Control sequences for HVAC systems
Follow these 10 steps to create a successful sequence of operation, one of the most important design aspects of any HVAC system.
- Learn how to create a successful sequence of operation.
- Recognize the importance of the sequence of operation as it relates to design, specification, and construction.
- Understand how the sequence of operation carries forward through commissioning and into the long-term operation of the building.
The sequence of operation is one of the most important design aspects of any HVAC system. Without a proper sequence, the system is left to operate wildly—or not at all. When approached methodically, the process can be broken into smaller segments. We’ll look at the steps required to create a successful sequence of operation using a single-zone variable air volume (VAV) air handling unit serving a convention space. These same steps can be applied to any piece of equipment.
Some information must be gathered before the designer can begin actually creating the sequence of operation. This data gathering and brainstorming process can be broken down into the following major steps:
Step 1: Create a flow diagram of the system.
Creating a flow diagram allows the designer to identify the components of the system. These are the components that must be controlled to achieve the desired operational results. The sequence can generally be written with a subsection for each of the major air handling unit components. Fan control may be addressed in one section, temperature control in another, and various safety devices and accessories detailed separately.
Figure 1 shows the main components of the air handling unit (AHU) being considered for our example. The unit has an exhaust fan, outside and supply airflow measuring stations, mixing box, pre-filter, final filter, heating hot water coil, chilled water coil, and supply fan. The flow diagram should also identify the airflow pathway and piping connections. Airflow and water flow rates do not need to be included as this information should be included on equipment schedules. The flow rates could be included if desired, or diagrams can be left more generic. The latter permits use of the same diagram for multiple units with similar configurations. Include all inputs and variables that must be controlled. Components that are not inputs or controlled variables should be left out to maintain a simple diagram that is easy to read.
Step 2: Categorize the purpose of the equipment.
One of the first questions to ask before moving forward is: “What is the purpose of the system?” Often the purpose is comfort heating or cooling for human occupants. Sometimes the purpose is maintaining acceptable temperatures for a process (e.g., a data center). Perhaps the system needs to maintain pressure relationships for a particular space or group of spaces. The designer should also identify any other equipment that is affected by the sequence. A makeup air unit, for example, needs to be interlocked with the exhaust fan(s) that create the need for the makeup air unit. Keep in mind that a system may have multiple purposes. An AHU may be designed for space conditioning during normal operation and also function as a smoke control system during a fire event.
Step 3: Identify the required inputs and outputs.
It was noted above that the flow diagram should include the inputs for the controlled variables. Inputs are those readings coming into the building management system (BMS). These include items such as space sensors, air temperature sensors, static or differential pressure sensors, and so on. When developing this list of input devices, the designer should note what inputs are already available for use in the control system. Are any of the required input devices included as a part of the equipment or already specified for other purposes? Additional devices should be indicated in the construction documents and specified at this time. Outputs should also be considered at this time in preparation for developing the full list of points. Outputs are those signals originating from the BMS to the controlled variable.
Step 4: List any code required functions of the system.
Energy codes (such as ASHRAE Standard 90.1) continue to become more stringent and demand ever more efficient systems. Identifying these requirements ahead of time helps to ensure the system complies with the applicable energy conservation code. Setback requirements, isolation dampers, demand controlled ventilation (DCV), economizers, reheat limitations, deadband, and supply air temperature reset are all examples of airside energy code requirements that, when required, need to be incorporated into the sequence. It is important to recognize the requirements and exceptions for your particular project location.
Other building, mechanical, and fire code requirements should also be reviewed at this point. For example, codes may require unit shutdown upon detection of smoke. Additional control requirements may come into play if the equipment serves a smoke control function.
HVAC equipment or features that are required by code must be identified early in the design process. This is one of the reasons it makes sense to develop the controls sequence early in the design process. Doing so allows for a complete and comprehensive coordination effort as the design is developed.
Step 5: Confirm the owner’s operational requirements and expectations.
