Which control system uses compressed air to send variable signals in HVAC systems?

The control system that uses compressed air to send variable signals in HVAC systems is pneumatic. Pneumatic control systems leverage compressed air to control valves, actuators, and other components within heating, ventilation, and air conditioning systems. The use of air as a medium for transmitting signals allows for rapid and precise control over system operations, which is crucial for maintaining desired temperature and airflow settings.

In pneumatic systems, pressure changes within the compressed air lines can correspond to different setpoints, allowing for automated adjustments in the HVAC system. This technology has been widely used in the industry due to its reliability, simplicity, and effectiveness in controlling HVAC equipment.

The other options do not involve using compressed air for signal transmission. Electric control systems rely on electrical signals to operate components, hydraulic systems utilize fluid under pressure, while thermal control systems are based on temperature changes. Each of these systems serves different applications and employs different principles for operation, differentiating them from pneumatic controls.