Outside air puts a great burden on the HVAC system. First, it’s usually hotter (colder) than the air returned from the conditioned spaces, so it must be cooled (heated) to the supply-air temperature. Second, it’s often more humid than the cool air returned, and less humid in the winter than the return air. Consequently, a lot of energy must be expended to dehumidify it, or to humidify it in the heating season. Finally, an equal volume of preconditioned return air must be exhausted, and the energy content of this volume is lost.
The consequences of having to triple the volume of outside air, as required in some instances by the new ASHRAE guidelines, can be extreme. If the existing chiller and air handlers are unable to handle the increased load, they may have to be replaced altogether.
There’s an alternative. The carbon dioxide content of the conditioned environment can be monitored either in the space itself or the return air path. When it drops below setpoint, the outside-air dampers modulate open to introduce more fresh air to displace a portion of the returned air.
The basis for this system is that an air handler is designed to supply outside air according to the maximum expected occupancy of a space. Many zones, such as meeting rooms, auditoriums, classrooms, and some offices, will oftentimes have many fewer occupants. Consequently, the outside-air supply can be safely reduced, as long as provision is made to accommodate the maximum occupancy whenever it happens. A carefully located and accurately calibrated carbon dioxide sensor will satisfy these requirements. It may even achieve a higher-quality air to the spaces than a preset system with minimal outside air supplied to it, and it conserves energy over a system with the full outside-air volume provided all the time.
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Excerpt from Energy Conservation Projects.