An energy saving air quality control system modulates supply fan speed by use of controlled, variable-frequency drive controls for automatic dampers or suction pressure to maintain adequate air flow across the evaporator coil at partial cooling loads. Demand ventilation in the system balances air quality and energy consumption by controlling the outdoor air damper in response to indoor CO2 levels. The control system variously either (1) produces a variable 0-10 VDC output signal to modulate the outside ventilation air damper as required to keep the CO2 concentration below a set point; (2) produces a 0 or 24 VAC signal to either open or close a two-position outside air damper for as long a time period as is required in order to keep the CO2 concentration below a set point; or (3) allows manually setting the outside air damper to provide proper ventilation air at maximum occupancy and maximum air flow and modulates the supply fan speed as required to keep the CO2 concentration as measured by a CO2 monitor below set point.