Demand controlled ventilation is the automatic linking of fan speed with one of the indicators of indoor air quality (IAQ), such as carbon dioxide (CO2) or humidity.  In this way, good IAQ can be maintained during occupied times with the least energy consumption by fans. 

For classroom ventilation, demand control is often arranged using a CO2 sensor, either integral to the unit or wall-mounted. With fan drive-motor speed arranged to modulate automatically, no teacher intervention is required for the control of classroom conditions.