The simplest model of the gaseous oxidant flow through the horizontal planar layer of porous condensed fuel, which appears in the gravity field because of the difference in the gas densities inside and outside the layer, was studied. The change in the molecular weight of the gas during the reaction was shown to result in the non-uniqueness of stationary filtration regimes that are established when the flows with different values and directions of the rate are possible at the same external parameters. It was found that, when the molecular weight of the reacting gas increased during the reaction, the self-ignition of porous fuel was possible only at the initial heating up higher than some critical value.