Although a typical HVAC system has many controls, the control of outdoor air quantity that enters the building can have a significant impact on IAQ, yet typically is not part of standard practice. Demand controlled ventilation is addressed as a method of humidity control, but is not otherwise discussed here because its primary use is to reduce the supply of outdoor air below the recommended minimum for the purposes of saving energy, not for improving IAQ.

Outdoor Air Volume Monitoring and Control

Supplying acceptable quantities of outdoor air to occupied spaces is a critical component of good indoor air quality. Yet nearly all school ventilation systems cannot indicate whether outdoor air is even being supplied to the school, much less gauge the quantity of that air. Virtually all existing school ventilation systems rely upon a fixed damper to regulate the amount of outdoor air. Yet wind, stack effect, unbalanced supply and return fans, and constantly changing variable air volume (VAV) systems can cause significant under- or over-ventilation, which can affect IAQ and energy costs. Combinations of these effects can even cause the intake system to actually exhaust air.

• Specify the addition of a measuring station that actively controls the amount of outdoor airflow by modulating the outdoor air damper and the return (recirculation) damper, if needed to overcome wind and stack effects.

These measuring stations are designed to work in limited duct space and with low air velocities. This is an easy task, as some manufacturers offer their airflow measuring stations in separate packages with dampers and actuators, and others are built into the AHU at the factory.