In the present numerical investigation, a transient numerical analysis for natural convection in air, between two vertical parallel plates (channel), heated at uniform heat flux, with adiabatic parallel plates downstream (chimney), is carried out by means of the finite volume method. The analyzed transient problem is two-dimensional and laminar. The computational domain is made up of the channel-chimney system, and two reservoirs, placed upstream the channel and downstream the chimney. The reservoirs are important because they simulate the thermal and fluid dynamic behaviors far away from the inflow and outflow regions. Results are presented in terms of wall temperature and air velocity profiles. They are given at different Rayleigh number and expansion ratios (chimney gap/channel gap) for a fixed channel aspect ratio (channel height/channel gap) equal to 10 and extension ratio (channel-chimney height/channel height) equal to 2.0. Wall temperature profiles over a period show the presence of overshoots and undershoots. The comparison among the maximum wall temperatures shows that the simple channel is the most critical configuration at steady state condition, but it is the best configuration during the transient heating at the first overshoot. As indicated by the temperature profiles, average Nusselt number profiles over a period of consideration show minimum and maximum values and oscillations before the steady state. Stream function fields allow to observe the development of fluid dynamic structures inside the channel-chimney system, particularly how and when the cold inflow is present and develops. (C) 2012 Elsevier Ltd. All rights reserved.