Field Investigation of Duct System Performance in California Light Commercial Buildings

Light commercial buildings, one- and two-story with package roof-top HVAC units, make up approximately 50% (of the number of buildings) of the non-residential building stock in the U.S. Despite this fact little is known about the installed performance of these package roof-top units and their associated ductwork. These simple systems use similar duct materials and construction techniques as residential systems (which are known to be quite leaky). This paper discusses a study to characterize the buildings, quantify the duct leakage, and analyze the performance of the ductwork in these types of buildings. Over the 1996 and 1997 cooling seasons, this study tested twenty-five packaged roof-top systems in sixteen different buildings located in northern California. All of these buildings had the ducts located in the cavity between the drop ceiling and the roof deck. In 30% of these buildings, this cavity was functionally outside both the building’s air and thermal barriers. The effective leakage area of the ducts in this study was nearly 3 times that in California residential buildings [ELA25 (cm’/m’ floor area) 3.7 for light-commercial compared to 1.3 for residential]. For these systems, the average supply-side leakage rate was 26% of the system flow rate. This paper looks at the thermal analysis of the ducts, from the viewpoint of supply effectiveness. Conduction effectiveness gauges the fraction of the capacity available at the supply-air plenum that is delivered to a supply-air register. Effectiveness calculations are done on a register basis and include the length of a cycle, and whether the fan is always on or if it cycles with the cooling equipment. Combining effectiveness and leakage numbers yields delivery efficiency. The ten systems tested in 1997 had an average delivery efficiency of 65%