Abstract: The unnecessary removal of heated or cooled room air by fume hoods is prevented during off-peak periods through a manual and automatic fume hood ventilating system. Ventilation (exhaust) air quantities are reduced in response to operator and/or automated programmed control of the exhaust fan shaft speed. The safety integrity of the fume hood system is improved by providing the operator with visual and audible dynamic feedback of the system operating status. Room air balance is maintained through subsequent reset of the room supply air coincident with exhaust air quantity adjustment. Building make-up air balance is possible using individual instantaneous exhaust flow summary signals to control the outside air flow rate.