Automation and controls for HVAC systems
- Assess basic design approaches for building automation systems (BAS).
- Interpret codes and standards that guide BAS design requirements.
- Understand key equipment for integration options to improve system efficiency.
The design of control systems for HVAC systems is generally based on the design engineer’s philosophy and experience level. This article cannot cover all possible HVAC systems in use today, nor can it cover all the control methods that can be applied to many of them to enhance their operation and efficiencies. However, the reader should understand that the HVAC system itself, its controls components, and the building in which it is installed should all be considered together as parts of a single, whole design. Designers should remain flexible in providing the best possible system that can control, to various degrees of comfort required, different applications within the same building. The system should provide this control with reasonable costs at the least possible energy use.
An overall goal for the control system should be to establish a method, or a system, of operating and controlling one or more processes by automatic means, using various devices that reduce the need for human intervention. A process can be defined in several ways and HVAC systems can have numerous processes, but it’s typically agreed that there are only a few basic elements of any control system:
- A process variable to be controlled
- A desired control setpoint for the variable
- A controlled device
- A controller that coordinates decision making
- A sensor to provide some type of feedback for a directed change.
Some of the control systems may be required for life safety reasons to prevent equipment from operating in an unsafe manner.
Each basic control system can be grouped together with others to form what can be considered a larger BAS for a building. In the case of a campus setting made up of more than one building, several different BAS could be gathered together in a larger energy-management and control system. Because each control system and/or BAS can be, and often is, proprietary or a "closed system" in terms of communication protocol used by the different individual manufacturers, it is often difficult to have "open" communication between control systems. System integration and interoperability have become more important as BAS options have developed, and this will continue to be a need in the future. BACnet protocol has been one of the driving forces for more open communication systems.
Regardless of whether the basic control system is stand-alone for a new piece of equipment (i.e., an air-handling unit [AHU] or a chiller) or it covers an entire building, an early review of codes, standards, and regulations is often necessary to allow for an expedient design and avoid conflicts that cost time and money to resolve. Groups such as ASHRAE, the Air Conditioning, Heating, and Refrigeration Institute (AHRI), the American Society of Mechanical Engineers (ASME), NFPA, the International Society of Automation (ISA), and many others all have standards to review for systems, equipment, and testing requirements.
A good primary resource for most engineers today is ASHRAE. The society’s various technical committees write standards and guidelines to establish a consensus for such items as methods of testing and classification, design, protocol, and ratings for systems and equipment components of those systems. ASHRAE has numerous technical sources of information including a series of four handbooks that are updated every 4 yr.
Two of these handbooks, 2012 ASHRAE Handbook—HVAC Systems and Equipment and 2015 ASHRAE Handbook—HVAC Applications, contain several chapters filled with information and basic criteria needed to fully understand various HVAC systems. In addition, ASHRAE has published various other standards and guidelines to assist designers on designing control systems. These include: ASHRAE Standard 135-2012: BACnet, A Data Communication Protocol for Building Automation and Control Networks; ASHRAE Guideline 13-2014: Specifying Building Automation Systems; ASHRAE Guideline 36P-2015: High Performance Sequences of Operation for HVAC Systems; and Fundamentals of HVAC Control Systems.
ASHRAE Standard 90.1-2013: Energy Standard for Buildings Except Low-Rise Residential Buildings is the reference standard for energy efficiency. This standard illustrates minimum efficiency and control systems requirements along with commissioning for building envelopes, HVAC, power, lighting, and other equipment, all of which is included in a building system design.
Maximizing energy efficiency with controls
The first step in designing any efficient, effective HVAC system for a building, or a campus full of buildings, is to perform accurate building load calculations and energy modeling. ASHRAE 90.1 provides methods and guidelines for these tasks. The type of HVAC system designed and installed, and its configuration, will certainly require one or more control schemes. The constant interaction and changes in HVAC loads within a building, or between multiple buildings, on a chilled-water system loop, for example, should be part of the system considerations so all equipment can be sized and controlled properly to account for all the energy impacts.
