A scalar model of turbulent incompressible convection is presented in which transfer of energy between modes at different scales or between horizontal and vertical vorticity modes is represented by quadratic nonlinear interactions. The growth rate of the modes is parametrized in order to take into account the buoyancy force and the mean temperature gradient. At a small Prandtl number (which is of astrophysical interest) it is found that the Nusselt number scales as (R&sgr;)1/3. The form of the spectrum at large wavenumbers is that of three‐dimensional turbulence (Kolmogoroff spectrum). At small wavenumbers the spectrum is determined by the growth rate of the unstable modes.