Continuous Commissioning® of an Office Building

The case study building is a typical 10-story office building. The continuous commissioning (CC) started in May 2004 and completed in March 2005. Major commissioning measures include: improvement of the VAV box fan operation; modification of the supply fan and return fan control algorithm; correction of the outside air intake setting; modification of the chilled water pump control sequence. The implementation results are presented. After CC, the supply fan speed was reduced, economizer operation hours were increased, the chilled water system operation period was shorten, chilled water pump speed control was optimized and indoor comfort was improved. In a nine-month period, the electricity demand was reduced by 18%, and electricity consumption was decreased by 1,177,857 kWh. Introduction The 10-story office building was built in 1987. The total gross area is about 200,000 ft. Most of the areas are occupied 40 hours per week. There are two floors occupied 24 hours on weekdays. A total of five main air handling units (AHU) supply the conditioned air to most area of the building. Among them, AHU1, AHU2, AHU3 and AHU4 are variable air volume (VAV) systems with fan powered boxes on floors except the executive level. AHU5 is a single zone and single duct VAV system serving Atrium. Another single duct constant volume system, AHU6 only serves the executive level. There are about 200 fan powered terminal boxes installed in the building. The VAV series fan powered boxes are installed in 10 floor, and the VAV parallel fan powered boxes are installed in other floors. The exterior zone boxes are equipped with hot water reheat coils, and the interior zone boxes do not have reheat coils. The central plant consists of one rotary chiller to produce chilled water to the chilled water secondary loop and two heat pumps to make ice for the ice storage system. They provide the required cooling to the entire building. The capacity of the rotary chiller is 200 Ton. The system is a constant primary and variable secondary chilled water flow distribution system. The capacity of each ice-making heat pump is 160 Ton. The total capacity of the ice-storage system is 2,850 ton-hours. Three identical tanks were installed in the basement mechanical room. The type of the ice-storage is external melt-on-coil. At the charging mode, the brine cooled by two heat pumps is fed to the coils submerged in tanks. At the discharging mode, the ice water in tanks is pumped through a frame-plate heat exchanger to cool the return chilled water from end users. The warm water returning from the heat exchanger is cooled by directly contacting with the melting ice. Direct digital control (DDC) is used in the energy management and control systems. The Continuous Commissioning® (CC) team from Energy Systems Laboratory at University of Nebraska-Lincoln and Omaha Public Power District initiated the CC project in May 2004 on the request of the owner. Most of the recommendations were implemented by December 2004. The CC process was started by verifying the existing control sequences and taking measurements of selected parameters under the existing operating schedules. Based on the actual site conditions, CC recommendations were developed. The major CC measures included: optimizing terminal box minimum flow set point, modifying the interior zone fan control sequence of the parallel fan powered boxes, improving the VAV air handler supply fan and return fan control, changing one constant volume ESL-IC-05-10-38 Proceedings of the Fifth International Conference for Enhanced Building Operations, Pittsburgh, Pennsylvania, October 11-13, 2005 2 system to VAV system, optimizing the chilled water system operation schedule, optimizing chillers, cooling tower and chilled water pump control . Main CC Measures The main CC measures are summarized below. Terminal boxes The terminal box CC measures include resetting the minimum airflow set points on fan-powered boxes and modifying the fan control for the interior zone parallel fan power boxes. Existing control When there was a heating requirement, the terminal box fan was on. Both interior zone boxes and exterior zone boxes used the same control sequence. For interior zones, the terminal box fans had never run since there is no heating requirement. Improved control For interior zone boxes, when the airflow is low at partial loads, because the parallel fans would not run, the throw from the diffuser is reduced. Reduced throw will result in poor air mixing. Cold air drops directly down to the occupants below the diffuser, which caused comfort problems. Turning on the fan when the terminal box flow is low can mix the cold air with the air in the ceiling better and provide more uniform air distribution. After CC, parallel fans will be on when box airflow is less than certain box flow low limit. This measure provides better indoor air quality, but increases electricity consumption. Variable Volume Air Handling Systems There are four variable air volume (VAV) systems, AHU1, AHU2, AHU3 and AHU4. These single duct (VAV) systems provide conditioned air to the fan powered boxes. In each unit, both the supply fan and return fan are installed with variable frequency drives. Figure 1 shows the air handling unit schematic. Figure 2 depicts the building main air conditioning system diagram. Commissioning of these units focused on supply fan control, return fan control and economizer control. VS VS