Designing a properly functioning building-control system can be a complex and challenging task, but even the most complex system will benefit from a good dose of the basics. Creating a successful system also is a task that takes more than just a controls engineer. It must be a team effort between designers and engineers, project managers, facility managers, building owners, and tenants. Here is a cross-disciplinary list of system-design “must-dos.”
As obvious as this may seem, our industry often works against conventional wisdom. Many engineering firms have dedicated control-system engineers and dedicated mechanical and electrical engineers. Therefore, the person writing control specifications may not understand how building equipment actually operates. If you want to control something, you must know:
• Its purpose.
• What it was designed to do.
• How it operates. What is its operational sequence? How does it start? How does it stop?
• What device or sensor controls it. Usually, there is at least one major control device (e.g., thermostat, pressure sensor, humidistat) for each piece of equipment.
Again, it might sound like common sense, but many projects start out with only a portion of the equipment defined or included. Understand why the building was designed, what its purpose is, and who is going to occupy it. Is it a school, an office, a hospital, or a laboratory? Is it industrial? Are you seeking to provide a process solution or a comfort-control solution? The equipment should complement the building and support the occupants.
Some designers consider the major equipment, but forget to outline the smaller equipment. The large equipment necessary to cool, heat, or ventilate a large building is critical and must be designed well. However, the ultimate success of a control system depends on the smaller systems. For example, you can design the best central-plant control system, but if the terminal unit control feeding the CEO’s office does not work well, the project will not be viewed as successful.
While the big equipment serves the building as a whole, the smaller equipment focuses on the comfort of individual occupants. If you forget about that once, I guarantee you never will again; the occupant will not let you. You will be called back and have to make it right.
Safety elements of a system’s operation need to be addressed as well. Ideally, a building’s systems may never need to operate in safety mode, but the reality is that it is bound to happen. Therefore, designers need to anticipate this aspect of their systems. An engineer tries to anticipate all of the modes of operation and design the way a system transitions into safety modes. Forgetting about these safety elements can be disastrous. For example, what happens in the event of a fire? What happens if the design pressure is exceeded? What happens when a system fails? In a building fire, most people die from smoke inhalation before the fire even gets to them, so your job is to control the smoke. In a large building, the smoke-control system can be complex, with multiple smoke zones. Matching the number of units to the zones is the key to minimizing complexity.
Do not forget the KISS (“Keep it Simple, Stupid”) principle. A complex control sequence is the kiss of death. Often, this is a result of poor system design. The system design should work to support simple operational sequences. If you start outlining a control sequence and find it becoming very complex, go back to the engineer and discuss how the design can be revised to simplify the control.
Some projects push the control system to solve a design problem. This is an indication that the KISS principle has been ignored. This happens because the equipment already has been installed, and targeting the controls that are operating the equipment is seen as the cheapest way to “solve” a problem—even if the problem lies in the system design. Control systems can do many things, but asking them to solve system-design issues often creates other problems and confuses the operator. Simple override actions can cause unwanted reactions. Just keep it simple.
Define the following:
• How is the equipment started? What needs to happen before the unit is started? Is there a startup (i.e., morning warmup) mode?
• Once started, how does each part operate, and what sets the standard operating mode?
• What changes the system from one mode to another, such as from heating to cooling?
• How is the equipment stopped? What needs to happen before a unit can be stopped? Is there a time delay? Is there a shutdown (i.e., evening cooldown) mode?
• Define the emergency sequences: over pressure, smoke, fire, equipment failure, alarms, etc.
The first thing you will need is a control diagram. Start by drawing the system. What major equipment makes up the system? What are the pieces of equipment? Remember to include the major and minor control devices. Include the safety devices. Include the monitoring and alarm sensors (Figure 1).
Next, you will need a points list. Looking at your control diagram, define each device, and evaluate how it will operate. Is it an input device? Is it looking for a signal for it to do something? Is it an output device? If it does not need a signal from something else, it most likely provides information to something else. Is it for control, monitoring, or alarming? Is it hard-wired, or is it integrated into the controller? This points list can be expanded into a point-list matrix or chart. This chart can provide a simple definition (name, type, and function) of each piece of equipment. The “type” of equipment should include whether it is analog input, analog output, digital input, digital output, etc. (Figure 2).
The final element is a sequence-of-control narrative (Figure 3).
Write out how the systems and equipment will operate. It is best to think of this like writing a computer program. Sequences are best if they have definite modes or actions, (on/off, open/closed, modulate open/closed, etc.). These used to be done with ladder diagrams, but now a graphical flow diagram illustrates it well. Spell out the sequences (“If X happens, then Y will result.”). Do not generalize; be specific. Refer to the control diagram and the points list. Was each item included in the narrative? If not, then you may have missed something. If you cannot figure out how a piece of equipment operates or cannot define its control, maybe that piece no longer is necessary.
Most control specifications call for energy-efficient or special control algorithms, such as optimized start/stop, economizer, temperature reset, and after-hours operation. Algorithms are great, but certain parameters still need to be provided. Even when control systems are provided, buildings can operate inefficiently, if the specifics, such as setpoints or start and stop times, are not defined clearly. Define specific space temperatures (occupied and unoccupied). Define the reset ranges and what specific states outline the range. Define start and stop times (include weekday and weekends and specific holiday operation). Remember, if you do not know what these parameters are, the control-system contractors most likely will not either.
These “must-dos” provide a great starting point for the beginner and everyday professional. Obviously, these are just starting points, and building-automation systems can include numerous additional items, such as trending definitions, open/closed controls, proportional-integral-derivative controls, direct-digital-control systems, and fuzzy logic, but sometimes it is best to KISS and step back to remember the basics that are the foundation of any successful building-control project.
J. Christopher Larry, PE, CXA, CEM, CEP, CIPE, LEED AP, is director of energy engineering for exp (formerly Teng Solutions). He has spent more than 25 years working to minimize the energy and environmental footprint of buildings through design, modeling, performance optimization, and intelligent controls. He has held numerous positions within ASHRAE, including chairman of the Chapter Technology Transfer Committee and chairman of Technical, Energy and Governmental Activities. He is past president of the Association of Energy Engineers (AEE) and has instructed the certified-energy-manager training course for AEE. Currently, he is chairman of the Building Intelligent Quotient within the Continental Automated Buildings Association and a member of the Zero Energy Consortium.