Building Controls: To Be (Centralized) or Not to Be, That is the Question

Building control systems have become highly sophisticated computer systems capable of operating nearly any piece of heating or cooling equipment. Achieving this control capability requires good input data and robust software programming. For certain systems, such as refrigeration systems, it is often advantageous to use decentralized controls for critical functions, such as compressor operation and refrigerant safety circuits. In a world where faster, better, and cheaper weigh equally, HVAC design engineers are faced with a choice between decentralized packaged control systems and whole-building direct digital control (DDC) systems to operate their equipment, each with its own integration challenges.

Decentralized Packaged Controls: Out of the Box Functionality

The main advantage for packaged controls is that they’re provided by the manufacturer for exactly the unit onto which they’re installed. Unlike DDC systems, in which off-the-shelf components must be custom-installed, programmed, and tuned, a packaged controls system is often ready to go right out of the box. All control logic, alarm conditions, and safety circuits are already in place. One only has to connect power, and the system is up and running.

This system works well for commodity equipment, and less well for custom equipment. For commodity equipment, such as boilers, chillers, and unitary air handling units, packaged controls provide an easy to use and safe interface through which the building DDC system can communicate. A boiler control panel, for instance, can stage the sequencing and firing rate for the attached boilers based on a supply header temperature set point input provided by the building DDC system. This saves the DDC programming from creating complicated, customized programming containing numerous PID loops and safeties to control the firing rate of the boilers under all possible operating scenarios. Packaged controls have a distinct advantage when operating proprietary equipment.

One disadvantage of this hands-off approach is the lack of flexibility or customization ability for the design engineer. Today’s highly efficient HVAC designs require sophisticated control schemes, such as daytime temperature or ventilation setbacks to reduce energy consumption. If these sequences are not provided by the equipment manufacturer, there may be no way to implement the specified sequence of operation. If this is true, it will require field modification of the packaged unit, resulting in a Frankenstein combination of packaged and DDC controls yielding unpredictable results.

A second disadvantage is the lack of external visibility to the operator through the DDC system. The decentralized packaged controls may not have provisions for accepting input from or providing outputs to the DDC system. This lack of visibility and control leaves building operators with little diagnostic information in the event of equipment failure. Trend log data may be nonexistent, and the only indication of alarm may be a blinking LED on the affected unit. This is a relatively primitive way to operate today’s high-performance buildings.

DDC Controls: Customization and Flexibility

A building-wide DDC system is a highly customizable computer system built with nearly any combination of controller inputs, outputs, and programming logic required to implement the HVAC design engineers’ control sequence of operation. This customizability leads to great flexibility on the part of the HVAC design engineer to refine the operation and system interaction of HVAC components like fans, dampers, and valves. The design engineer will not find him- or herself surprised or limited by the “standard offering” of a unit’s packaged controls.

Besides allowing for greater flexibility in design and construction, building-wide DDC systems can easily provide operators with exactly the kind of information they want to see when operating a building in a customized graphical interface. Energy performance monitoring for Measurement and Verification plan compliance and fault diagnostics are some of the software features not found in packaged controls that can be integrated into the DDC system and improve the building operator’s ability to monitor the facility.

One disadvantage of DDC systems is that they’re as custom and prototypical as the mechanical systems they operate, which can result in complicated operation and interaction. The HVAC design engineer creates a schematic control sequence of operation, but it’s up to the DDC programmer to implement the design intent. This allows some latitude and creativity on the part of the programmer, and the results can be unpredictable. A high degree of collaboration and communication between the design engineer, the DDC programmer, and the Commissioning Authority is required to ensure the DDC system provides the desired operation and required functionality.

A second disadvantage of DDC systems is cost. Packaged controls are an economic solution for small unitary equipment, such as a furnace or rooftop air-source heat pump. If the packaged controls sequence of operation meets the design engineer’s requirements, the additional expense of duplicating the programming and providing independent control components, such as current sensing relays and motorized actuators, is hard to justify. Packaged controls have their place; DDC is not the only or ultimate solution.

Achieving Integration

A high efficiency HVAC system will often include a building-wide DDC system that must integrate with some packaged controls, such as a chiller or boiler control panel. This integration can be challenging. Care must be taken to ensure that the packaged controls have the readable and writable points available to integrate and communicate with the DDC system. A communication interface, such as BACnet or Modbus, is an economical option. This communication interface provides a lot of readable data to the DDC system, but not each of the points is writable, or, “commandable.”

The HVAC design engineer should clearly specify which points are to be provided, and show these requirements in the contract documents, either as text in the project manual, or as a table on the contract drawings. The HVAC design engineer should also indicate whether each point is a part of the packaged control system or the DDC system. Equipment submittals should be carefully reviewed to verify the expected interface between the DDC system and the packaged system, such as a terminal strip, is present. Finally, the Commissioning Authority should review and understand the demarcation between the packaged controls and the DDC system so he or she can verify the control responsibility during start-up and functional testing. This sequence of events will result in reduced start-up times and a smoother turnover experience.

Better Together

Today’s HVAC design engineers face integration challenges when specifying the combination of decentralized packaged control systems and whole-building DDC systems that high-performance buildings require. Balancing the use of efficient packaged control systems with robust and capable DDC systems, and making the two systems work together, leverages the strengths of each to the best advantage of the building’s operation.