In general, the "O" terminal on a heat pump thermostat is energized whenever the system selector switch is set to ______?

The "O" terminal on a heat pump thermostat is associated with the reversing valve, which is responsible for switching the heat pump between heating and cooling modes. When the system is set to "Cool," the thermostat energizes the "O" terminal, which activates the reversing valve to change the direction of refrigerant flow, allowing the system to operate in cooling mode. This process is essential for the heat pump to efficiently extract heat from the outside air and release it indoors as cool air.

In contrast, when the system selector is set to "Heat," the "O" terminal is not energized since the system needs to operate in heating mode through a different configuration of the reversing valve. Similarly, the "Auto" and "Fan" settings do not specifically energize the "O" terminal, as "Auto" would determine whether the system operates in heating or cooling based on indoor temperature, and "Fan" simply activates the blower without affecting the refrigerant flow or the mode of operation. Thus, the correct association is that the "O" terminal is energized when the thermostat is set to "Cool."