Which hot water system relies entirely on natural forces for operation?

The gravity-fed system relies entirely on natural forces for its operation, making it the correct answer. In a gravity-fed system, the flow of hot water is driven by the force of gravity rather than relying on pumps or mechanical components. This system utilizes the principle that water will flow from a higher elevation to a lower elevation, which means that water is heated in a hot water cylinder positioned above the draw-off points, typically close to the attic or loft area.

As the hot water is used, colder water from the storage tank is drawn in to replace it, maintaining the water supply in a flow that's sustained naturally by gravity. This setup is widely used for its simplicity and minimal mechanical failure risk, as well as reducing energy needs typically associated with pumping systems.

In contrast, the other heating systems such as the vented system, combi boiler, and unvented system all involve various levels of mechanical operation or depend on pressurized systems, which eliminates them as options that operate solely on natural forces.