The natural convection heat transfer and ventilation characteristics of heated and vented parallel wall channels have been studied numerically. The flow is assumed to be laminar and steady, and the governing two-dimensional Navier-Stokes equations are solved by a finite volume formulation to calculate the chimney effect that draws cooler ambient air from the lower opening. The fluid-dynamic and heat-transfer characteristics of vented vertical channels are investigated for both symmetric isothermal and constant heat-flux boundary conditions for the Rayleigh number ranging from 10(3) to 10(5) and the aspect ratio in the 5-20 range. The non-dimensional entrance and exit pressure losses and the induced mass flow are correlated with the Rayleigh number. The results indicate that although inlet- and exit-loss coefficients may vary significantly with Rayleigh number and aspect ratio, the total pressure-loss coefficient is a weak function of Rayleigh number. It is also shown that the total pressure loss coefficient and non-dimensional mass-flow rate results are better correlated with the modified Rayleigh number that is obtained by dividing the Rayleigh number by the aspect ratio. The elliptic flow results, obtained from the present procedure, are compared with fully developed flow results and with boundary-layer calculations of previous authors. Numerical results also yielded important information regarding the placement of the free pressure boundary. The results for the present geometry indicate that the effect of free-boundary location is negligible if it is placed at a distance of four times the channel width or greater.