A variable-air-volume system for modulating the speed of a supply fan by measuring a plurality of inlet static pressures within the inlet of a corresponding plurality of terminal boxes within the system; calculating a plurality of setpoint errors equal to the difference between the plurality of inlet static pressures and a corresponding plurality of terminal box setpoint pressures; selecting minimum setpoint error out of all of the calculated setpoint errors; and modulating the fan speed as a function of the minimum setpoint error. The system can also utilize a power adjustment factor to modify the terminal box set point pressures.