Incomplete testing and evaluation of VAV system requires reset

An overhaul of the air handlers at a community college reaped energy savings and improved occupancy comfort.

12/27/2013


Through the local electric utility’s energy reduction program, exp US Services was hired to perform retro-commissioning of three of the buildings at a suburban community college. Although the buildings were less than 10 years old, they were not energy efficient and were a source of tenant complaints. The buildings used three large variable air volume (VAV) air handlers with a total fan motor horsepower of 770 hp. The investigation of the HVAC systems revealed that the VAV boxes were never properly commissioned, and the minimum settings were all above the 50% flow setting. Each classroom had a designated VAV box, but because the rooms were often sparsely occupied, these spaces were getting overcooled due to the high airflow. Additionally, the school was paying a double penalty in that the reheat coils in many of the boxes were coming on even in summer, to reheat the overcooled spaces.

Figure 1: The energy consumption of the large VAV air handler before implementation of any control changes is shown. Courtesy: exp U.S. Services

Through some minor changes in the building automation system (BAS), the commissioning team was able to reset the VAV flow minimums to much lower settings, and in some cases to zero when occupancy sensors signaled that the room was unoccupied, which was in keeping with the original design intent. The electric energy savings were substantial, and although the engineers did not verify the gas savings, it is estimated they were about 35,000 therms/year.

Figure 2: This shows the energy consumption of the large VAV air handler after implementation of the recommended control changes. Courtesy: exp U.S. Services

The facility provided utility data for the before and after periods of September 2009 to August 2010 and March 2011 to February 2012 (see Table 1). This provided an excellent comparison of year-to-year electricity usage and savings.

Table 1: The total annual savings were 1,202,716 kWh and $91,960. Courtesy: exp US Services

The Cx team installed amp loggers on an air handling unit (AHU-1), and the before and after data are shown in Figures 1 and 2. It is clear that the new VAV minimum settings allowed the AHU fans to drop to less than 30% of their peak loads, whereas previously the minimum load was never less than about 60% of the peak. 


Terrence Malloy is project manager for the energy solutions group at exp US Services Inc. He focuses on energy conservation, retro-commissioning, and renewable energy systems.



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