Energy modeling and analysis of inherent air leakage from parallel fan-powered terminal units using EMS in EnergyPlus

Parallel fan-powered terminal units (FPTUs) are widely considered more energy efficient compared with series FPTUs because of the intermittent terminal unit fan operation. Recent studies found that the energy savings potential of parallel FPTUs may be overestimated due to the primary air leakage through the backdraft dampers when terminal unit fans are not operating. However, the lack of an effective modeling approach in building simulation programs has prevented the thorough investigation of how the primary air leakage from parallel FPTUs affects the energy consumption of variable air volume (VAV) systems. In this paper, a modeling approach was developed and implemented in EnergyPlus using the Energy Management System (EMS) for the comparative analysis of energy impact of primary air leakage from parallel FPTUs. A series of building energy simulations were also conducted at four air leakage levels under five different U.S. climates. Although the percentage energy increases caused by the primary air leakage vary with local climates and leakage levels, simulation results show consistent increases in the energy consumption of central cooling coil, central supply fan, and FPTU supplemental heating throughout all the climates as the air leakage level increases. Compared with series FPTUs, the energy savings potential of parallel units significantly decreases as air leakage level increases. At the highest air leakage level of 15% simulated in this study, series FPTUs may use less energy than parallel FPTUs.