Characterizing Air Leakage in Parallel Fan-Powered Terminal Units

Air leakage data from six parallel fun-powered terminal units (FPTUs) that used electronically commutated motors were evaluated to determine if simple models of air leakage could be developed for applications in building energy simulation programs. The data were from an earlier investigation by Edmondson et al. (2011). Units with both 8 and 12 in. (20.3 and 30.5 cm) primary inlets from three manufacturers were evaluated. The analysis included the impact of downstream static pressure, upstream static pressure, and primary airflow on the leakage from the units. Data for downstream static pressures ranged from approximately 0.1 to 0.5 in. w.g. (25 Pa to 125 Pa) and ranged from approximately 0.4 to 2.0 in. w.g. (100 Pa to 498 Pa) for upstream static pressures. Because the original data included some primary airflows outside the expected range of operation of the 8 and 12 in. (20.3 and 30.5 cm) FPTUs, the data set was reduced to include only data in the expected primary airflow operating ranges of these sized Units. All the original leakage data were collected with the FPTU fan off and with only primary air flowing through the unit. No attempt was made to quantify how much leakage was coming through differing sources of leakage, such as the seams, penetrations, or backdraft damper. Leakage was found to be primarily dependent on downstream static pressure. A simple regression of leakage and downstream static pressure yielded correlations with R-squared values from 0.80 to 0.96. Leakage airflow showed little variation with either upstream static pressure or primary airflow when the downstream static pressure was held constant Three leakage classifications were identified low, medium, and high based on the measured data. Correlations that describe the leakage as a function of downstream static pressure for each classification were developed. Calculations for a system with downstream static pressures varying with the square of primary airflow were performed and results presented. These leakage correlations can be used in building simulation models that use mass and energy balance approaches to modeling FPTUs.