The designer should become familiar with ASHRAE 90.1-2013, Section 6, which includes various requirements and exceptions that affect HVAC design. Section 6.4.3 is titled Controls and begins stating various requirements within the standard that must be considered and included in a BAS. Other sections provide additional information for control requirements. For example, there are some requirements, with necessary exceptions, for the following:
- Off-hour controls on HVAC systems with automatic shutdown of HVAC systems for start/stop under different time schedules, or based on occupancies, or for life safety and security reasons.
- Setback or other controls on heating and cooling systems with optimum start, which will also prevent mixing or simultaneously supplying air that has been previously mechanically heated or cooled, either by mechanical cooling or by economizer systems.
- Motorized shutoff damper controls for outdoor air intake and exhaust systems, so they can be automatically opened or closed when the systems or spaces served are not in use and/or to reduce energy costs or meet code requirements.
- Ventilation fan controls for fans with motors greater than 0.75 hp shall have automatic controls capable of shutting off fans when not required.
- Controls to prevent simultaneous operation of humidification and dehumidification equipment.
- Ventilation controls for high-occupancy areas using demand control ventilation (DCV).
- In Section 220.127.116.11.3., there is a requirement to include variable air volume (VAV) pressure optimization in systems by using static pressure reset based on the zone requiring the most pressure. The static pressure control setpoint(s) are modulated to the lowest operable pressure to control the VAV terminal unit damper positions in the system, thus reducing fan power requirements.
- In Section 18.104.22.168, multiple-zone HVAC systems must include supply-air temperature reset controls to respond to building loads, or outdoor air temperatures depending upon the existing climate zone.
- Section 22.214.171.124 indicates a requirement to include pump-pressure optimization in systems where the total pump-system power exceeds 10 hp. This reduces pump energy by varying control valve positions in a hydronic system, thus providing variable fluid flows.
Control system items to consider
A designer’s goal is to meet the client’s system-performance needs by deciding how to operate the HVAC system to control the desired outcome. There are several basic items that a designer must provide in a design if this is to happen efficiently. These consist of the following components:
- An effective specification
- A control schematic or diagram of the system showing the relationships between equipment and control components
- An input/output (I/O) points list defining all analog and digital (sometimes referred to as binary) inputs and outputs as well as virtual points
- A sequence of operation that describes the actual system to be controlled and custom-tailored to meet the goals and requirements of the project.
Most design firms already have a set of standard specifications that are used on their projects. Some are more descriptive or robust, but a designer who wants to pursue a more effective specification should review ASHRAE Guideline 13. ASHRAE Technical Committee (TC) 1.4, Control Theory and Applications worked on providing this guideline to designers of a BAS with recommendations for developing specifications for direct-digital-control (DDC) systems in HVAC applications. This guide also includes some additional guidance on different performance-monitoring and management levels as well as on overall system architecture, hardware performance, programming configuration, and system installation, communication, and testing.
Guideline 13 provides a very good overview of the benefits of a BAS and discusses the characteristics of a DDC system. One important aspect for consideration of the BAS design is the integration of any other building systems such as fire and life safety, elevators, security access and cameras, and electrical-related items like lighting, sub-metering, and emergency generation. Each subsystem—whether HVAC-related or otherwise—may use a different protocol for communication, and this can create some difficulties in the overall integration. Building controllers, advanced-application (custom) and application-specific controllers, routers, and gateways all become important aspects of the BAS in terms of its entire functionality, with each component having inherent benefits and limitations. The functions performed include automatic control through programmed sequencing of the various building’s system components. This occurs through data-sharing communication and scheduling, trending, processing control variables, and managing alarms and events.
A building controller is a general-purpose controller that is field-programmable and carries out a building’s systems automation and control tasks. It may or may not have I/O points, and usually connects a subnetwork of other controllers to coordinate BAS functions to the field devices on its portion of the overall network.
Advanced-application controllers, also referred to as custom-application controllers, are devices that normally control custom equipment, such as a particular manufacturer’s chiller or AHU. This controller is typically programmed in the manufacturer’s propriety language and usually can only control its particular associated piece of equipment. This controller could be a peer device on the subnetwork with the building controller, or could be in a "master/slave" relationship to it. This master/slave arrangement means that the controller must communicate with the building controller for data transfer and for information such as outdoor air temperature.
