The effects of variable properties and non-uniform heating on laminar air flows induced by natural convection in vertical channels are investigated numerically. A full-elliptic model which accounts for variations in viscosity and thermal conductivity with temperature and which determines the density from the state equation has been applied to cases in which variable property effects cannot be neglected (including conditions for which flow reversals may occur). The influence of the Rayleigh number and non-uniformity of the wall heat flux distribution on the critical heat per unit time transferred from the walls in symmetric and asymmetric channels is analyzed for a wide range of Rayleigh numbers. It is shown that the maximum wall temperature can be reduced substantially by selecting an appropriate wall heat flux distribution, although at the cost of slightly more restrictive critical conditions. (C) 2004 Elsevier Ltd. All rights reserved.