Abstract: The invention provides an instantaneous water heater which includes safety devices separately or in combination to prevent the explosion of the heat exchanger in the event of an excessive heating of the water in the heating coil and to shut-off the flow of gas to the burner in the event the atmosphere surrounding the water heater presents a dangerous decrease of its oxygen content or an excess CO/CO.sub.2 content, thus avoiding the risk of asphixiation or intoxication to the user. Additionally, the main gas flow control valve includes a settling chamber for separating the solid particles entrained in the combustible gas and preventing the obstruction of the internal gas passages in the gas control valve, as well as the plugging of the gas injectors of the pilot flame and/or burner nozzles.