An application-specific controller (ASC) can be used to control relatively smaller pieces of equipment within an HVAC system, such as VAV boxes, rooftop units, heat pumps, and others. The control of this equipment is typically done through preprogrammed routines prepared and installed into the ASC by the manufacturer with propriety program language.
Most systems can be designed with an interoperability that allows for open communication, or the basic exchange of data, between components and devices, but many owners still procure systems with varying levels of proprietary protocol. This is typically due to the need to extend an existing system, but the designer should strive to prevent this as much as possible. This is where a good understanding of the intent of ASHRAE 135 would be helpful.
An effective specification will clearly spell out the design intent of the BAS. A generic reference to an open system is not only unacceptable, but also could create major issues when an existing system is modified to include a different and competing manufacturer’s product that includes its own proprietary communication protocol. A specification that requires manufacturers to design/build products that can integrate all information between different systems and manufacturers is becoming a necessity. In addition, it provides a level playing field on performance and pricing options, and allows for future system expansions along with easier training and understanding for system operations staff.
The more difficult task is deciding with the owner the degree of interoperability to provide within the system architecture, and how far down into the system the open protocol is to be carried out. These decisions affect costs and system performance, as communication may have to occur through workstations and various controllers (i.e., primary or secondary) based on their arrangements and how they are linked together via wires or data-link communication buses (typically defined in three tier levels), often called local area networks (LAN; e.g., Ethernet, TCP/IP, MS/TP).
Figure 1 is a sample of one possible network architecture. The system may require any number of routers that pass messages between networks with the same communications protocol, or gateways, that require some translation communication between devices with two different protocols. The primary goal is to determine how best to design the network so that multiple products from several potential manufacturers can be interoperable.
Control schematic diagram
The designer should begin with a list of systems to be included in the project’s BAS. These systems could include a chiller and/or boiler plant with primary and secondary pumps; cooling towers and condenser water pumps; AHUs with or without VAV boxes or other air-distribution devices’ exhaust fans; energy- or heat-recovery systems; and other systems and products. For each system, the designer should produce one or more schematic diagrams and/or piping and instrumentation diagrams/drawings. The level of detail required for these diagrams is dependent upon the project, process, and/or owner needs; the diagrams should show the interconnection of equipment, the critical control components, and the instrumentation used to control the process or system. The symbols used to prepare the diagrams should be from a recognizable standard or be defined within the diagrams themselves.
Figure 2 is an example of a control diagram for an AHU. The inclusion of such a control schematic shows the control contractors the number and type of control devices (e.g., valves, sensors), and their relative physical location of the system components, to include in their bid. All variables to be controlled, with their respective inputs and outputs, should be identified on the schematic diagram. To keep the diagram simplistic, any other component that either is not to be controlled or does not add to the informational aspect of the system representation should be left off so it is easy to understand. This assists in providing a clear definition of the project scope of work and helps define the design intent during construction and final installation and commissioning.
I/O points list
For each system schematic diagram, the designer needs to provide a list of objects or points that are to be included in the BAS controls design. This points list is typically shown in a tabular format (MS Excel is an excellent tool to use and can be incorporated into an AutoCAD drawing) and illustrates all the points required to match up to the system diagram. This listing is a valuable coordination tool for communication to the controls contractor, and can also provide key functional information for the system operation.
Because this listing shows the physical points (hardware) that are part of the system where signal information to/from (in/out) occurs related to those points, it is very often referred to as an I/O points list. The hardware point types are noted as digital (or binary) and analog. The digital points are simple two-option points (software 1 or 0) that can be set up to read on/off or other two-condition change of value (COV) for inputs or outputs. The analog I/Os are proportional values that modulate or vary due to a change in voltage (0 to 10 Vdc), amperage (4 to 20 mA), position (0% to 100% open), pressure (0 to 160 psia), or some other potential range of values.
