HVAC control sequences of operation have a direct impact on the effectiveness and efficiency of building systems. When specified and programmed correctly, control sequences of operation enable HVAC systems to efficiently meet the loads placed upon them. When designed and implemented incorrectly, sequences of operation can cause HVAC systems to either fail to meet building loads or waste energy (e.g. a system trying to simultaneously heat and cool the same space). In order for HVAC systems to operate at their peak efficiency, the control sequences of operation need to be appropriate for both the equipment type and the application.
ASHRAE RP (Research Project) 1455 represents a step toward defining “best-in-class” sequences of operation for variable air volume (VAV) air handlers and terminal units. An important anticipated outcome of the project is to specify control sequences that can be readily implemented by design engineers and controls contractors. The scope of RP 1455 is focused on high performance buildings, rather than minimally code-compliant buildings – the reason being that “best-in-class” controls requires more than the minimal amount of control sensors, actuators, and programming. The sequences developed through RP 1455 are intended to help building owners avoid unintended results in the start-up and operation of their HVAC equipment. Additionally, the sequences include hierarchical alarm suppression code (e.g. suppression VAV zone temperature alarms when AHU is shutdown due to a fan failure alarm). The RP 1455 sequences are also designed to support automated fault detection and diagnostics (AFDD; see http://buildingenergy.cx-associates.com/2012/07/fault-detection-and-diagnostics-in-direct-digital-control-systems/). The sequences developed through ASHRAE RP 1455 are accompanied by “English Language Pseudocode,” which will be helpful for both design specification and commissioning design review of control sequences.
One of the most impactful activities in the commissioning process is design review of HVAC systems in the design documents. Design review is an opportunity early in the project life cycle to check that the system design satisfies the design intent (e.g. Owner’s Project Requirements). Our design reviews include a thorough review of the HVAC control sequences of operation, which invariably includes the development of recommendations for improved system efficiency and effectiveness. Most of the design sequences of operation that we review are in need of more detail, specificity, clarity, and consideration of energy efficient operation. The design and delivery of efficient and effective of HVAC systems could benefit from standardized sequences of operation for common HVAC systems, and I hope that ASHRAE RP 1455 helps to improve the quality and consistency of sequences of operation in the HVAC industry.