A HVAC control system is a computerized system for climate control in buildings. Stand alone control devices may be pneumatic or electronic. Some may have microproccessors, but to be considered a "control system" for the context of this article, computerized and networked are expected requirements. HVAC stands for heating, ventilation , air-conditioning. Often, these integrate fire, security, and lighting controls into one system. These systems typically use one or more central controllers to command and monitor the remote terminal unit controllers, and they communicate with one or more personal computers that are used as the operator interface. These control systems are typically used on large commercial and industrial buildings to allow central control of many HVAC units around the building(s). The latest systems use the building ethernet for communications between central controllers, and allow operator access from a web browser. It has been suggested that this article or section be merged with Control theory. ... All-air and air-water HVAC systems use air ducts, outlets, and inlets installed throughout their buildings. ... Return inlet (left)Supply outlet (right). ... In the broadest sense air conditioning can refer to any form of cooling, heating, ventilation or disinfection that modifies the condition of air, typically for thermal comfort. ... Ethernet is a large, diverse family of frame-based computer networking technologies for local area networks (LANs). ... An example of a web browser (Mozilla Firefox), displaying the English Wikipedia main page. ...

Direct digital control

Central controllers and most terminal unit controllers are programmable, meaning the direct digital control program code may be customized for the intended use. The program features include time schedules, setpoints, controllers, logic, timers, trend logs, and alarms. The unit controllers typically have analog and digital inputs that allow measurement of the variable (temperature, humidity, or pressure) and analog and digital outputs for control of the medium (hot/cold water and/or steam). Digital inputs are typically (dry) contacts from a control device, and analog inputs are typically a voltage or current measurement from a variable (temperature, humidity, velocity, or pressure) sensing device. Digital outputs are typically relay contacts used to start and stop equipment, and analog outputs are typically voltage or current signals to control the movement of the medium (air/water/steam) control devices.(Valves/dampers/motor speed) Setpoint is the target value that an automatic control system, for example PID controller, will aim to reach. ... An analog or analogue signal is an allergy continuous in both time and amplitude. ... A digital system is one that uses discrete values (often electrical voltages), especially those representable as binary numbers, or non-numeric symbols such as letters or icons, for input, processing, transmission, storage, or display, rather than a continuous spectrum of values (ie, as in an analog system). ...

History

It was only natural that the first HVAC controllers would be pneumatic, as the engineers probably understood fluid control. Thus mechanical engineers could use their experience with the properties of steam and air to control the flow of heated or cooled air. There are still pneumatic HVAC systems in operation in some buildings, such as schools and offices, which can be a century old. Pneumatics, from the Greek πνευματικός (pneumatikos, coming from the wind) is the use of pressurized air in science and technology. ... Mechanical engineering is the application of physical principles to the creation of useful devices, objects and machines. ...

After the control of air flow and temperature was standardized, the use of electromechanical relays in ladder logic, to switch dampers became standardized. Eventually, the relays became electronic switches, as transistors eventually could handle greater current loads. By 1985, pneumatic control could no longer compete with this new technology. Ladder logic is a method of drawing electrical logic schematics. ... A zone damper is a specific type of damper used to control the flow of air in an HVAC heating or cooling system. ... Assorted discrete transistors A transistor is a semiconductor device, the most common type of which acts as a current amplifier. ... 1985 (MCMLXXXV) was a common year starting on Tuesday of the Gregorian calendar. ...

By the year 2000, computerized controllers were common. Today, some of these controllers can even be accessed by web browsers, which need no longer be in the same building as the HVAC equipment. This allows some economies of scale, as a single operations center can easily monitor thousands of buildings. 2000 (MM) was a leap year starting on Saturday of the Gregorian calendar. ... Economies of scale characterizes a production process in which an increase in the number of units produced causes a decrease in the average cost of each unit. ...

See also

• Building Automation
• EPICS

This article or section contains information that has not been verified and thus might not be reliable. ... The Experimental Physics and Industrial Control System (EPICS) is a software environment used to develop and implement distributed control systems to operate devices such as particle accelerators, telescopes and other large experiments. ...

External links

• HVAC - North American Directory for Heating, Ventilation and Air Conditioning Contractors and Consultants.
• HVAC News & Directory New product features, news, events, training calendar and directory for hvac industry professionals.
• Heating Control System Glossary Clear & Concise glossary for heating control systems.