This study investigates the energy performance of chiller and cooling tower systems integrated with variable condenser water flow and optimal speed control for tower falls and condenser water pumps. Thermodynamic-behaviour chiller and cooling tower models were developed to assess how different control methods of cooling towers and condenser water pumps influence the trade-off between the chiller power, pump power, fan power and water consumption under various operating conditions. Load-based speed control is introduced for the tower falls and condenser water pumps to achieve optimum system performance. With regard to all example chiller system serving an office building, the optimal control coupled with variable condenser water flow could reduce the annual system electricity use by 5.3% and operating cost by 4.9% relative to the equivalent system using constant speed fans and pumps with a fixed set point for cooling water temperature control. (C) 2008 Elsevier Ltd. All rights reserved.