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Keys to Designing More-Effective Control Systems

Oct. 2, 2012
21st-century building-control systems should be better than they are. The good news is that they can be

Building-control systems are notorious for failing to meet the performance expectations of their designers and operators. Much of the problem can be attributed to the convoluted process through which they are implemented, which separates those who design the system from those who install it and those who install it from those who operate it. As control systems become more complex, this fragmentation adds huge degrees of difficulty to getting building controls to meet performance expectations. Also, it has contributed to the lagging technical development of control systems. But there are some things designers can do right now, despite this defective design and implementation process, to ensure building owners and operators are satisfied with their building controls once a system is up and operating.

This article will suggest three things designers scan do that will almost certainly improve both the end performance of an HVAC system and the owner’s and operators’ satisfaction with it.

Add More Instrumentation to Occupied Spaces
It is an embarrassment to our industry that here in the 21st century, many occupied commercial spaces—even in new buildings—are not monitored even for their temperature. The old adage that you can’t control what you don’t measure certainly applies and explains why comfort continues to be such a problem. But most designs still employ only a single space-temperature sensor for every HVAC-system zone, even when that zone serves multiple offices or large open areas. This is a throwback to the pre-digital control days, when one temperature sensor was all that could practically be employed to control an entire HVAC zone. Control for zones serving multiple spaces has not really been updated since that time.

Here is how designers can most effectively incorporate more occupied-zone instrumentation to improve comfort, efficiency, and ease of building operations. First, ensure that a building-control-system design incorporates at least a space-temperature sensor and an occupancy sensor (to provide occupancy status to the control system) in every office or area separated with high partitions. In large open areas, install temperature and occupancy sensors in at least every 500 to 800 sq ft of occupied area. Locate them wisely so they will not be affected by direct sunlight or obstructed by furniture and so the occupancy sensor will be able to detect occupancy throughout the area monitored by the temperature sensor. This instrumentation will provide vastly improved space-temperature and occupancy information to the control system.

Along with each temperature/occupancy sensor, consider including a humidity sensor. And at least some of the subzones (i.e., each space served by a temperature/occupancy sensor) should be instrumented with carbon-dioxide sensors. This is especially important in spaces that may be subject to large variations in occupancy. The result is that nearly all HVAC zones, including open office areas, will have more than a single temperature/occupancy sensor, and, thus, each zone typically will have multiple subzones. To finish off this subzone instrumentation, include a separate switching circuit for the general lighting in each subzone operated by the building control system—if possible, in conjunction with the occupancy sensor.

From a design/installation standpoint, each HVAC zone is designed, installed, tested, and balanced very much the same as in the past. But from a control standpoint, consider how much more effective spaces can be operated. Comfort in occupied areas can be greatly improved with space-temperature and occupancy data from each subzone. The zone is operated in accordance with the average space temperature of only those subzones that are occupied at any time; if all spaces are unoccupied, the zone operates at setback temperature limits. If one occupied subzone becomes out of range (usually more than 1°F above or below space-temperature setpoint) its weight in the average calculation increases. With this automatic zone control, occupied areas of the building are much more effectively maintained within the desired space-temperature range. Occupant satisfaction is improved significantly, and operators find it easier to oversee building operation.

Lighting control should be accomplished separately for each subzone. The occupancy sensor is applied to control lighting as well as the temperature control described above. Providing the occupancy signal to the control system to provide final lighting control allows a big improvement in the effectiveness of lighting and reduces lighting energy. For example, the time delay between the end of occupancy sensing and switching the lighting off can be reduced after-hours so the lights do not remain on long when the service staff makes its rounds at night to empty the wastebaskets.

Especially in more humid climates, space humidity sensors can be very useful in regulating supply-air-temperature-adjustment limits so that the sufficient latent cooling is provided in mild weather, and wasted dehumidification is avoided in warmer weather. The carbon-dioxide sensors are applied for demand ventilation to ensure adequate ventilation for comfort is provided at all times without wasting energy conditioning unneeded outside air.

Essential in this increased instrumentation is some means for occupants to interact directly with the controls to satisfy their preferences. This process needs to be very different than the old setpoint-adjustment button or knob on the temperature sensor. Instead, occupants should be empowered to easily indicate preferences for warmer or cooler space conditions or for having lights on or off for specific periods despite subzone occupancy status. While there are many approaches to how occupants interact with the controls, designers are urged to consider approaches that allow occupants to indicate their current preference (and/or complaints) through their computer or smartphone. Apps for computers and phones have been (and continue to be) developed for this purpose. A simple diagram illustrating the elements of this subzone control strategy is shown in Figure 1.

Some designers believe occupied-area instrumentation is too time-intensive to incorporate into the design process, and is too costly for construction budgets. But if designers concentrate their efforts on this critical aspect of building controls, they can find economical approaches and ways to reduce what is now largely wasted effort, as will be discussed later.

Incorporate More Effective and Reliable Control Networks
Building-control-system hardware and firmware development continues to lag considerably behind control advances in other industries. For networks, our industry continues to rely largely on outdated RS485 serial communications to connect to devices beyond air handlers. Standards for this network are incomplete, and performance is notoriously slow and unreliable, especially when equipment of more than a single vendor resides on a network. This is illustrated by the fact that variable-speed drives and other equipment still often are hardwired for control when a simple network connection would be so much easier and less expensive. While the industry needs to press controls manufacturers to develop and incorporate more-robust device networks (and provide incentives/preferences for those that do), designers can minimize the potential for problems with systems that employ this older device networking by minimizing the number of components on each such network and especially avoiding large numbers of third-party components on a single network. Many controllers have multiple serial networks. Designers should require to the greatest extent possible that networked components be distributed among all those available, with third-party components segregated on networks with fewer components when older networks are employed.

