This experimental study deals with natural convection flow in a water channel with a symmetrical isoflux wall heating. Wall temperature, velocity, bulk temperature and fluctuating quantities are measured for a modified Rayleigh number Ra *=6.7x10^7. The analysis of the mean values shows a change of behaviour at the two-third of the height of the channel. This change turns out to be a transition from a laminar heat transfer in the bottom part of the channel to a turbulent one in the upper part. Due to isoflux wall boundary conditions, shear layers develop and thicken in each half-channel. It is shown that the meeting of these layers at the centre of the channel triggers the transition from a laminar heat transfer to a turbulent one. This experimental study is followed by a scaling analysis.