What is the difference between 24V AC and 24V DC actuator power?

Most HVAC actuators historically used 24V AC because transformers were simple and reliable, and the AC signal allowed early 2–10V position feedback without separate power supplies — the full-wave rectified AC provided inherent isolation. Modern actuators increasingly use 24V DC because brushless DC motors are more efficient, provide higher torque at low speeds, and enable battery-backup fail-safe operation without needing a separate spring. The Belimo fail-safe SuperCap technology, for example, uses 24V DC to charge capacitors that can drive the actuator to its fail position on power loss, eliminating the mechanical complexity of spring-return mechanisms. When retrofitting, note that 24V AC actuators cannot typically be powered from 24V DC supplies (the internal rectifier expects AC zero-crossing for timing control), but many modern 24V DC actuators accept 24V AC as well — always check the manufacturer's specifications before mixing power types.