Temperature and flow patterns in Benard-type scenarios of fluid flow and heat transfer in porous media are currently studied for the Rayleigh number (Ra) values above those typical of the onset of full convection due to the engineering interest in the heat transferred in the process. The interesting results for the range of Ra numbers below the onset are presented in this paper. The emergence of cells in this range of Rayleigh numbers and their evolution as Ra increases is numerically studied using the network simulation method. The influence of the height and length of the rectangular domain, H and L, which define the aspect ratio and the particular Ra below critical, give rise to singular scenarios of irregular cells whose development decreases from the lateral boundaries - where the first rolls emerge - towards the center. Some of these interdependences are shown through the flow patterns of different cases.