In referring to Figure 3, Tags A-5 and A-17 are the return and supply air duct detector alarms, respectively, which are physical devices but act as digital inputs to the BAS. These would normally be "off," and can turn "on" when needed. Correspondingly, Tags A-1 (analog input) and A-2 (analog output) are related to varying outside air temperature and a modulating outside air damper, respectively.
If the I/O point is to be sampled and trended in a log, and/or shown in a graphical representation, then this should be noted in the table. The desired trending time interval or differential-value change for the analog points and the COV for digital points should also be noted here, if not in the specifications. The I/O list will typically include related software points, which also help to ensure that graphical displays and trending are fully understood. However, note that trends are at the mercy of how much memory the network or building controllers have available, so these need some design consideration. These types of points can be noted as analog or digital values, but are often shown simply as virtual points. Even though it can be hard to list all the required software points related to a particular manufacturer or control system, the designer should include anything that is necessary for the required sequences of operation or other system functions for interoperable operations. However, the designer should not just add points to the system list if they are not needed, as this will tend to increase costs. Increasing the number of points will require additional storage capacity within the BAS controllers.
Some controls professionals think that a points list is nice but not necessary, and consider the control schematic a far better source to communicate the project needs. Both items are important and should be included in the construction documents, but the points list must be accurate. A major advantage of having a detailed I/O list is to assist the building operations staff in monitoring the systems and the various alarm points. This assists in overall system performance, allowing for greater comfort for the occupants, security, safety—and, in most cases, energy-efficient operation. Note that there may be some physical indicating devices, such as temperature sensors or filter pressure gauges, which may be shown on the schematic diagram but not included in the I/O listing.
Sequence of operation
A control sequence is a key element in the overall functionality of a system, and is necessary for achieving any energy-management and savings goals. The designer should also ensure that control sequences are in compliance with any life safety requirements or any related comfort, indoor air quality, and energy codes in effect at the time for the designated project. There are a multitude of codes with differing requirements and based on the local authority having jurisdiction’s adoption timelines. In addition, the client should be involved in the final review of any control sequences to ensure conformance to any particular facility requirements for their safety or operational standards, not the least of which may require input from, and education of, the owner’s controls technicians and operation staff.
The sequence should be complete, and should cover all operational modes for occupied and unoccupied time frames. Failure modes of operation either for specific control devices—such as outside air dampers or chilled-water control valves—or for the entire system under control should also be included in the sequence of operation. The system and/or equipment should perform exactly as the sequence indicates, including any emergency modes with alarms or safety trips as applicable. Under functional testing during commissioning, the sequence of operation should be tested and confirmed to operate as defined, including all failure modes.
The sequence of operation can be written in one of two manners: by operating mode or by component. Either manner will work, but the overall sequence should be as simple as possible, be organized in a comprehensive manner to clearly illustrate the operation of the system and its components, and be easily understood by the maintenance and operations staff. It might even be helpful to describe the system at the beginning of the sequence to help the reader further understand the system. If the system is a subset of a larger system, e.g., a cooling tower sequence as a portion of the overall chilled-water system sequence, there may be a need for a certain hierarchical descriptive approach. For this example, a good sequence could include:
- A list of reference drawings beyond the specific system or process control schematic (i.e., the chilled-water system overall schematic, the specific chiller schematic)
- An overall system description and purpose (i.e., "the function of the condenser water system is to provide the chillers with 85 F [maximum] condenser water")
- Condenser water system sequence of operations
- Cooling tower start/stop
- Condenser water temperature and fan speed control
- Condenser water pump start/stop
- Condenser water solids separator
- Instrument list
- Failure effects analysis.
This may lead to longer control sequences that take time to write, but the designer should take this time and fully understand all the benefits and limitations of the specific controls items. The worst thing that happens on some projects is the provision of a control sequence that was "cut and pasted" from a previous project, resulting in a system that does not meet the owners’ safety, efficiency, or operational requirements.
Randy Schrecengost is a senior project manager/senior mechanical engineer with Stanley Consultants. He has extensive experience in design and project and program management at all levels of engineering, energy consulting, and facilities engineering. He is a member of the Consulting-Specifying Engineer editorial advisory board.