What is the difference between the Fan 'Auto' and Fan 'On' settings?
In 'Auto' mode, which is the most commonly used setting, the fan runs only when the heating or cooling system is actively on. In 'On' mode, the fan runs continuously, which is useful for improved air circulation or more efficient air cleaning. (Page 6)
From the official Honeywell TH3000 Programmable Thermostat manual, page 6
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