Expert Advice on Green Buildings

How Do Building Automation Systems Work?


In the green building industry, we throw around acronyms frequently. But probably no better example for acronym confusion exists than with building automation systems (BAS) since they’re also referred to as building management systems (BMS), energy management systems (EMS), direct digital control (DDC) systems, as well as building automation and control systems (BACS)!

All of the references above are oftentimes used interchangeably in the controls industry, but for all intents and purposes this article will attempt to cover all of these acronyms, in brief, under the guise of building automation systems (BAS) because this is the term that I typically use.

Automatic HVAC control systems (building automation systems) are primarily designed to control heating, ventilation and air conditioning equipment to maintain building temperature, humidity, pressure, energy use, control exhausting of indoor contaminants and manage equipment scheduling. As buildings become more and more intelligent, it’s becoming common to see a BAS integrate with control of energy use, life safety, power, security and lighting systems as well.

The prevalence and availability of technology has produced an associated exponential growth of direct digital control (DDC) building automation systems (compared to air-driven pneumatic systems of the past). Direct digital control (DDC) is a control loop in which a microprocessor-based controller directly controls equipment based on sensor inputs and set point parameters. In a DDC system, the programmed control sequence determines the output 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 additional maintenance and upkeep versus DDC systems. DDC control systems offer and advantages of computerized control, full modulation, start/stop and staged control strategies, higher levels of accuracy, quicker response times and 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 Corporation) 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 buildings mechanical equipment.

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 include temperature, airflow, fluid flow, pressure, current, CO2, wattage, etc.

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

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

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

Ultimately a BAS is a multi-layered, complicated, and very important component of managing a high-performance building. Having the most advanced or energy-efficient equipment and technology installed cannot alone assure energy-savings or indoor environmental quality without also integration of an advanced BAS.

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Expert Advice and Comments
clairemolo's picture

Re: How do Building Automation Systems Work?

Thanks for this great article about Building Automation Systems!

I am a recent graduate of Cornell University, where I studied Environmental Science and concentrated in Sustainable Development.