A simple electromechanical time clock can operate the ventilation equipment on a constant schedule with on-off times for a 7-day routine. This prevents the equipment from running after hours or on weekends. The scheduled times can be easily modified to suit individual circumstances.
A little more elaborate system puts all the HVAC equipment on a central time clock, with an override timer located in the conditioned space. This device turns the equipment on for a fixed time, countermanding the central timer. This feature may be needed, at a high school gymnasium, for example, where the coach may schedule after-hours practices, games, and other activities.
Timers for other spaces, such as a high school locker room, can serve yet another purpose. The exhaust fans can be turned on several times after hours to cycle out stale, humid air and replace it with fresh, clean air. This can be nearly as effective as operating the ventilation system all the time and is much more cost effective.
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Excerpt from Energy Conservation Projects.