Pre-1980: Simple and inefficient. Every piece of equipment (furnace, fan, pump, boiler, light, etc.) had its own standalone controller. These controllers were usually very simple switches and thermostats and they operated in ignorance of what other equipment was doing (Figure 1).
1980s through 2000s: Complex and efficient. Central 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 (Figure 2).
Today: Complex and less efficient. Some equipment is arriving from the factory as self-contained systems with all sensors, control devices, DDC panels, and software in place. Onboard 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, VFDs, compressors, burners, etc. Other equipment continues to rely on the BAS for all of its inputs and outputs (Figure 3).
Today’s systems are, on the surface, more cost-effective for a new construction project. The standardization and quality control inherent to onboard equipment controllers 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 onboard 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 rooftop unit (RTU) on-board controller will modulate its return air fan speed to maintain a setpoint building pressure. That’s fine if there is only one RTU serving a building. When there are multiple RTUs, building pressure cannot easily/successfully be accomplished by individual rooftop units, which do not communicate with each other.
Even if the RTUs are all controlling to the same building pressure sensor and setpoint (which is possible with most RTU-to-BAS interfaces), the individual control loops on the RTU will invariably result in different output signals to their respective return fans. As soon as the return fans start operating at different speeds, they will potentially start fighting each other, and you can 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 RTUs.
In “complex and efficient” all-BAS systems, this was not a problem because the BAS could send exactly the same output signal to all return fans for building pressure control. This is just one example of how the shift to a “complex and less-efficient” approach has resulted in a step backwards with respect to systems integration. Building controls will undoubtedly continue to evolve but this is a current reality that project teams must address. ES