When building owners want their building systems to be designed for resiliency, the discussion between the owner and the engineers typically includes redundancy and temporary HVAC systems that can support all building HVAC systems or portions thereof. This discussion is most important, as it will be used by the mechanical engineer to establish the design criteria and associated mode of operation of the building HVAC systems during an emergency. What exactly constitutes an emergency? To find the answer to this question, one first needs to have a good understanding of the relative code requirements. However, since code requirements represent a minimum set of performance criteria, it is not unusual for the building owner to ask for increased resiliency. In this scenario, determining what needs to stay operational beyond what is required by code is a balance between cost and the owner’s risk tolerance. Hospitals, BSL-4 research laboratories, and data centers are some of the facility types that typically require redundancy and resiliency levels beyond code requirements.
Figure 1 shows a riser diagram of a large building (i.e., greater than 200,000-square-foot) that is being served by a water-cooled chilled water plant and a heating plant with high-efficiency condensing boilers. The chilled water plant serves the air-handling units (AHUs), various IT rooms, and a data center on the first floor. The heating plant serves the AHUs and any terminal units that are provided with a hot water heating coil. The terminal units and other HVAC system components are not shown in the figure nor described in this article.