Direct Digital Controls Can Be a Commissioning Solution

By Mark Kirby, P.E., Senior Mechanical Engineer, TLC Engineering for Architecture, Orlando, Fla. -- Consulting-Specifying Engineer, 3/1/2004

One of the hottest topics these days among engineering professionals is building commissioning. But when it comes to communicating this enthusiasm for commissioning to owners, I've faced the same question every time: "Why should I have to pay extra for you to commission my building?"

A lot of people—including so-called commissioning agents—struggle with the answer to this question. The reaction from architect, engineer and contractor is often to think, "Here we go again. We're going to have to provide additional services for minimal or no compensation."

But what if you could commission a building at virtually no extra time or cost? That's a no-brainer, right? Well, direct digital control (DDC) systems may well be the commissioning tool that makes it possible.

To understand how DDC systems can be the commissioning solution, one must first understand, on a project-by-project basis, what commissioning means and what the objectives are. For example, is the owner trying to obtain certification for energy efficiency—maybe for U.S. Green Building Council's Leadership in Energy and Environmental Design certification? Or does the project require proof of energy consumption, as in performance-contracted improvements? Is the project trying to qualify for a financial incentive offered by the local energy company? Does the owner want to verify that additional dollars for energy upgrades are well spent and that the upgrades perform as anticipated? Or—and this is perhaps the biggest reason commissioning has become so hot—is it required to make sure everything works the way it's supposed to?

Once the project team determines the commissioning objectives and requirements, strategies can be developed to incorporate DDC into the process. Nowadays, microprocessor-based DDC systems are part of almost every project. In fact, it's a safe bet that projects without DDC systems are probably simple enough not to require commissioning.

But in order to understand how it can function as part of the commissioning process, a brief description of DDC is in order.

DDC systems are typically made up of distributed processing control units mounted at the device being controlled and networked to a central operator's interface station. The systems are typically used to control and monitor environmental, plumbing, lighting, fire-alarm and security systems, which coincidentally happen to be the main systems that require commissioning.

DDCs have evolved to the point that they are very easy to install, with almost all the control logic preprogrammed. Modern DDC systems require only minor adjustment of set points or activation of subroutines to function. The use of plug-and-play DDC systems is one of the main reasons why commissioning has become so important.

However, even though DDC systems have become easier to install and program, because building systems have become increasingly complex, control systems also have become more complicated. What this means is that A/E/C professionals must do a better job of training other players to understand, on a holistic basis, how these systems work and to understand the project objectives. If we demand improved attention to detail in the execution of all aspects of a project, our projects will be more successful and more profitable. Utilization of DDC systems for commissioning demands better project execution, which is easily verified if the systems are designed and installed with this objective in mind.

When using a DDC commissioning strategy, the design professional must not only define commissioning objectives, but also achieve building team concurrence on these objectives as early as possible. It can't be stressed enough that the commissioning objectives must be defined before the system specifics can be developed. Once the objectives are defined, the DDC systems can be developed to achieve the commissioning objectives through inclusion of transmitters such as temperature, humidity, CO₂ and pressure.

Trend logs—time-based logs that track a measured system parameter—should be used to store and present these system parameters for commissioning verification. Trend logs are the key to commissioning with DDC. Most DDC building systems are provided with trend-log capabilities through canned subroutines, which only need to be activated and have the required system parameters attached. Consequently, using DDC systems for commissioning need not burden the cost or complexity of the project if the process is executed properly.

The overall objective of using the DDC system for commissioning should be to add as few devices to the system as possible while meeting the commissioning requirements. The same holds true for the trend logs. The designer should require only those trend logs that are required to verify the commissioning objectives.

Careful thought should be given to the information which is required to verify the commissioning objectives, and the design documents must clearly state how this information is to be trended and presented. Extra effort must be made to ensure that all A/E/C professionals clearly understand the commissioning objectives and requirements before any work begins.

An example of a commissioning trend log for a variable-air-volume air handler might include: the discharge air temperature, discharge air pressure, fan speed, chilled-water valve position, chilled- water supply temperature, space CO₂ level and outside air quantity. Each parameter could be trended every 15 minutes and presented on a graph that would be used to verify system commissioning objectives and serve as a diagnostic tool to determine system deficiencies. The same types of trend logs and graphs could be set up for all building systems to verify system operation and the commissioning objectives.

Commissioning with DDC is a low-cost solution to a big concern. Assuming that objectives are developed properly, conveyed concisely and executed accordingly, DDC is the best commissioning tool you probably didn't know you had.