The main objective of this paper is to analyze the heat and mass transfer in a ventilation duct filled with encapsulated paraffin wax RII-56. A mathematical model and its numerical solution are presented. A new approach for approximating the specific heat of the PCM as a function of its temperature is proposed for all range of operating conditions. The interpolating cubic spline function method is used for determining an effective specific heal in each time step of temperature calculations. Equations for three-dimensional transient thermal analysis with associated boundary and initial conditions are solved by the control volume finite difference method with the fully implicit scheme. Proper trends and a good agreement between the results of theoretical modeling and physical reality for the solidification/melting process are obtained. Finally, 24 h operation of the proposed short term latent heat storage system is analyzed and the influence of the PCM capsules geometry on the thermal performance of the tested storage module is shown. (c) 2006 Elsevier Ltd. All rights reserved.