Applying Fan Energy Index in Variable-Air-Volume Systems Fan energy index (FEI) is more than a rating metric. It also is a design tool, an indicator of the ability of a fan system (fan, motor, drive) to serve a duty point efficiently. With FEI beginning to take hold in model energy codes and standards, including ASHRAE 90.1-2019, this article provides guidance for using FEI in VAV-system design. 2 20 19 A M CA i n m o t i o n w w w. a m c a .o r ghttp://www.amca.org