Energy reducing retrofit method and apparatus for a constant volume HVAC system

A method reducing energy consumption by retrofitting a preexisting single zone constant volume HVAC system that provides heating, cooling, and/or ventilation to an indoor space within a single zone of a building that has periodic occupancy, wherein the heating, cooling, and/or ventilation is initiated based upon preset conditions or on occupancy command and wherein the preexisting HVAC system includes a preexisting fan assembly having a fan and a fan motor that operates in a range of not operating to a maximum operation, a thermostatic device, heating assembly and cooling assembly having preexisting equipment control terminations, and an economizer assembly comprised of at least one outside air and one return air damper with an associated damper actuator;

• the energy reducing method comprising:
• providing a programmable logic controller that can receive input from the preexisting thermostatic device and send heating, cooling, and ventilation instructions to the preexisting fan assembly for ventilating in an indoor building space in a ventilation mode, and to the heating assembly that can send heat to the same indoor building space in a heating mode, and to the cooling assembly that can send cooling to the same indoor building space in a cooling mode, and to the preexisting economizer assembly for controlling the damper actuator to control opening of the damper in an economizer mode;
• and operably connecting the programmable logic controller to the preexisting equipment control terminations;
• providing a variable frequency drive (VFD), controlled by the programmable logic controller, and operably connecting the VFD to the fan motor;
• providing an input sensor to sense outside air and return air conditions and return economizer air and the sensors communicate their sensed conditions to the programmable logic controller;
• operating the HVAC system in the ventilation mode, the a heating mode, the cooling mode, and the economizer mode controlled by the programmable logic controller, and operating the fan motor at a reduced speed during the ventilation mode when less than maximum ventilation is required and operating the fan motor at a reduced speed during the heating and/or cooling mode;
• and
• providing fault detection via a discharge air sensor and/or a current status switch.