4" to 3" Sheet metal HVAC Duct Reducer for flexible or metal HVAC Ducts and air vents. Our sheet metal reducers increasers are manufactured from the highest quality of galvanized steel. They are the perfect sheet metal fitting for changing duct sizes and utilized for maintaining the best airflow for maximum efficiency in your HVAC duct system. These reducers are made of 26 gauge galvanized sheet metal up to 12" or 24 gauge galvanized sheet metal from 14" to 24". Perfect for use with either flexible duct or rigid sheet metal duct.