The size of the plenum, as in any duct system, depends on the length of the plenum. In most cases, where the plenum duct length is less than 150 feet (45 m), the plenum will be 7-inch (178 mm) diameter or 8.5 x 8.5-inch (216 x 216 ,mm) square. However, in certain instances you will need to use larger duct.
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