A caliper control system for controlling the thickness and sheet quality of paper passing between calender rolls of a calender stack includes a distribution chamber which houses a supply of heat transfer fluid. A series of nozzles extends across the width of the unit in the cross machine direction, and each of these nozzles are selectively actuable. An actuating means, which may be a piston connected to a solenoid, opens and closes an opening in the wall between the distribution chamber and the nozzle for each nozzle of the unit. In a preferred embodiment, the caliper control system includes two such units. One of these units will supply hot air to the roll surface, and the roll to which the hot air is applied to an unheated roll. The second unit will supply cold air to a heated roll. Each of the nozzles is covered with a plate which includes a large number of apertures for expelling the heat-transfer fluid from the nozzle. The travel of the heat-transfer fluid is such that an impingement flow is created through the apertures. The heat transfer fluid is heated or cooled by a heat exchanger which includes a number of internally finned tubes over which a steam or cold water flow is applied. The air forced through the tubes by a fan is then either heated or cooled.