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Another type of water heater developed in Europe predated the storage model. In London, England, in 1868, a painter named Benjamin Waddy Maughan invented the first instantaneous domestic water heater that did not use solid fuel. Named the geyser after an Icelandic gushing hot spring, Maughan's invention made cold water at the top flow through pipes that were heated by hot gases from a burner at the bottom. Hot water then flowed into a sink or tub. The invention was somewhat dangerous because there was no flue to remove heated gases from the bathroom. A water heater is still sometimes called a geyser in the UK.
Maughn's invention influenced the work of a Norwegian mechanical engineer named Edwin Ruud. The first automatic, storage tank-type gas water heater was invented around 1889 by Ruud after he immigrated to Pittsburgh, Pennsylvania (US). The Ruud Manufacturing Company, still in existence today, made many advancements in tank-type and tankless water heater design and operation.
Water heating is a heat transfer process that uses an energy source to heat water above its initial temperature. Typical domestic uses of hot water include cooking, cleaning, bathing, and space heating. In industry, hot water and water heated to steam have many uses.
Domestically, water is traditionally heated in vessels known as water heaters, kettles, cauldrons, pots, or coppers. These metal vessels that heat a batch of water do not produce a continual supply of heated water at a preset temperature. Rarely, hot water occurs naturally, usually from natural hot springs. The temperature varies with the consumption rate, becoming cooler as flow increases.
Appliances that provide a continual supply of hot water are called water heaters, hot water heaters, hot water tanks, boilers, heat exchangers, geysers (Southern Africa only), or calorifiers. These names depend on region, and whether they heat potable or non-potable water, are in domestic or industrial use, and their energy source. In domestic installations, potable water heated for uses other than space heating is also called domestic hot water (DHW).
Fossil fuels (natural gas, liquefied petroleum gas, oil), or solid fuels are commonly used for heating water. These may be consumed directly or may produce electricity that, in turn, heats water. Electricity to heat water may also come from any other electrical source, such as nuclear power or renewable energy. Alternative energy such as solar energy, heat pumps, hot water heat recycling, and geothermal heating can also heat water, often in combination with backup systems powered by fossil fuels or electricity.
Densely populated urban areas of some countries provide district heating of hot water. This is especially the case in Scandinavia, Finland and Poland. District heating systems supply energy for water heating and space heating from combined heat and power (CHP) plants, waste heat from industries, incinerators, geothermal heating, and central solar heating. Actual heating of tap water is performed in heat exchangers at the consumers' premises. Generally the consumer has no in-building backup system, due to the expected high availability of district heating systems.
In the United States today, domestic hot water used in homes is most commonly heated with natural gas, electric resistance, or a heat pump. Electric heat pump hot water heaters are significantly more efficient than electric resistance hot water heaters, but also more expensive to purchase. Some energy utilities offer their customers funding to help offset the higher first cost of energy efficient hot water heaters.