Automatic Control Systems

HVAC systems are dynamic:

Sized for extreme conditions

Most operation is part load / off-design

Deviation from design => imbalance since Capacity > Load

Without control system, HVAC would overheat or overcool spaces.

Automatic control system

“A system that reacts to a change or imbalance in the variable it controls by adjusting other variables to restore the desired balance.”

Modem computer-based systems manage system resources (“supervisory”):

• Reduce energy use

• Identify maintenance problems

Essential components of a control system:


Controlled variable is a characteristic of system to be regulated.

“set point” is desired value “control point” is actual value

“set point”- “control point” = “error” or “offset”

Sensor measures actual value of controlled variable.

Controller modifies action of controlled device in response to error.

Controlled device acts to modify controlled variable as directed by controller.

Example: Water tank level control


Automatic Control Systems


Simple water-level controller. The set point is the full-water level; the error is the difference between the full level and the actual level.

Automatic Control Systems



Example: Steam heating coil.

подпись: example: steam heating coil.FIGURE 12.2a

Simple heating coit control system showing the process (coil and short duct length), controller, controlled device (valve and its actuator), and sensor. The set point entered externally is the desired coil outlet temperature.

Posted in Fundamentals of Heating. Ventilating, and Air-Conditioning