Heat transport in a porous medium is governed by molecular diffusion and hydrodynamic dispersion. The latter factor, represented by a second-order tensor which depends on the fluid velocity, is taken into account for the first time in the problem of thermal convective currents in a layer heated from below. The linearized and the energy analysis yield the same results and value of the critical Rayleigh number as those obtained with neglect of dispersion. Finite amplitude convective currents are influenced by dispersion, which has a stabilizing effect. Dispersion may have an influence for relatively coarse materials or at high values of Rayleigh number.