressed in cubic feet.
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Determine whether the duct you are calculating the volume of is round or square. Round and square duct use separate formulas for calculating the volume of the duct system. For a round duct, skip to step 4.
Measure the height and width of the square duct. Multiply these numbers together to get the area of the duct. Convert this number to square feet by dividing by 144. (for example, 12-inch-by-10-inch duct would have an area of 120 square inches. Divide this by 144 and you end up with 0.833 square feet).
Measure the length of the duct. Multiply this number together with the area to get the volume in cubic feet. (for example, if the 12-inch-by-10-inch duct runs for 20 feet, the volume of the duct run would be 16.66 cubic feet).
For a round duct, measure the diameter. Calculate the area by dividing the diameter by two to get the radius. Then square the radius and multiply it by pi (3.14). Convert the result to square feet by dividing by 144. (for example, 8-inch-round duct would have a radius of 4. Four square is 16, and 16 multiplied by 3.14 is 39.44 square inches. Divide that number by 144 and you get 0.349 square feet).
Measure the length of the round duct run. Multiply the area by the length to get the volume in cubic feet. (If the 8-inch duct run is 50 feet long, the volume of the duct would be 17.4 cubic feet).
When working around HVAC ducts, always wear work gloves to avoid getting severe cuts because HVAC ducts are extremely sharp.