After identifying the minimum code required functions of the unit, the designer should confirm whether the owner has any specific operational requirements and understand how the owner intends to use the equipment. These requirements may be identified in the owner’s project requirements (OPR) or a request for proposal that explained the project scope. If an OPR was not developed, the designer should still consult with the owner to verify the intent of the systems. The team should discuss which desired system features may conflict with overall successful operation or code requirements. The system should be reviewed for additional components necessary to suit the owner’s desired operation.
The sequence of operation should be tailored for how the building will be operated, as well as the experience of the facilities maintenance staff. Sequences developed for a large casino resort with a full-time, highly experienced, on-site maintenance staff may be more complex than those developed for a small office building with no dedicated maintenance staff. Sequences should always be as simple as possible while still meeting the performance requirements. Unnecessarily complex control sequences can overwhelm even the most experienced operator because they are more difficult to operate and maintain. A lack of operator understanding or need to override often leads to the building operating differently than the designer intended.
Once this information has been gathered, the designer can begin to actually create the sequence of operation. This becomes the baseline upon which the requirements for the sequence of control are further identified and developed.
Step 6: Develop a list of points.
The information gathered in the previous steps allows for the creation of a points list. The points list identifies all the inputs and outputs that are controlled or monitored by the BMS. A matrix similar to Table 1 is often the best method of identifying these points. The matrix should identify all inputs and outputs for the controlled system. The points can be classified as digital or analog. Digital inputs and outputs are a simple on or off (0 or 1) condition. Analog inputs and outputs represent a value within a range corresponding to a change in voltage (e.g., 2 to 10 Vdc) or amperage (4 to 20 mA), or in the era of pneumatic controls, a change in air pressure. A dirty filter alarm from a differential pressure switch is an example of a digital input to the controller. Chilled water valve position is an analog output as it modulates from 0% to 100% open position. The system should be designed to permit expansion and be capable of handling at least 125% of the number of points currently specified. Allowances should also be made for virtual points. These are points that are calculated or passed through the controls system as opposed to hardwired physical points.
It may be necessary to revisit step 3 as the points list is developed. The designer may realize that he or she does not have all of the required input and output devices to achieve proper control of the system. It is better to identify these changes during design so that they do not become costly changes in the field.
Monitoring capability and alarms should also be reviewed at this time. These points provide additional information to the operator, allowing the operator to monitor the system performance. This can be valuable information, but an excessive number of points can be overwhelming, costly, and of no real benefit to the operator. Controls should be kept simple wherever possible. The operator should have all of the necessary information at a glance, but additional information becomes ”noise” and distracts the operator from focusing on the important points. It is possible to monitor and trend almost any value within a system. The designer needs to ask whether or not a point is actually needed for the particular system.
The storage capability of the system must also be specified. Identify how long the data should be retained (e.g., 30, 60, or 90 days). The frequency of the trends must also be evaluated. Is it necessary to sample and record the readings every 15 seconds or every 15 minutes? Trending hundreds of points every few seconds may lead to network performance issues.
Consider best practices for the specific region in which you are working. In some areas, control specifications may be performance based where the temperature control contractor will be responsible for providing all hardware components and points necessary to achieve the engineered sequence of operation. Other geographical regions or particular projects may require the designer to specify the exact details and points list for all system components that the control contractor is to provide.
Step 7: Identify the setpoints.
Setpoints are values the system tries to maintain during operation. Space temperature is a common example of a setpoint. The space sensor or thermostat is the input device that measures the current space temperature. The control system evaluates this condition against the setpoint value. Setpoints are not limited to temperature. The duct static pressure sensor controlling fan speed will also have a setpoint. Likewise, a setpoint must be identified for the carbon dioxide (CO2) sensor that serves as the input for the demand controlled ventilation strategy.
Step 8: Work through the actions and functional responses.
The initiation and functional response are the key aspects of the sequence of operation. It is probably what comes to mind for most people when they hear the term “sequence of operation.” It is best to work through these as a numbered list.
