Using a vectorized finite-difference marching technique, the steady-state continuity, momentum, and energy equations are solved numerically to evaluate the effects of buoyancy-induced secondary flow on forced flow in a horizontal rectangular duel with a four-row array of 12 neat sources flush mounted to the bottom wall. Secondary flows, in the form of longitudinal vortices, initially develop at spanwise positions corresponding to the edges of the heat sources. Additional plumes and vortices subsequently develop across the heater width, leading to the eventual formation of large-scale circulation patterns. For a fixed Rayleigh number and decreasing Reynolds number, the row-average Nussell numbers decrease, reach a minimum, and subsequently increase due to buoyancy effects. Thus, due to buoyancy-induced secondary flow, conditions exist for which heat transfer may be enhanced by reducing the flow rate and hence the pump power requirement. Heal transfer enhancement above the forced convection limit, as well as the Reynolds number range for which enhancement occurs, increases with increasing Rayleigh number. Appropriate scaling parameters are introduced to characterize the strength of the buoyancy-induced secondary flow and to delineate conditions for which it is significant.