'' to 4 '' Sheet metal HVAC Duct Reducer for elastic or metal HVAC Ducts and air vents. Our sheet metal reducers increasers are from the highest social station of surfaced steel. They are the tense sheet metal for changing plant structure sizes and utilized for maintaining the best airflow for limit efficiency in your HVAC plant structure system. These reducers are of 26 gauge surfaced sheet metal. for use with either elastic plant structure or unmoving sheet metal plant structure.