After I moved to Florida, I designed one of the first “cold-air” distribution systems for an elementary school. The project was fast-track and had to be constructed in nine months. Because cold-air distribution was used with 45-degree supply air, all of the ductwork and supply fans were smaller than ones for a normal system. After the project was completed and as it was being tested and balanced, there were shortages of airflow in several areas. The mechanical contractor, controls subcontractor, and TAB contractor were convinced that there was a design error in my static-pressure calculations and that larger fan motors were required to alleviate the low airflow.
After several meetings, the maintenance personnel checked some of the VAV terminals' inlet-area factors in the controls software. The area of the inlet of a VAV terminal is used in the velocity calculation and determines the amount of airflow supplied by the VAV terminal. It turned out that the inlet diameter, not the area, had been input, so the VAV terminals had false readings and were closing, not allowing the proper airflow.
Once all of the area factors were input correctly, the system operated as designed and left the three contractors embarrassed.
Keith J. Couch, PE
Construction Engineering Group
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