A simple way to supply fresh air to a space is to introduce it at the air handler. There it’s mixed with the return air, and then supplied in equal proportion to all spaces supplied by that air handler. A packaged rooftop unit, for example, has an outside-air hood integral to the equipment. This hood can supply up to 25 percent outside air to the unit. Motorized dampers make it possible to control the outside airflow.
One option possible with motorized outside-air dampers is the warmup cycle. In the winter, the dampers are closed when the unit is first started up in the morning. No energy is spent warming cold outside air, until the air in the conditioned space is heated to setpoint. Then the outside-air dampers are opened, and the correct volume of fresh air is supplied to the unit.
Another feature is to close the outside-air dampers during the night when a building isn’t occupied. A minimum of energy is thus used to maintain comfortable conditions, without compromising occupant comfort. One precaution, however, about maintaining indoor-air quality: Buildings with a high occupancy can build up a high carbon dioxide level during the day, even with constant outside air supplied via the HVAC system. Such facilities depend on the night cycle to circulate the air out of the building, so if the dampers are closed, the indoor air quality will suffer.
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Excerpt from Energy Conservation Projects.