Numerical multiphysics coupling is gaining more and more importance in engineering. Partitioned algorithms are among the most efficient methods to solve the coupled problem. However, they present some drawbacks, with the stability of the algorithm itself being the crucial one. The principal factors that lead to instability are the physical and numerical properties of the system. In this paper, a thermal coupling between a fluid and a solid, performed through a parallel partitioned algorithm, is considered. A condition that ensures the stability of the algorithm is derived.