An energy-reducing method and apparatus for
retrofitting a 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 controller is programmed for the reduction of fan speed in the heating and cooling
modes. 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. The fan speed may be further reduced upon a reduction in sensed occupancy levels of the space, such as with a CO2 sensor. Additionally, the fan speed may be reduced in the heating and / or cooling mode to further reduce
energy consumption. The controller may also be programmed and equipped with sensors to allow simultaneous mechanical compressor cooling and
economizer free cooling operation, for the operation of the economizer in response to the monitoring of outside air and return air to determine the preferred source for cooling operations, to keep the outside air
damper closed during the unoccupied heating
modes such as
morning warm-up, and / or to provide fault detection. The retrofit method and apparatus may be employed in a stand-alone or networked version.