Which three main components make up a DDC loop in a typical HVAC application?

• A. Sensor, controller, and actuator.
• B. Sensor, display, and actuator.
• C. Sensor, sensor, and controller.
• D. Transmitter, controller, and sensor.

Answer: (A)

In a DDC loop, the essential flow is sensing, deciding, and acting. The sensor measures a process variable like room temperature and converts it into a signal the controller can read. The controller compares that measurement to the setpoint, runs the control algorithm, and outputs a command that represents how much the system should change. The actuator takes that command and actually changes the HVAC equipment—modulating a valve, moving a damper, or adjusting a fan speed—to drive the process variable toward the setpoint. A display is for monitoring, not part of the control path, and a transmitter is often just a way to convey the sensor signal to the controller rather than a core control element.