How does the FAN Auto/On switch or key work?

The FAN selection key has two or three positions: Auto, On, and on select models, Circ (Circulate). The Auto position will allow the system fan to operate whenever the heating or cooling system is turned on by the thermostat. The fan will not blow air unless there is a call from the system. This is the normal setting for a system fan. The On position will turn the fan on to run continuously, regardless of whether the heating or cooling system is turned on by the thermostat or not. This is the setting used to move air in your application all the time instead of when the system is running. The Circ (Circulate) position, if available, will run the fan approximately 35% of the time, roughly 20 minutes each hour, minus any time the fan is already running with the heating or cooling system. This setting is normally used when there is low demand seasonally, such as fall or spring, or due to setback, either "leave/away" or "sleep", to help move air around the application.