Final answer:
The fan switch on a residential low-voltage thermostat controls the operation of the HVAC system's fan. Setting the switch to AUTO enables the fan to run only when the heating or cooling system is running, while setting it to ON makes the fan run continuously.
Step-by-step explanation:
The purpose of the fan switch on a residential low-voltage thermostat is to control the operation of the HVAC system's fan. When the fan switch is set to AUTO, the fan will only run when the heating or cooling system is actively running.
This helps to conserve energy by preventing the fan from running continuously. On the other hand, when the fan switch is set to ON, the fan will run continuously, even when the heating or cooling system is not actively running. This can help with air circulation and filtration.