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At its most basic level, control system verification is a process that seeks to ensure the proper operation of controllers, sensors, and actuators in a variety of conditions. The purpose of a control system seems simple: to make a system behave as intended. However, this gets more complicated when one takes note of the many points of variability. A system should return to its desired state in the event of a disturbance, but should adapt intelligently if the disturbance occurs on a regular schedule. A control system must command the machinery properly, but be able to notice and adapt if the machinery does not respond exactly as intended. These complex decision protocols govern control systems design.
The verification of a control system starts with a clear understanding of the goals of the project. While some buildings, such as laboratory environments, must maintain a strict temperature range to ensure safety, other buildings such as offices may seek to reduce cost by swaying more towards ambient air temperature within a comfortable range. Some systems seek to reduce cost by investing in “smart” sensors at every terminal, and others avoid this up-front cost by accepting more imprecision in building regulation. HVAC control systems can be as diverse as their applications.
Even within one HVAC control system, there is diversity. There may be a sector of the equipment and controllers dealing with humidity levels, another sector for air temperature, and a third for air quality. Not only that, these systems interact with each other as well. The humidity in the air affects how long it will take to stabilize a building at the desired temperature. HVAC control system verification also seeks to ensure that these various parts are working correctly in relation to each other. They must operate not only with a priority hierarchy, but also in the correct sequence and at the correct respective speeds.
Finally, even control systems that appear to be operating alright can benefit from control system verification. A system may be achieving the correct internal air conditions due to perfect controls, or it may be vulnerable to disturbances of a type which have not occurred recently. An HVAC controls system must not only operate when conditions are normal, but also in the event of a large disturbance like an air quality emergency. For example, the recent wildfires that swept California left a trail of wildfire smoke that exposed weaknesses in otherwise functional air quality systems.
As stated in the “HVAC Controls Evaluation Protocol” from NREL, “The type of HVAC system to which [the most recent federal protocol] applies is common in newly constructed commercial and institutional buildings… [h]owever, numerous older buildings have… older systems with less-efficient control sequences that can benefit from this measure.”1 This is key; both older an newer systems can benefit from an upgrade to the most recent control systems protocols, and this doesn’t always involve replacing much or any hardware. It’s worth considering whether a control system verification procedure would benefit your building.