Let’s look at the temperature control for our example. The space temperature sensor is the input device. It measures the space temperature and sends the value to the brain of the system. In stand-alone packaged equipment, this will likely be an equipment controller with preset control sequences. In larger, more complex systems, the value is reported to a BMS. The BMS acts as the brains of the operation and evaluates whether the measured value is within the operational parameters—at setpoint. Assume the system is operating in the cooling mode with the chilled water control valve partially open and the heating coil valve fully closed. If the value is above setpoint, then the space temperature is higher than desired. The chilled water control valve must modulate open to provide additional cooling and lower the space temperature.
The sequence of operation should concisely list these evaluations and how the system needs to respond. Recommended language for the last example may be similar to the following:
The designer needs to systematically work through all of the ways the system may be required to modulate. Consider all of the modes in which the system must operate and what system components need to operate differently in these various modes. Think about the supply and exhaust fans in our system. Our example assumes that the air handling unit exhaust fan also functions as a smoke exhaust fan. The fans and control dampers will operate differently in smoke control mode than they will in normal operation. The sequence of operation should specifically identify requirements for each of these modes.
Note that normal operation mode may also have under it several modes. In our example, we have economizer operation and recirculation operation. A matrix like the excerpt shown in Table 2 is an easy way to identify the required parameters for the various operating modes. While this matrix does not necessarily need to be included in the construction documents, it provides the designer with an overview summary that helps develop a written sequence of operation. The various analog and digital inputs and outputs should, in some form, be clearly identified in the construction documents with a corresponding written sequence of control.
Step 9: Identify failure scenarios.
At some point, system components will fail. Quality products help reduce the frequency of failures, but they are still inevitable. If the designer plans for these failures in the sequence of operation, then he may be able to reduce the resulting operational impact when a failure does in fact take place. Again, be careful to not over specify. Resiliency requirements for a typical office building will be substantially different from those of a data center. Life safety requirements should also be considered.
Failure considerations should look at both the input devices and the controlled system components. The failure of a supply duct static pressure sensor may lead to improper control of the supply fan variable frequency drive (VFD) speed. If the value measured at this sensor varies significantly from the expected value, then a false measurement may be received. The sequence of operation could specify that this reading be ignored if the value is some percentage outside of the expected value. Some input devices may also have an invalid reading function built into the sensor.
Consider a low static pressure sensor reading. A sequence that identifies the failure of this component can reset the supply fan to some fixed speed that keeps the system in operation and provides at least partial capacity until the maintenance team can properly address the problem. A system that does not anticipate this failure will continue to control the system using the erroneous static pressure measurement. This system will likely increase the supply fan speed until the system eventually shuts off on high static pressure if a high static pressure setpoint was considered in the original points list. This typically requires a manual restart and the system will have a longer downtime compared to the one that incorporated fail-safe scenarios. This is an important consideration for systems where environmental conditions are critical or safety could be compromised.
Now consider the failure of a controlled device such as a control valve actuator. Failure of this device will lead to loss of space temperature control. The importance of a fail-safe position for this actuator can be debated, but a spring return open actuator may be considered for a chilled water coil in a hot climate. Although the system no longer has accurate control, this arrangement errs on the side of caution and will overcool the space until the system can be repaired. This may be more important for cooling equipment that serves telecomm rooms, data centers, or other process loads where a loss of cooling has significant consequences.
A supply fan motor failure in a single supply fan system has no real fail-safe position. However, a system with multiple supply fans and motors may be able to respond to this failure scenario with no decrease in supply airflow rate. This is a prime example of how developing a sequence of operation may lead to changes in which hardware components are specified for the system. The impact of the supply fan motor failure may have been overlooked prior to this stage of the design.
Step 10: Review the sequence.
At this point, the designer has completed the first pass to developing the sequence of operation. A successful sequence is iterative and often requires revisiting the previous steps. It was mentioned earlier that a designer may begin writing the actual functional responses of the system and realize he does not have all the required input and output devices. This may require an update to the flow diagram initially developed. The process of developing the sequence of operation may also identify features or options that were not originally specified. This is the time to adjust and refine those specifications.
