We developed a phase-field model for Marangoni convection in a liquid-gas system with a deformable interface, heated from below. In order to describe both Marangoni instabilities (with short and long wavelengths), an additional force component must be considered in the Navier-Stokes equation. This term describes the coupling of the temperature to the velocity field via the phase-field function. It results by minimizing the free-energy functional of the system. For a bidimensional problem in linear approximation we performed a numerical code that successfully computes both Marangoni instabilities. In the limit of sharp and rigid interfaces, our results are compared with the literature.