The application of heat and enthalpy wheels in HVAC systems has been steadily increasing, powered by the cost reduction associated with the decrease of the thermal load they provide. Heat wheels are usually employed to pre-cool the fresh air stream before it is admitted to the fan coil. When impregnated with a hygroscopic material, heat wheels are often referred to as enthalpy wheels, and are also capable of drying the make-up air supply, thus providing a further thermal load reduction. Accordingly, the present work aims at developing a simple mathematical model to describe the heat and mass transfers in rotary exchangers, which is validated by comparing the present with independently obtained results. An Effectiveness-number of thermal units (NTU) analysis is carried out, the results showing that heat wheels can be far less efficient than enthalpy recovery wheels, depending on the atmospheric conditions. (C) 2008 Elsevier Ltd. All rights reserved.