The question remains as to how to connect the subzone instrumentation. This instrumentation must be network- connected to keep installation and control costs down. The answer for the short term is to bypass the slower control-system networks and employ one or more of the newer networks that have been developed for equipment of a specific type. New wireless mesh networks provide reliable wireless communication, and some of these newer devices are self-powered, eliminating the need for electrical connections.

There are two important issues the designer needs to consider in configuring such a system. The first is device location. While auto-locate features for devices such as these are an imperative for the future, for now, ensuring subzone devices are clearly labeled and located for easy access—preferably in plain sight so the operator can visibly confirm each device’s name and location simply by walking through the occupied spaces is recommended.

The second critical issue in such a wireless-system configuration is network speed. Depending on the precise occupancy-sensor configuration that is chosen, one or more of these networks may be relied upon to turn on the lights. If that is the case, the network connection(s) between the occupancy sensor and the lighting- power relay must be speedy to ensure lights turn on quickly when occupancy is detected. To ensure this quick response, designers should pay particular attention to how occupancy sensors and lighting relays for each subzone are configured. If they cannot communicate directly, both relays for each circuit should be connected to the same controller whenever possible, and networks involved in their communications should be limited in size and operate at the highest speed.

Work With Owners to Develop Operations-Support Programs
Building operators are almost always divorced from and lack understanding of, optimization regimens developed during the design of their control systems. The resulting failure to adequately understand and support optimization in operations is the primary reason control-based optimization so seldom succeeds over time. Designers need to recognize that neither control subcontractors nor building operators are optimization experts. Instead, optimization is fundamentally a high-level operations function that requires special support. Designers who expect to deliver high-performance systems must work with the owner during the design phase to ensure adequate support is available for operators to achieve the full optimization potential of the systems being designed.

To be certain this needed support is provided once the building systems are in operation, designers should, as part of their design duties, introduce owners to a new group of firms geared toward working with building operators and managers to achieve, maintain, and verify optimized HVAC-system operation. Designers also should be prepared to assist owners in recommending or selecting the form of operations support that will ensure building systems meet all performance objectives.

Operations-support (OS) firms have several approaches to achieving and maintaining optimized building operation. Some employ remote connections to mine data from building controls and tune controls and/or adjust setpoints based on certain operating criteria. Others employ advanced control strategies they have developed for optimization. Most incorporate some level of artificial intelligence in tuning their control strategies and apply varying degrees of fault detection to ensure systems continue to operate as effectively as possible even while operating circumstances are less than ideal. These firms usually provide periodic performance reports to operations management and some degree of direct support to the operations staff.

The common thread among all OS firms is that they provide much of their computing services remotely and communicate directly with the building-control system. Thus, the term “cloud” services frequently has been applied to them. This utilization of remote analysis is an important new strategy that can infuse greater power into the control capabilities of modern buildings—capabilities that otherwise are inhibited by the limitations of building controls as well as the disconnected processes of their design, construction, and operation. Incorporating an OS firm to work with the owner to develop operations regimens for a building under design means a design team can focus more effectively on ensuring fundamental structural elements of the mechanical system and controls are adequate to support the levels of performance intended for the building. This is a task that is far more likely to succeed than expecting that a sequence of operations with untested advanced control strategies as part of the design documents can be implemented through the fragmented implementation and turnover phases with any success.

Instead, applying cloud-based analysis and optimization permits OS firms to develop customized control strategies from more general algorithms that have been tested and used in many applications. This “mass customization” of control software simplifies implementation and startup and encourages the reliability, effectiveness, and sophistication of control sequences to develop over time.

Because they are cloud-service providers, OS firms can incorporate connections to the serving utilities and provide features such as demand response and time-of-use electric-cost-adjustment features, that reduce operating costs as well as energy use. A diagram of how OS firms typically connect and interact with building HVAC controls and management is shown in Figure 2.

An Industry at a Critical Point
The building industry is at a critical point. It simply must change its processes and approaches to better accommodate available technologies that will vastly improve both the comfort and energy performance of buildings. The disjointed processes and fragmentation of responsibility the industry still relies on to implement its systems are entirely inadequate. For the short term, system designers can greatly improve the chances for success in the application of newer control technologies by following three recommendations: add more instrumentation and control in occupied spaces, provide a much greater emphasis on building control networks, and work with the owner to ensure operations-support that ensures the energy and comfort performance of the design is achieved and maintained over time is provided.

Thomas E. Hartman, PE, is principal of The Hartman Co., an engineering and technology development firm. He can be reached at 254-793-0120 or by e-mail at [email protected].

About the Author


Principal of The Hartman Co., an HVAC engineering and technology-development firm, Thomas Hartman, PE, is an internationally recognized expert in the field of advanced high-performance building-operation strategies. His accomplishments include development of Hartman Loop, an integrated approach to chiller-plant control that dramatically improves operating efficiencies as plant load decreases; Terminal Regulated Air Volume, a network-based, variable-air-volume control technology that coordinates central-fan-airflow and supply-air-temperature control with actual zone requirements; the Dynamic Control family of software strategies and algorithms, which were among the first to employ integrated strategies to take advantage of microprocessor-based control systems; and the Hartman Energy Valuation System, one of the first hourly building-energy simulation programs.