When does the reversing valve's pilot solenoid pin carrier change position?

The correct answer is based on the operational mechanics of the reversing valve in HVAC systems, typically used in heat pump applications. The reversing valve has a pilot solenoid that controls the movement of a pin carrier, which dictates whether the system is in heating or cooling mode.

The pin carrier shifts position when the solenoid coil is either energized or de-energized. Energizing the solenoid activates the electromagnetic field that pulls or pushes the pin carrier to change its position, thereby altering the flow of refrigerant through the system. Conversely, when the solenoid is de-energized, the absence of the electromagnetic force allows the pin carrier to revert to its original position, switching the operational mode again.

The other options suggest scenarios that do not directly influence the functioning of the solenoid pin carrier’s position. High pressure in the system (first option) does not inherently trigger a change in the solenoid's state. Maintenance checks (third option) are performed for inspection and repairs but are not factors in routine operational changes. Lastly, reaching a desired temperature (fourth option) does not impact the solenoid's energization state directly, but rather it is the system's operational commands that dictate when the solenoid is activated or deactivated. Thus,