What is a 24VAC (non-communicating) control system?

A 24VAC thermostat sends a one way signal from the thermostat relays to the HVAC equipment.

24VAC control systems are widely used in control systems around the world for things like irrigation controls, low voltage lights, CCTV systems, thermostats and HVAC zone control systems. They’re safe to work on and easy to diagnose faults.