States that accurately controlling the amount of outdoor air brought into a building is a major factor in ensuring good indoor air quality. However, controlling this minimum ventilation volume is difficult in variable air volume systems. While a number of proposed control systems work well in theory, they fall short in practice, mainly because of unrealistic assumption of control accuracy which can lead to gross errors in ventilation volume. Reviews several options for controlling the minimum ventilation volume in a VAV system. Demonstrates the inaccuracies inherent in several widely-accepted fan tracking systems and goes on to propose two methods that offer much greater accuracy - the injection fan method and plenum-pressure control.