Honeywell RTH4300B Manuel D'utilisation page 9

Select the fan setting About your new thermostat
Set FAN switch to choose whether the fan
runs continuously or only as needed.
RTH4300B Programmable Thermostat
Set the FAN switch to Auto or On.
In "Auto" mode (the most commonly used
setting), the fan runs only when the heat-
ing or cooling system is on.
If set to "On," the fan runs continuously.
Use this setting for improved air circula-
tion, or for more efficient air cleaning.