In this paper, a dynamic cooling coil unit (CCU) model is developed by extending the CCU engineering model and by combining the model with the mass and energy balance equations. Commissioning information is then used to estimate, at most, six model parameters by a nonlinear on line identification method. Unlike other existing dynamic CCU models for control and optimization, this modeling method is relatively simple and exactly captures the nonlinear characteristics over a wide operating range of the CCU without requiring geometric specifications, which is very convenient in real time engineering control practice. Experiments on a centralized heating, ventilation and air conditioning (HVAC) pilot plant are conducted to show that the method is robust and gives an accurate match to real performance over the wide range of operating conditions. This method is expected to work well for real time control of an operating HVAC system. Even more, the modeling methodology can also be extended to other heat exchangers. (c) 2005 Elsevier Ltd. All rights reserved.