Vent Controller Incrmental Incremental Vent Controller Choose this control if you have a non-computerized control and you want your vent to open in increments. Opens in increments, closed, 1/4 , 1/2 and full open. Primarily used with STEP50, STEP500, M-1608. This is a master control. If placed in the center it can have six 2R units, 3 on each side External limit switches