<P> Two conflicting safety issues affect water heater temperature--the risk of scalding from excessively hot water greater than 55 ° C (131 ° F), and the risk of incubating bacteria colonies, particularly Legionella, in water that is not hot enough to kill them . Both risks are potentially life - threatening and are balanced by setting the water heater's thermostat to 55 ° C (131 ° F). The European Guidelines for Control and Prevention of Travel Associated Legionnaires' Disease recommend that hot water should be stored at 60 ° C (140 ° F) and distributed so that a temperature of at least 50 ° C (122 ° F) and preferably 55 ° C (131 ° F) is achieved within one minute at points of use . </P> <P> If there is a dishwasher without a booster heater, it may require a water temperature within a range of 57--60 ° C (135--140 ° F) for optimum cleaning, but tempering valves set to no more than 55 ° C (131 ° F) can be applied to faucets to avoid scalding . Tank temperatures above 60 ° C (140 ° F) may produce limescale deposits, which could later harbor bacteria, in the water tank . Higher temperatures may also increase etching of glassware in the dishwasher . </P> <P> Tank thermostats are not a reliable guide to the internal temperature of the tank . Gas - fired water tanks may have no temperature calibration shown . An electric thermostat shows the temperature at the elevation of the thermostat, but water lower in the tank can be considerably cooler . An outlet thermometer is a better indication of water temperature . </P> <P> In the renewable energy industry (solar and heat pumps, in particular) the conflict between daily thermal Legionella control and high temperatures, which may drop system performance, is subject to heated debate . In a paper seeking a green exemption from normal Legionellosis safety standards, Europe's top CEN solar thermal technical committee TC 312 asserts that a 50% fall in performance would occur if solar water heating systems were heated to the base daily . However some solar simulator analysis work using Polysun 5 suggests that an 11% energy penalty is a more likely figure . Whatever the context, both energy efficiency and scalding safety requirements push in the direction of considerably lower water temperatures than the legionella pasteurization temperature of around 60 ° C (140 ° F). </P>

When was the first hot water heater made