We consider the effect of suddenly applying a uniform heat flux to a vertical wall bounding a porous medium which is saturated by a Bingham fluid. We consider both an infinite porous domain and a vertical channel of finite width. Initially, the evolving temperature field provides too little buoyancy force to overcome the yield threshold of the fluid. For the infinite domain convection will always eventually arise, but this does not necessarily happen in the vertical channel. We show (i) how the presence of yield surfaces alters the classical results for Newtonian flows and (ii) the manner in which the locations of the yield surfaces change as time progresses. (C) 2015 Elsevier Ltd. All rights reserved.