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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.
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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...