BACnet: Setting the new standard

BACnet has to be specified correctly to be fully interoperable. The BACnet Protocol Implementation Conformance Statement is designed to aid in this process.

By Terry Hoffmann, Johnson Controls Inc., Milwaukee August 4, 2010
At a glance

BACnet – A Data Communication Protocol for Building Automation and Control Networks was developed under the auspices of the ASHRAE. BACnet is an American standard, a European standard, and a national standard in more than 30 countries. It also is an ISO global standard.

As part of a push toward a more energy-efficient nation, building efficiency codes and standards are in the spotlight. Energy policies to improve building efficiency often favor voluntary, incentive-based approaches. But at the beginning of a building’s lifecycle, the permitting process for new construction and retrofits increasingly involves mandates that embed efficiency into the infrastructure.

The goal to improve equipment efficiency from the start has put some manufacturers, integrators, and engineers in the hot seat. For this reason, codes and standards have been identified as important steps toward more efficient buildings.

In Assessment of Electricity Savings in the U.S. Achievable through New Appliance/Equipment Efficiency Standards and Building Efficiency Codes (2010–2020) by the Institute for Electric Efficiency, the institute explores the impacts of existing and proposed codes and standards on electricity consumption in the United States. The potential impacts of existing and proposed codes and standards on electricity consumption are large (3% to 9% reduction in energy consumption). However, experts estimate that compliance with building codes across the United States is less than half. To improve that percentage our buildings and the systems they rely on will have to work together to achieve the lofty goals set for them. BACnet forms the foundation for communication and device development to do so.

Back to BACnet basics
Before we look to revolutionize every corner of the commercial building sector, let’s review the basic principles that have existed for almost two decades. BACnet was the first protocol developed by the building automation industry purely for building system integration. In general terms, BACnet broadly supports the interoperability of HVAC, lighting, electric metering, fire management, and other building systems over a common network, and is widely adopted by manufacturers of building control devices.

 Importantly, BACnet devices do more than just report information when it is requested. They ask each other to perform services. For example, a temperature controller can “talk” to other HVAC devices or building systems. The “conversation” consists of encoded messages, common to all BACnet devices, which exchange information and commands that do things together. In many cases this eliminates the need for central workstation applications and increases reliability.

Why consulting engineers need BACnet background
Most HVAC-savvy engineers are well versed in BACnet, but designers and integrators of all system types need the assurance that all devices will perform the functions that are required for complete interoperability in efficient buildings. Consulting engineers must be able to specify BACnet controls from different manufacturers with great assurance that systems will work together as designed. The BACnet Protocol Implementation Conformance Statement (PICS) is designed to aid in this process.

 Specifically it outlines:

  • Basic information identifying the vendor and describing the BACnet device
  • The BACnet Interoperability Building Blocks supported by the device
  • The standardized BACnet device profile to which the device conforms, if any
  • All nonstandard application services that are supported, along with an indication for each service of whether the device can initiate the service request, respond to a service request, or both
  • A list of all standard and proprietary object types that are supported and any restrictions on the range of data values
  • The data link layer options, both real and virtual, supported
  • Whether segmented requests and responses are supported.

Setting high-performance codes and standards
The proper BACnet infrastructure is critical as we consider the new codes and standards embedded in the nation’s built environment. For more than 35 years, ASHRAE has set the bar for energy conservation with Standard 90 and later 90.1─the basis of building design and construction governance since the energy crisis of the 1970s.

Now in 2010, the stakes have been raised as the building industry pushes toward the promise of a net-zero commercial building market. The new ASHRAE Standard 189.1 takes efficiency a step further, providing minimum requirements for the design, construction, and operation of high-performance green buildings. More specifically, ASHRAE’s website defines Standard 189.1 as a “total building sustainability package” for those who strive to design, build, and operate green buildings. From site location to energy use to recycling, this standard sets the foundation for green buildings by addressing site sustainability, water use efficiency, energy efficiency, indoor environmental quality, and the building’s impact on the atmosphere, materials, and resources.

 Reading the finite details of 189.1 it is clear that an integrated building system infrastructure is critical, and therefore proper understanding and implementation of BACnet interoperability is paramount.

