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.


Learning objectives

  1. Learn how to create a successful sequence of operation.
  2. Recognize the importance of the sequence of operation as it relates to design, specification, and construction.
  3. 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:

Figure 1: A schematic diagram shows the control components of the example air handling unit (AHU). Courtesy: JBA Consulting EngineersStep 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.

Figure 2: Deadband is a setpoint differential to avoid simultaneous heating and cooling. Energy codes and efficiency standards specify setpoint overlap restrictions. Courtesy: JBA Consulting EngineersStep 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.

Figure 3: This sample building management system (BMS) graphic shows various points. The graphic overview provides a summary of the unit status in a clean, simple appearance. Courtesy: ABS Systems Inc.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.

Table 1: A list of points for the air handling unit (AHU) example is shown. All desired inputs and outputs should be listed and classified. Courtesy: JBA Consulting EngineersStep 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.

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