Differential Pressure Rise Measurements and Impact in EnergyPlus Modeling for Series VAV Fan-Powered Terminal Units Using PSC Motors
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2017 ASHRAE. When modeling an HVAC system using series fanpowered terminal units (FPTUs) with permanent split capacitor motors in EnergyPlus, the user must enter values for fan total pressure rise and fan/motor efficiency. Commonly recommended fan total pressure values for use in EnergyPlus simulations are much higher than those measuredfor an 8 inch inlet (203 mm) series fan-powered terminal unit (Bryant and Bryant 2015). Values cited in that study were on the order of 0.25 in. w.g. (63 Pa) and not 1.5-3 in. w.g. (374-747 Pa) as found in the literature. This paper presents manufacturers' performance data that verify and extend variable-air-volume (VA V) series fan-powered terminal unit total pressure rise measurements. For the two most common series fan-powered terminal unit inlet sizes, 8 and 12 in. (203 and 305 mm), the fan total pressure rise was found to be in the range of 0.20-0.35 in. w.g. (50-87 Pa). An EnergyPlus five-zone model was used to determine the impact of using "poor" versus "good" input values when modeling a VAV system with series FTPUs. Results show considerable impacts in yearly fan and heating energy. However, HVAC total system energy differences are quite small, less than 2% between recommended and "poor" inputs. The value for fan/motor efficiency for a series fanpowered terminal unit with a permanent split capacitor motor could essentially be eliminated as long as an appropriately low value of fan total pressure rise is selected for the simulation inputs.
