  We consider a general compressible viscous and heat conducting fluid confined
between two parallel plates and heated from the bottom. The time evolution of
the fluid is described by the Navier--Stokes--Fourier system considered in the
regime of low Mach and Froude numbers suitably interrelated. Surprisingly and
differently to the case of Neumann boundary conditions for the temperature, the
asymptotic limit is identified as the Oberbeck--Boussinesq system supplemented
with non--local boundary conditions for the temperature deviation.
