Commissioning Fan-Powered VAV Terminal Units
Don’t just focus on the large equipment. Keep in mind the small systems and their variety of configurations.
FIGURE 1. SERIES SUPPLY AIR REHEAT
FIGURE 2. PARALLEL SUPPLY AIR REHEAT
FIGURE 2. PARALLEL RETURN AIR REHEAT
This month, I begin a few columns about technical commissioning issues. It is intriguing to me that some of the most common and “simplest” HVAC equipment can be some of the trickiest to get right. This may not be so much because they are actually tricky, but more because we do not take them seriously enough.
Commissioning professionals focus a great deal of attention on a project’s few large and complex central systems, such as boiler plants, chiller plants, air handling units, and energy recovery equipment. This is how it should be, but there are plenty of things to keep in mind and to stay on top of with the many small and simple systems. Failing to optimize the scores or hundreds of terminal units in a building can add up to significant energy and performance issues.
A case in point is fan-powered VAV boxes (FP-VAVs). The first thing for the commissioning professional to keep in mind is that some people do not realize there are two fundamentally different types of FP-VAVs, i.e., series and parallel. For the many who understand that much, there are far fewer who realize that parallel FP-VAVs can be provided with their reheat coils upstream in the return air intake or downstream in the discharge air duct. Please refer to Figures 1, 2, and 3 for schematic depictions of each of these varieties.
During the design phase, the commissioning professional needs to make sure it is clearly understood which of the three varieties is being specified. After that, the sequence of operation needs to align with the features and capabilities of that variety. For example, in most cases:
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Series box fans are intended to run continuously whenever the space needs to heated, cooled, or ventilated.
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Parallel box fans with supply air reheat only need to be energized for heating when the central AHU is off.
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Parallel box fans with return air reheat need to operate whenever the zone is calling for heat, regardless of the status of the central air handler.
During the construction phase submittal review process, it is critical to confirm that the submitted FP-VAVs are of the type intended by the design engineers. After that, the controls contractor needs to submit a graphics package which illustrates the actual configuration of the FP-VAVs and the specified control sequences. Many controls manufacturers have standard “canned” sequences of operation for FP-VAVs, which may or may not be what the design engineers have customized for a particular project. This potential disconnect needs to be revealed and corrected on paper before it is programmed into all of the terminal units and then discovered during commissioning testing. ES
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