The best way to review the sequence of operation is to step through all of the actions and responses. Try to break the system by identifying scenarios that your sequence of operation cannot properly respond to. Rewrite the sequence as necessary to address these scenarios. A peer review is a great way to ensure the intent of the sequence is clear to others.
In complex systems, consider how individual pieces of equipment interact with the sequence of operation of the other equipment. A multi-zone VAV system will have a sequence of operation for the individual terminal units and the central air handling unit. These sequences must be coordinated to ensure they work in harmony to provide the most efficient operation. The successful operation of one is dependent on the other.
Functional testing during commissioning helps ensure the constructed project operates according to the design intent. The tests are largely based on the designer’s sequence of operation. The equipment should not be expected to perform functions that were not required by the sequence.
Discrepancies noted during the commissioning phase of the project should be reviewed with the designer. It may be necessary to update the sequence of operation based on data gathered during the functional testing. Refer to the static pressure sensor example mentioned earlier. Commissioning is the appropriate time to verify the fail-safe strategies function as expected. The intent was to keep the system in operation. The commissioning authority should test the operation of this feature and the team should modify setpoints as required to achieve the desired results.
Commissioning is the last chance to evaluate the sequence before turning over the project to the owner. The designer should be involved in the commissioning process and review the final commissioning report. The sequence may need to be modified based on observations during the commissioning period. Changes this late in the project schedule may have large cost and schedule impacts. That being said, the designer should not rely on the commissioning process to make up for lack of adequate foresight during the design phase.
The building operator should fully understand the sequence of operation. This ensures the facilities maintenance group operates the equipment consistent with the design intent to recognize the full benefits of the system they have been provided. The building operator may override the supply air temperature in response to space temperature complaints. He or she should understand the consequences of this override as they relate to sacrifices in energy efficiency. Identify root causes of operational deficiencies and solve problems at the source.
Although the designer should consider the operational requirements during the design phase, these specific details may not always be available. The sequence may need to be refined as the building operation evolves over time. The sequence of operation should be considered a living document that is continuously maintained throughout the life of the system. Doing so allows for seamless transfer of knowledge within the operations group. Understanding the control logic for existing equipment is important for designers working on building renovations or tenant improvements within an existing space. Without this knowledge, the new design may work against the base system instead of in sync with it.
An up-to-date sequence also becomes a benchmark for how the system should be operating. The sequence for existing equipment may be modified to optimize energy efficiency and better suit the evolved building functional and operational requirements. Retro-commissioning and energy audits are great ways to identify deficiencies in the sequence of control for existing equipment. Existing equipment without a well-defined sequence of operation may be a good target for energy optimization.
HVAC systems use considerable amounts of energy in commercial buildings. Developing a well-thought-out sequence of operation helps minimize the energy consumption of these systems. In addition, it allows the system to meet the criteria for which it was designed. The designer must develop the sequence to a level of detail that is appropriate for the project at hand and maximizes the success of that particular project.
Although it is not uncommon to see sequences included in project specification manuals, the best location for this information is often directly on the construction drawings. Keeping the sequence of operation closely tied to the equipment schedules, plans, and control diagrams increases the transparency of information throughout the project history. This arrangement is advantageous as the project specification manual is not always available in the field and often becomes separated from the drawings.
The steps outlined can be translated to almost any system regardless of size and complexity. The important thing to remember is that the sequence of operation should not be written in haste as the project is going out the door. An effective sequence of operation begins early in the design process when the systems are being developed and equipment is being selected. Doing so allows the designer to develop the most effective system.
Jason A. Witterman is a mechanical project engineer with JBA Consulting Engineers. He has experience in various market sectors including data centers, commercial office, aviation, medical, and government projects. His expertise is data centers, sustainability, and energy codes. Ed Butera is chairman of the board at JBA Consulting Engineers and has more than 40 years of experience. He specializes in master planning and design of complex systems for health care, high-rise buildings, central utility plants, and large hospitality resort projects.