Direct digital control

Central controllers and most terminal unit controllers are programmable, meaning the direct digital control

Direct Digital Control

Direct digital control is the automated control of a condition or process by a digital device .A very early example of a DDC system meeting the above requirements was completed by the Australian business Midac in 1981-1982 using R-Tec Australian designed hardware...

 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

Analog signal

An analog or analogue signal is any continuous signal for which the time varying feature of the signal is a representation of some other time varying quantity, i.e., analogous to another time varying signal. It differs from a digital signal in terms of small fluctuations in the signal which are...

 and digital

Digital

A digital system is a data technology that uses discrete values. By contrast, non-digital systems use a continuous range of values to represent information...

 outputs for control of the transport 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 such as valves, dampers, and motors.

Groups of DDC controllers, networked or not, form a layer of system themselves. This "subsystem" is vital to the performance and basic operation of the overall HVAC system. The DDC system is the "brain" of the HVAC system. It dictates the position of every damper and valve in a system. It determines which fans, pumps and chiller run and at what speed or capacity. With this configurable intelligency in this "brain", we are moving to the concept of building automation.

Building Automation System

More complex HVAC systems can interface to Building Automation System (BAS)

Building automation

Building automation describes the functionality provided by the control system of a building. A building automation system is an example of a distributed control system...

 to allow the building owners to have more control over the heating or cooling units. The building owner can monitor the system and respond to alarms generated by the system from local or remote locations. The system can be scheduled for occupancy or the configuration can be changed from the BAS. Sometimes the BAS is directly controlling the HVAC components.
Depending on the BAS different interfaces can be used.

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.

After the control of air flow and temperature was standardized, the use of electromechanical relays in ladder logic

Ladder logic

Ladder logic is a programming language that represents a program by a graphical diagram based on the circuit diagrams of relay logic hardware. It is primarily used to develop software for programmable logic controllers used in industrial control applications...

 to switch dampers became standardized. Eventually, the relays became electronic switches, as transistor

Transistor

A transistor is a semiconductor device used to amplify and switch electronic signals and power. It is composed of a semiconductor material with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current...

s eventually could handle greater current loads. By 1985, pneumatic control could no longer compete with this new technology.

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

Economies of scale

Economies of scale, in microeconomics, refers to the cost advantages that an enterprise obtains due to expansion. There are factors that cause a producer’s average cost per unit to fall as the scale of output is increased. "Economies of scale" is a long run concept and refers to reductions in unit...

, as single operations center can easily monitor thousands of buildings.

External links

• ASHRAE - The American Society of Heating, Refrigerating and Air-Conditioning Engineers