Abstract: A hot air modulating valve for mixing hot air with cold air flowing in a duct, the modulating valve having a torque motor operated pilot valve to position the modulator valve to maintain a predetermined temperature of the hot air-cold air mixture. The temperature of the mixture is sensed by a thermistor which in conjunction with a tri-mode (proportional plus integral plus derivative) electronic controller which provides an input current signal to the torque motor to position the modulator valve in proportion to the input current signal, produces zero steady state errors, and provides an anticipation effect to compensate for control system time lags. The controller has auxiliary inputs which when exceeding predetermined values switch the air mixture temperature at the thermistor to a lower setting. The modulating valve has a relatively low gain during the initial portion of its opening stroke to improve system dynamic stability and has a position feedback to the torque motor operated pilot valve.