The transient thermal analysis of a natural convection solar air heater is presented. The heater consists of a single-glazed flat plate solar collector integrated with a paraffin type phase change material (PCM) energy storage subsystem and a rectangular enclosure which serves as the working chamber. The PCM is prepared in modules, with the modules equispaced across the absorber plate. The underside of the absorber plate, together with the vertical sides of the PCM module container, serve as air heating vanes. Air flow through the system is by natural convection. Energy balance equations are developed for each major component of the heater and linked with heat and mass balance equations for the heated air flowing through the system. The airflow rate is determined by balancing the buoyancy head resulting from thermally induced density differences and the friction head due to various flow resistances. The predicted performance of the system is compared with experimental data under daytime no-load conditions over the ambient temperature range of 19-41 degreesC and daily global irradiation of 4.9-19.9 MJ m(-2). Predicted temperatures at specific locations on the absorber plate, heat exchanger plate, glazing, and heated air agree closely with experimental data to within 10, 6, 8, and 10 degreesC, respectively. Maximum predicted cumulative useful and overall efficiencies of the system are within the ranges 2.5-13 and. 7.5-18%, respectively. Correlations of the predicted efficiencies are presented. (C) 2003 Elsevier Science Ltd. All rights reserved.