An energy-reducing method and apparatus for retrofitting a single zone, constant volume HVAC system, with or without an economizer, that provides heating, cooling, and ventilation to occupants within a building space. The present invention includes the introduction of a programmable logic controller and variable frequency drive (VFD) that takes control of the existing fan, heating, cooling, and optional economizer operation. The reduction of the fan speed in the ventilation mode when the 100% operation is not needed saves significant energy of the existing constant volume HVAC system where the fan motor is designed to run 100% of the time.