The effect of local thermal non-equilibrium on the onset of convection in a porous medium consisting of two horizontal layers, each internally heated, is studied analytically. Linear stability theory is applied. Variations of permeability, fluid thermal conductivity, solid thermal conductivity, source strength in the solid and fluid phases, interphase heat-transfer coefficient and porosity are considered. It is found that heterogeneity of permeability, fluid thermal conductivity and source strength in the fluid phase have a major effect; heterogeneity of interphase heat-transfer coefficient and porosity have a lesser effect, while heterogeneity of solid thermal conductivity and source strength in the solid phase are relatively unimportant.