Lambro 3 1/4 Inch x 10 Inch to 7 Inch Aluminum Transition is for use with rigid ductwork. Range hoods must be vented using metal ductwork. The minimum side duct allowed on most range hoods is 3-1/4-inches by 10-inches, which is equivalent to a 6-inch round. Lambro manufactures aluminum and galvanized ductwork for range hood and downdraft venting applications. Lambro represents innovation, quality, service and reliability for over 40-years.