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Commissioning: Is Systems Integration Getting Worse? 

In very simplistic terms, building systems controls have evolved as follows:

Pre-1980 = Simple & Inefficient

Every piece of equipment/system (furnace, air conditioner, fan, pump, boiler, light, etc.) had its own stand-alone controller. For the most part, these controllers were very simple switches and thermostats and they operated in complete ignorance of what other equipment was doing.

1980’s through 2000’s = Complex & Efficient

Central building automation systems (BAS) monitored all data and calculated all control signals. As such, the BAS computer contained all of the input and output signals and algorithms for every piece of equipment/system.

Today = Complex & Less Efficient

Some equipment is arriving from the factory as self-contained systems with all sensors, control devices, direct digital control panels, and control software already in place. On-board equipment controllers share input and output points and setpoints with the BAS but typically do not allow the BAS to dictate output signals used to control equipment devices such as dampers, variable frequency drives, compressors, burners, etc. Other equipment continues to rely on the BAS for all of its inputs and outputs.

Today’s systems are, on the surface, more cost effective for a new construction project. The standardization and quality control inherent to on-board equipment controllers clearly drives down the total first cost of equipment, installation, and startup. However, we are at a very challenging place in the evolution of building systems control because many of today’s on-board controllers are not designed to communicate with each other and they limit the level of integration the BAS can accomplish.

One example of this is building pressure control. A single air handling unit (or rooftop unit) on-board controller will modulate its return air fan speed to maintain a setpoint building pressure. That’s fine if there is only one air handling unit serving a building. When there are multiple air handlers serving the same building, for example four rooftop units serving vertical slices of a multi-story building, building pressure cannot easily/successfully be accomplished by individual rooftop units which do not communicate with each other.

Even if the rooftop units are all controlling to the same building pressure sensor and setpoint (which is possible with most rooftop unit-to-BAS interfaces), the individual control loops on the rooftop units will invariably result in different output signals to their respective return fans. As soon as the four return fans start operating at different speeds, they will potentially start fighting each other and you can easily end up with one or two return fans at 100% speed and the other fans at minimum speed. This plays havoc with ventilation and temperature control at all of the rooftop units.

In the Complex & Efficient all-BAS systems, this was not a problem because the BAS could send exactly the same output signal to all rooftop units for single building pressure control. This is just one example of how the shift to a Complex & Less Efficient approach has resulted in a step backwards with respect to systems integration. Building controls will undoubtedly continue to evolve but, in the meantime, this is a current reality project teams in general and commissioning professionals in particular face.  

 

Commissioning: Getting It Right

Engineered Systems, September 2012

by
Rebecca Ellis, PE, LEED AP BD+C, CCP, CPMP, CxA
President
Questions & Solutions Engineering
1079 Falls Curve
Chaska, MN  55318
rteesmag@QSEng.com