If the heating is provided by electric resistance elements, the lower energy required to warm the space causes lower rate billing charges and lower peak costs as well. Utilities typically see a high peak in the morning in the heating seasons as homes and businesses are warmed after the chill of the night. The peak demand charge encourages people to reduce their electricity use at this time.
The peak load in the cooling season is toward the end of the business day, late in the afternoon when the sun is a few hours past its zenith. Reducing outside-air volume to the cooling coils reduces the load on the coils. The air is much hotter than returned air from the space, and often much more humid as well since warm air can hold more water vapor than cool air. This humidity must be extracted at the coils before the air can be cooled down, at great cost.
Again, this energy being used by the air handler is at the peak billing rate. Reduction of outside-air intake for an hour or two at the end of the day can lower the daily peak demand, without causing unhealthy conditions in the occupied spaces. Natural infiltration overnight, or for a short period of forced ventilation of interior zones, makes up for any fresh-air deficit that may result. This air, also, is typically lower in water-vapor content, so the load on the unit the following morning will be less.
The implementation of this project is straightforward. Installing dampers in the outside-air duct with a pneumatic or electronic actuator is easily done. A central or a dedicated timer for remote air handlers can open and close the dampers to correspond to the occupancy period. The same time clock system can be used for on/off controls of the air handler for even more substantial savings.
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Excerpt from Energy Conservation Projects.