This paper addresses the problem of optimizing the plate separation of an open, parallel-plate channel that is cooled by natural convection air Bow. The plates are symmetrically heated by uniform heat flux. The I-shaped computational domain comprised two subdomains : the actual physical domain between the plates, and two large rectangular reservoirs placed upstream of the entrance and downstream of the exit. The aggregate subdomains accommodated the diffusion phenomena by momentum and energy that occur outside the channel. The full elliptic Navier-Stokes and energy equations are solved numerically in the composite domain. Correlations of the optimal values of the plate spacing as a function of the Gr(L) number and of the induced mass flow rate, as well as thermal and velocity profiles, are presented for air.