Fan switches are normally open switches that close when the indicated temperature is reached. For example, a fan switch located near a heat exchanger will close and turn on a fan when the temperature reaches 120 degrees farenheit. Temperature settings are indicated with an F. An example would be F120-30. The switch would close at 120 degrees and automatically open when the temperature drops 30 degrees to 90 degrees farenheit. Opposite of a fan switch is a limit switch.