Which type of water heater heats water as it passes through a heated coil?

An instantaneous water heater heats water on demand as it flows through a heated coil. When a hot water tap is opened, cold water travels through the coil, where it is heated almost immediately. This design eliminates the need for a storage tank because the system provides hot water instantly and continuously as long as there is a demand.

This feature is particularly beneficial in situations where space is limited, or where there is a high demand for hot water without the wait time associated with traditional storage tanks that must first heat and store water. Additionally, it offers the advantage of energy efficiency, since it only uses energy when hot water is needed, unlike storage water heaters, which continuously maintain a reservoir of hot water.

While options like storage, solar, and tankless may involve some similar principles, they do not provide the instant heating characteristic of instantaneous water heaters. Tankless systems, while they are similar to instantaneous water heaters, typically refer to those which can deliver hot water continuously without a pre-existing store, whereas 'instantaneous' directly describes the heating method in use. Thus, for the specific nature of the question regarding heating water as it passes through, 'instantaneous' is the most accurate description.