Performance Diagnostics of Thermal Distribution Systems in Light Commercial Buildings: Final Report for CIEE/CEC
This paper presents findings from field performance testing of five thermal distribution systems in four light commercial buildings. The systems studied are located in Sacramento and Pleasanton, California, and are connected to typical rooftop packaged units serving small office spaces. All of the buildings are single-story office-buildings with floor areas less than 1,000 m2. The field study included distribution system characterization of the five duct systems, and short-term refrigerant-side monitoring on two of the packaged units. The air leakage results are presented in terms of specific effective leakage areas (ELAs), the ASHRAE-defined duct leakage classes, leakage flowrate (from flow subtraction or derived from ELA and operating pressures), and air leakage ratios (i.e., fractional loss of fan flow). Comparisons are made with results from previous studies on light commercial buildings. The specific ELAs ranged from 0.8 to 5.3 cm2 per m2 of floor area served, and the leakage classes ranged from 232 to 414, making these duct systems much leakier than the "unsealed ductwork" classification by ASHRAE. The air leakage ratios were approximately 10% of fan-supplied airflow on average, which was considerably lower than previous studies. Conduction energy losses calculated by temperature measurements in the duct systems were found to be significant, ranging from 9% to 24% of capacity. Refrigerant-side monitoring of temperature and pressure were used to calculate equipment efficiencies. The results show that there were considerable energy losses by air leakage and conduction through ducts, and that the losses varied from system to system. Frequent on-off cooling cycling was common with the systems we tested and additional losses were also found due to improper system control. These diagnostic results suggest that efficiency improvement in thermal distribution could be achieved via better system control, design and sizing, and by duct sealing.