The air-handling units provide heating, cooling and ventilation to the entire facility, blending fresh air and return air to a fixed mixed air temperature setpoint. This provides temperature and humidity control in the space, but tends to bring in more ventilation air than needed, especially during partial occupancy periods. Any outside air brought into the building has to be conditioned by heating or cooling equipment, so it is best to minimize it.
CO2 control on the air handlers would ensure that ventilation air volumes are matched to occupancy under all conditions. CO2 sensors would be installed in occupied areas and return air ducts, as appropriate for each air handler. ASHRAE Standard 62 recognizes that ambient (outdoor) CO2 levels can fluctuate, and that a better measure of ventilation requirements in the space is to use the difference between indoor and ambient levels. An outside sensor would also be installed to determine the ambient CO2 levels. The new sensors would be connected to the building automation system, which would be reprogrammed to maintain the indoor air CO2 levels at 700 ppm above ambient.
The outside air and relief dampers would be replaced with low leakage dampers.
CO2 to ventilation rate conversion assuming 400ppm outside ambient level.
ref: U.S. Federal Energy Management Program