The effect of a rectangular obstruction of different sizes on natural convection heat transfer in the case of a vertical channel has been analysed for T boundary conditions on the walls. A comparison of the Nusselt number values with those for plane channel is presented. For smaller obstruction depths and for asymmetric heating, there is not much variation of the results from a case of channel with a baffle for asymmetric heating. For large obstruction depths, the flow conditions show a behaviour similar to that of a channel with a backward-facing step. © 1993, MCB UP Limited. All rights reserved.