Current BACnet specifications
BACnet has to be specified correctly to be fully interoperable. A specification should include all information that facility executives want to be able to access via BACnet. What are the goals for the BAS system? What information needs to be visible? How will visibility be achieved? Knowing the answers to these questions up front will help specifying engineers take these needs into consideration during the design process.

The BACnet protocol by itself does not determine how a BAS is programmed. Information in the spec should include the sequence of operations, a points list, and input/output points that should be accessible via BACnet.

Good specifications are product-based as well as performance-based. BTL-listed and BACnet-conforming are two of the terms that can be used to describe BACnet devices. A BTL Mark indicates that the product has been independently tested by the BACnet Testing Laboratories (BTL) and found to implement the required capabilities. However, not all devices have a testing program defined and implemented as yet, so they cannot be BTL-listed. Other devices might be considered IT components and, depending on system design, might not be logical choices for BACnet listing. Specifying BTL tested and listed controllers throughout the building wherever possible might be the best rule of thumb.

Where did the project go wrong?
Often systems don’t work as effectively as they should because care was not taken in specifying the objects, attributes, and services that are necessary. They overlook the correct level of detail in the device’s Protocol Implementation Conformance Statement (PICS), which provides the needed information about its capabilities.

Or, the contractor with the low bid decided that the intent of the specification could be served by a product that does not meet all of the demands of the specified devices and that fact was not caught on submittal review or the time of system commissioning.

Sometimes complaints from users about open systems interoperability revolve around a lack of standards in the logic of control for devices from different manufacturers. Factory-mounted controls, while highly desirable for economy and standardization, might expose all their information using BACnet, but might not allow a different controller to intervene in the sequence of operations that the OEM supplier programmed into the controller. So, if the OEM controller does not have an economizer, you can’t add one from a supervisory device using BACnet.

Another example involves BACnet controllers that expose BACnet variables without the use of the optional priority array tables that make it possible for automation systems to override them in a logical fashion. So, should the consultant be expected to specify that priority arrays must be supported, or should he just specify that the BAS must be able to override the lighting controller?

Achieving energy standards and controls
Interoperability is a concern of all consulting engineers. For example, a requirement may call for the automatic shutoff of all interior lighting in certain commercial buildings based on square footage. Using compliant BACnet devices, correctly specified, this can be achieved, allowing the security controller to signal the lighting controller over a common network.

Space control is another example. Spaces having schedule-based HVAC control are usually required to allow occupants to override the schedule, turning the air conditioning on when it might otherwise be turned off. With a BACnet solution, an HVAC override can also be shared with other building control systems. In response, the building controllers command the lighting system to remain on as well.

Specifying BACnet in the future
BACnet is a trusted standard today, but it is also designed to support the technology innovations that lie ahead. The impact of Standard 189.1 is yet to be measured, but rest assured that new committees will continue to be formed to consider the upgrades necessary to handle it. For example, earlier this year, a new BACnet specification for Extensible Markup Language (XML) was announced. The XML language makes building data more IT friendly and, in turn, allows BACnet to extend its capabilities to a wider range of applications. Moving forward, XML can simplify the exchange files between systems, communicate with the Smart Grid, and expand Web services.

In the midterm (6 to 18 months), specifying BACnet as it exists today will serve to support the future too.            

Why your customers care
Why are building owners integrated in efficiency? The bottom line. Utility providers often penalize commercial and industrial customers for exceeding peak demand allowances. Therefore, owners and facility managers are eager to find ways to avoid high demand charges.

A recent Johnson Controls Energy Efficiency Indicator survey of more than 1,400 North American decision makers responsible for managing investments and energy use in commercial buildings reinforced the strategic value of green design. In fact, 65% say they are paying more attention to energy efficiency than they were one year ago. Among those who have prioritized their strategies to reduce their carbon footprint, a large majority point to improving energy efficiency in their buildings as their top climate solution. Again, more evidence that the proper BMS infrastructures supported by BACnet devices, and reinforced by standards and codes, are the best solution.

Hoffmann is director of BAS marketing at Johnson Controls where he has worked in the BAS, fire, and security markets. He holds a bachelor of science degree in electrical engineering from Marquette University and a master’s degree in engineering management from the Milwaukee School of Engineering, where he serves as an adjunct professor. He has written numerous articles in various industry trade publications.