Water heaters that are manufactured for swimming pools come in several forms, most of which require an electrical input for a source of power. Passive-circulation systems, however, require no electrical power input because fluid circulation occurs as a result of thermal gradients. In solar-based systems, thermal gradients are developed by energy collected from sunlight. The combination of solar collection and passive circulation yields a system in which fluids, particularly water, are heated and circulated without need of assistance from external mechanical or electrical sources.
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