The only thing constant about the green building industry is its capacity for innovation. And with great change, unfortunately, comes great debate over appropriate vocabulary, especially for building automation systems.

As a result, we tend to throw around acronyms frequently. Probably no better example of acronym confusion exists than building automation systems (BAS). They’re also referred to as building management systems (BMS), energy management systems (EMS), direct digital control (DDC) systems, and building automation and control systems (BACS).

All of the references above are used interchangeably in the controls industry, but for the sake of simplicity this article will deal strictly in building automation systems (BAS) — mostly because this is the term that I typically use.

Building automation systems control heating, ventilation and air conditioning equipment. This in turn controls exhausting of indoor contaminants, manages equipment scheduling and maintains building temperature, humidity, pressure and energy use. As buildings become more and more intelligent, it’s not unusual to see a BAS that integrates life safety, power, security and lighting systems as well.

Direct Digital Control (DDC) Building Automation Systems

The prevalence and availability of technology has produced an exponential growth of direct digital control (DDC) building automation systems (different from the air-driven pneumatic systems of the past).

Be warned — the term “direct digital control system” is often used to describe to a building’s entire BAS. However, this wasn’t always the case. Direct digital control actually refers to a communications protocol developed in the 80s that expedites signal transmission between BAS components.

Confused yet? Maybe going over the nuts and bolts of DDC will help.

DDC is a control loop in which a microprocessor-based controller uses sensor inputs and setpoint parameters to program equipment accordingly. In a DDC system, the programmed control sequence determines outputs to the equipment.

While many pneumatic systems remain in operation today, over time their hoses and components begin to leak, and their associated compressors require more maintenance and upkeep. DDC systems, on the other hand, offer the following advantages:

  • Computerized control
  • Full modulation
  • Start/stop and staged control strategies
  • Higher levels of accuracy
  • Quicker response times
  • Greater flexibility for future expansion

While many early DDC systems utilized proprietary programming language, the development of communication protocols such as BACnet (developed by ASHRAE) and LonWorks (developed by Echelon) have helped standardize the industry. This was a huge step forward, since one of the most important characteristics of a DDC system is the transmittal of information.

But it’s important to remember that these systems are scalable, and only as valuable as the programming behind them. Once data is collected from sensors or devices, it’s up to the BAS programming to interpret this data and respond accordingly to control, monitor and optimize the building’s mechanical equipment.

How a DDC Control Loop Works

The basic DDC control loop requires three components:

1) An input

2) A processor/logic system

3) An output

The input comes from sensors or devices and can be either analog or digital. Examples of inputs include readings of temperature, airflow, fluid flow, pressure, current, CO2, wattage, etc.

The processor comes in the form of a DDC controller. This uses programmed control algorithms to process information from the input device, and then responds (if necessary) with an output signal.

The output, which can also be analog or digital, is the actual device that is being controlled by information from the input. Output devices include valve and damper actuators, relays, variable frequency drives (VFDs), airflow stations, etc.

The analog input/outputs mentioned above are modulating signals sent to or from the controller, while digital (or binary) input/outputs are either ‘on’ or ‘off’, ‘1’ or ‘0’.

Ultimately, a good BAS is multi-layered, complicated, and a very important part of managing a high-performance building. Having the most advanced or energy-efficient equipment and technology installed cannot guarantee energy savings or indoor environmental quality. Only the integration of an advanced BAS can bring a building’s many systems together.

For more information on the ins and outs of DDC control systems, check out DDC Online’s Guide to DDC.


imageSarah Gudeman

Sarah Gudeman is a mechanical design engineer and licensed EIT in the state of Nebraska. Ms. Gudeman also is a LEED Accredited Professional (BD+C) and an active member of the USGBC Nebraska Flatwater Chapter’s board of directors. She specializes in building energy modeling and audits, sustainable design and environmentally-friendly practices.

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