  We study the scaling properties of heat transfer $Nu$ in turbulent thermal
convection at large Prandtl number $Pr$ using a quasi-linear theory. We show
that two regimes arise, depending on the Reynolds number $Re$. At low Reynolds
number, $Nu Pr^{-1/2}$ and $Re$ are a function of $Ra Pr^{-3/2}$. At large
Reynolds number $Nu Pr^{1/3}$ and $Re Pr$ are function only of $Ra Pr^{2/3}$
(within logarithmic corrections). In practice, since $Nu$ is always close to
$Ra^{1/3}$, this corresponds to a much weaker dependence of the heat transfer
in the Prandtl number at low Reynolds number than at large Reynolds number.
This difference may solve an existing controversy between measurements in SF6
(large $Re$) and in alcohol/water (lower $Re$). We link these regimes with a
possible global bifurcation in the turbulent mean flow. We further show how a
scaling theory could be used to describe these two regimes through a single
universal function. This function presents a bimodal character for intermediate
range of Reynolds number. We explain this bimodality in term of two dissipation
regimes, one in which fluctuation dominate, and one in which mean flow
dominates. Altogether, our results provide a six parameters fit of the curve
$Nu(Ra,Pr)$ which may be used to describe all measurements at $Pr\ge 0.7$.
