Setting Up Controls for Success
Feinzig was reporting at the National Conference on Building Commissioning (NCBC) meeting on "Controls Commissioning: When Good Isn't Good Enough," a paper he authored with Arthur Nichols, a senior mechanical commissioning engineer, and Dan Sullivan, an Automated Logic Controls independent dealer.
Determining how smart your BMS is, and how smart it needs to be, can challenge many an engineer or commissioning provider, Feinzig said.
"What aspects of the BMS should be included in the owner's project requirements, basis of design, construction documents, and specifications?" he asked. "Does the redundancy in the controls match the given examples from the point of view of a critical facility where reliability and redundancy are essential? All too often, the reality is that owners don't know what they're getting, the designers don't understand what they're specifying, and only someone with a strong controls background is qualified to make sense of it all. Otherwise, the controls contractors will deliver the lowest-cost, lowest-functioning system."
BUILDING MANAGEMENT BRAINSIn these types of facilities, he said, life safety and the long-term mission often require limited downtime, if any. "Downtime is simply not acceptable," Feinzig said.
In the cases he referred to, "The controls sequences were clear and well-written and they met the basis of design. Where the process ran into difficulty was in how the BMS vendor implemented the design, and what the owner was willing to accept."
The BMS needs to function properly in order to meet the owner's project requirements, he pointed out. And in order to do that, the commissioning authority (CxA) "must be intimately familiar with various manufacturers' control systems, programming methods, and what information ... will affect cost, flexibility, and the overall controls design in a way many owners, and even design professionals, are not familiar or comfortable with.
"The engineers can design a $20 million mechanical infrastructure," Feinzg said, "but without a comparable control system, the full benefit of this investment will not be realized. The BMS is the brains of the building, and a knowledgeable CxA can ensure the owner gets as smart a building as he is willing to pay for.
"Clear control sequences are not enough to ensure project success," he said. The CxA must ensure that the installation matches the design, and that must be done during the actual construction.
MATCHING THE MECHANICALSIn a critical facility, such as a lab or medical facility, redundancy and reliability are more important than energy efficiency, Feinzig pointed out. "Air quality may have a vital importance as well, particularly where pressurization and/or the mitigation of chemical, biological, and radiation hazards are integral to the facility's mission and the life safety of the occupants."
The key to success, he said, "is to get the controls design to parallel the mechanical design in these areas." Does the system have redundant mechanical systems in place? Make sure the controls are designed to match those multiple units.
The controls system also needs to be tolerant of maintenance and fault, Feinzig said. "How will the controls design meet a comparable level of reliability, availability, and maintainability?" These items need to be spelled out in the owners project requirements. "The CxA should push for clear statements about reliability, availability, maintainability, and fault tolerance," he said, "specifically related to the control system."
DEDICATION AND HEARTBEATControl system redundancy that matches the HVAC equipment's redundancy is the goal. It's accomplished "by providing a lead and a redundant controller for every critical system," Feinzig said. It's called a manager.
"Programming is mirrored between the two managers," he explained. "The redundant controller is always on line to perform an instantaneous changeover on loss of the lead manager." A chiller plant example shows all the chillers being controlled by the lead chiller manager. The lead chiller manager is connected to the redundant chiller manager through a transfer relay panel.
"Each individual chiller has a single control module," Feinzig said. "In this arrangement, each chiller has one controller. Failure of a controller is equivalent to failure of a chiller, and the redundancy matches."
Then there's the "heartbeat." This hardwired signal originates from the lead manager, and it connects the two managers. As its name implies, the signal pulses.
"The heartbeat continuously informs the redundant manager that the lead manager is healthy," Feinzig said. "If the signal is interrupted - the heartbeat is lost - the redundant manager immediately assumes control functions." Primary and secondary pumps, condenser water pumps, cooling towers, etc., use a similar arrangement.
What you don't want to do, he added, would be to control all chillers on a single controller. "The loss of a single control transformer or circuit breaker would render all chillers inoperable."
"The controls design is integral to the success of the project," Feinzig said. "The key question to answer is, â€˜Who is driving the bus?' "
Is it the mechanical engineer, a dedicated controls design firm, or the controls vendor? "Most mechanical engineers don't have the depth of experience of a dedicated controls designer to evaluate vendors, their products, and personnel," he said.
"The commissioning authority's responsibility is to the owner," he concluded. "It is not necessary that the CxA be a DDC expert, but if he or she has enough of an understanding of control systems hardware and software, then he can ensure the owner gets what he expects. Then, good enough becomes great."
Publication date: 09/25/2006