What component shuts off the compressor when the thermostat opens?

The contactor is an electrically controlled switch that plays a critical role in the operation of HVAC systems by managing the power supply to the compressor. When the thermostat calls for cooling or heating, it sends a signal to the contactor, engaging it to close and allowing electrical current to flow to the compressor and other essential components. Conversely, when the thermostat opens—indicating that the desired temperature has been reached or that the heating or cooling demand has ceased—the contactor opens, cutting off the power to the compressor. This action prevents the compressor from running unnecessarily, helping to maintain system efficiency and longevity.

The other components mentioned, while important in various HVAC functions, do not directly manage the power to the compressor in response to thermostat signals. The capacitor is primarily involved in starting the compressor and maintaining its operation. The fan motor operates the indoor or outdoor fan but does not control the compressor's power. The relay switch also has specific functions in an HVAC system, but it is not the primary component that directly controls the compressor based on thermostat signals.