[en] Full text: In this paper an algebraic delay model is extended in order to study the dynamic of boiling cooling channels, modeling the friction, acceleration and gravity terms. The linear and non linear stability analysis were analytically and numerically performed, and the gravity term influence over the Density-Waves and Ledinegg instabilities was studied. It was found that the gravity causes an unstabilizing effect over the density-wave stability margin and it increases the density-waves frequency. This study suggests a new classification of the dynamic behaviors of such systems: gravity flows or friction ones. This classification may be done considering a new dimensionless number, (called by the authors the 'gravity number') that relates the gravity and friction terms, the last instead of the inertia term considered on the well known Froude number. Indeed, in the 'gravity number' remains the two major terms of most of the actual boiling systems (such as BWR nuclear reactor fuel channels or steam generators), whereas the inertial term generally is neglected. The linear frequency of the density-waves are analytically obtained, and its gravity number dependence shows good agreement with the completely differential model results