Good design practice requires separate air-conditioning systems for areas that are regularly 8-hour occupancies and those that are 24-hour occupancies. This applies to hospitals, schools, factories, and other facilities on an extended work schedule. If the business offices, for example, are on a different unit from the factory floor, then either area can operate after hours without having to provide air-conditioning to other large spaces that are not in use.
Another superior design practice is to operate systems that serve unoccupied areas intermittently and with as little outside air as possible. Air handlers usually mix a fixed volume of fresh air with the return air stream. The amount of outside air required is a function of the number of people in the space, their age, activity level, and other factors. If the area is not occupied, it is not necessary to provide as much outside air. The energy saved accrues from the lack of heating, cooling, and dehumidification needed to condition the outside air.
The amount of carbon dioxide in a busy building builds up over the course of a working day and peaks at the close of business. The continued circulation of air through the night—slowly replacing stale building air with outside air—brings the carbon dioxide content down to tolerable levels by the next morning. This should not be necessary if the original HVAC system was properly designed to provide adequate outside air during normal operating conditions. Many older systems may have been under-designed by modern standards, so it’s vital that any alterations in outside-air controls do not result in unforeseen consequences to the total building environment.
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Excerpt from Energy Conservation Projects.