Abstract: The apparatus may be implemented in software which performs the following steps: calculating a desired air velocity in the exhaust duct from a first input signal representative of the size of a variable sash opening and a first user defined setpoint representative of a desired face velocity; comparing successive values of the desired air velocity to determine any absolute and percent change therebetween; comparing the absolute change to a user defined reset deadband value; producing output signals, responsive to a second input signal representative of the measured air velocity in the exhaust duct and the second comparing step, for input to the fan so as to force the measured air velocity toward the desired air velocity. The production of the output signal is accomplished by adaptive, feedforward techniques when the absolute change is greater than the reset deadband and is accomplished by proportional/integral techniques when the absolute change is less than the reset deadband.