When the fan selector switch on a thermostat is set to "on", which terminals will close?

When the fan selector switch on a thermostat is set to "on," it activates the fan continuously, irrespective of whether the heating or cooling system is currently operational. The terminals involved in this configuration are the "R" and "G" terminals.

The "R" terminal represents the power supply from the transformer, typically 24 volts, while the "G" terminal is connected to the fan. When the switch is set to "on," it creates a closed circuit between these two terminals, allowing power to flow from the transformer to the fan, thereby turning it on continuously.

In this setup, the "Y" terminal is generally associated with the cooling system's compressor activation, which is not engaged simply by turning on the fan. The "C" terminal, usually linked to the common side of the transformer, is not relevant in the context of directly activating the fan.

Thus, the correct understanding is that closing the "R" and "G" terminals ensures that the fan operates regardless of heating or cooling demands, offering constant airflow throughout the conditioned space.