Abstract: The invention provides a geyser (210) for heating water in domestic or industrial applications. The geyser (210) includes a first end section (212), a second end section (214), and a central section (216) between the first and second end sections. The central section (216) includes a plurality of modular components (218 and 220) defining an inner wall (316) which together with the first and second end sections (212 and 214) forms a primary, sealed chamber (C) for containing fluid within the geyser (210), in use, and an outer wall (318), spaced from the inner wall (316), which together with the inner wall (316) and the first and second end sections (212 and 214) forms a secondary, safety chamber (D) for containing fluid expelled from the primary chamber (C) upon failure of the geyser (210). An outlet (331) is provided on the geyser (210) for allowing fluid to discharge from the secondary, safety chamber (D), typically via a vent pipe, to a desired location.