A device for heating water on-demand, usually a gas burner with a spiraling water pipe surrounding/above it. As opposed to a boiler, which pre-heats water and stores it for later use (and also needs to keep reheating the water as it cools if not used).

Spanish wikipedia is the only with a picture of the internals: https://es.wikipedia.org/wiki/Calentador_de_agua_circulante?...