Which principle allows for water flow in a gravity hot water system?

The principle that allows for water flow in a gravity hot water system is the difference in water density. In this type of system, when water is heated, it becomes less dense and rises, creating a natural circulation due to this density difference. The hot water ascends to the higher points of the system, while the cooler, denser water descends to be heated again. This cycle establishes a continuous flow driven purely by gravity and the natural thermodynamic properties of water, without the need for mechanical pumps.

Other principles mentioned, such as pressure differential or temperature change, do play roles in various heating systems but do not specifically account for the natural flow mechanism seen in gravity hot water systems. Circulation pumps, while important in some hot water systems, are not applicable in a gravity system where the movement is entirely dependent on the physical properties of water.