Hot-water heating systems circulate water by either a pump (forced hot-water heating system) or by _______ (gravity hot-water heating system).

In a gravity hot-water heating system, water circulation relies on the natural force of gravity to move heated water from the boiler to the radiators or heating units. When water is heated, it expands and becomes less dense, causing it to rise. As it circulates through the system and loses heat to the environment, it cools down, becomes denser, and subsequently falls back to the boiler for re-heating. This cycle continues without the need for mechanical pumps, relying solely on the gravitational pull and the natural flow of hot and cold water.

Choosing gravity as the answer accurately reflects the fundamental principle that defines how this type of heating system operates, differentiating it clearly from forced hot-water systems, where pumps actively circulate the water. The other options do not relate to the method of water circulation in gravity hot-water systems, with centrifugal force being a mechanism that could apply in specific pump systems, mechanical advantage referring to a concept in physics related to force and movement, and pipe insulation serving as a material that helps minimize heat loss but not involved in the circulation method itself.