In this study a thin liquid film on the horizontal plane is considered. It is heated isothermally from the lower rigid boundary and its upper deformable, free interface is open to the air. Through the interface the heat flows upward. The hydrodynamic instabilities of liquid films are analyzed theoretically when the long-wave type disturbances are introduced. This analysis is performed for non-wetting systems with varying the film depth from the order of nm to that of mm. The disturbance equations are obtained by applying the linear stability theory under the lubrication approximation and the result is applied to a liquid film of water or ethanol. When the film depth is very small, it is shown that the liquid film is unstable owing to the intermolecular forces. But when the depth of a liquid film is above a certain value, the liquid film becomes stable against long wave instabilities due to gravity forces.