In this paper, heat transfer and air flow in a composite-wall solar-collector system with a porous absorber has been studied. The 'unsteady' numerical simulation is conducted to analyze the performance of heat transfer and air flow in the composite wall. The excess heat is stored in the porous absorber by the incident solar radiation, which leads to a temperature gradient in the porous layer, so that the absorber can work as a good thermal-insulator when sunlight is not available. The influences of the particle size and the porosity of the porous absorber on the air temperature in the heated room are significant. The results show that all these factors should be taken into account for a better design of the passive solar-heating system. (C) 2004 Elsevier Ltd. All rights reserved.