Gravity is a force acting on any physical body near the surface of an astronomical object (planet, star) and consisting of the force of gravitational attraction of this object and the centrifugal force of inertia caused by its daily rotation.
A liquid at rest or in motion is influenced by gravity, atmospheric pressure, and applied forces. The fluid in motion is also affected by the forces of inertia and friction, and temperatures (dynamic factors).
The natural circulation of the heat exchanger in private houses works according to this principle.
Ordinary water usually acts as the heat-carrying fluid, which moves along the circuits from the boiler to the radiators and vice versa due to the change in its thermodynamic characteristics. When heated, the density of the liquid decreases, and the volume increases; it is squeezed out by the cold flow coming from the return pipeline and rises through the pipes. As the heat-carrying fluid is distributed by gravity along the horizontal branches, the temperature decreases, and the heat-carrying fluid returns to the boiler.