HVAC control system, from the simplest room thermostat to the most complicated computerised control, has four basic elements:
•    Sensors: It measures actual value of controlled variable such as temperature, humidity or flow and provides information to the controller
•    Controllers: It receives input from sensor, processes the input and then produces intelligent output signal for controlled device
•    Controlled devices: It acts to modify controlled variable as directed by controller
•    Source of energy: It is needed to power the control system. Control systems use either a pneumatic or electric power supply.