The article concerns the hydraulic version of Boussinesque free convective motion model adapted to the flow in the vertical plane slit. The author determines the limiting values of friction and heat transfer medium speed, pressure thrust, speed adjustments. It is proved that the average speed of a free-convective motion is proportional to the height of the gap and drop of temperature. Under the limited air velocity, heat exchange rate is maximal. Under the limited heat exchange, air velocity is minimal. The deformation of the flow velocity along the length of the slot provides a minimal pressure change: the movement is organized in such a way that the deviation from the equilibrium state is minimal.