Dual thermostat control apparatus with dead band response

Abstract

A dual thermostat apparatus includes two thermostats which generate fluid control signals proportional to room air temperature. The thermostats are matched leakport pressure sensors having a signal orifice and a bimetal lid element to generate back pressure signals linearly related to temperature. The bimetal elements have adjustment cams to set the reference position with respect to the orifice. The cams are connected to a common shaft and dial to provide simultaneous and opposite readjustment with respect to the reference pressures. The signals are applied to separate signal chambers of the duplex relay, with a pressure regulator connected to the heat signal chamber to limit the operative pressure level of the heating signal to a selected maximum switchover pressure. The duplex relay controls a pressure supply valve for supplying a proportional pressure signal to a common output line to pneumatic operators of a heating valve and a cooling valve. The regulator and duplex relay create a control in which the heat pressure signal operates to one side of said reference pressure and the cooling signal controls to the opposite side of such reference pressure. The position of the cams sets the required temperature related movement before the operative back pressure signal is established and thus defines a dead band region in which neither heating or cooling is provided. The signal switchover becomes operative only in accordance with the dead band setting and thus a range in which neither the heating valve or cooling is actuated to conserve energy.