Turbulent combined flow of forced and natural convection was investigated using large eddy simulations (LES) for a vertical channel flow with significant heat transfer. The walls were maintained at constant temperatures, one heated and the other cooled, at temperature ratios of 1.01, 1.99 and 3.00. Results showed that aiding and opposing flows emerge near the heated and cooled walls, respectively, while the pressure gradient drives the mean flow upwards. Buoyancy effects on the mean velocity, temperature, and turbulent intensities were observed near the walls. In the aiding flow, the turbulent intensities and heat transfer were suppressed and the flow was relaminarized at large values of Grashof number. In the opposing flow, however, turbulence was enhanced with increasing velocity fluctuations.