The increasing economic advantage of replacing traditional two-speed fans with variable-frequency drive (VFD) fans has been gaining popularity in the industry. However, concerns regarding frequent ON/OFF switching and the lack of a well-devised controller have discouraged widespread adoption. In this study, a temperature zone method is proposed to replace the set-point method of fan control. Additionally, the highest output water temperature allowed in the process is set as the upper limit of a zone in order to further conserve energy. Both strategies are comprehensively analyzed for a virtual cooling tower that uses operational data from an existing VFD-fan-based cooling tower system in Taiwan. The results show energy savings of 38% for a 0.75 degrees C zone without increasing the ON/OFF switching frequency. The proposed strategies were further verified via an on-line field experiment. The proposed methods can be universally and easily applied to any existing cooling tower, and have significant implications for energy conservation if adopted globally. (C) 2015 Elsevier Ltd. All rights reserved.