In this paper a benchmark solution for the conjugate heat transfer of backward-facing step flow is computed using the stream function vorticity formulation. The numerical solution is obtained using the multidomain Boundary Element Method. A significant difference was found when comparing the results with the prior benchmark solution computed by Kanna and Das (2006). Similar disagreement has also been reported in the work done by Teruel and Fogliato (2013). The new benchmark temperature and Nusselt number values were obtained using Richardson extrapolation to zero-sized mesh. The presented results have excellent agreement when compared to the third numerical code. (C) 2015 Elsevier Ltd. All rights reserved.