Water heating – Wikipedia, the free encyclopedia

Water heating is a thermodynamic 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 based on 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, 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 and Finland. District heating systems supply energy for water heating and space heating from waste heat from industries, power plants, 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.

Hot water used for space heating may be heated by fossil fuels in a boiler, while potable water may be heated in a separate appliance. This is common practice in the US, especially when warm-air space heating is usually employed.[1]

In household and commercial usage, most North American water heaters have traditionally been the tank type. Also called storage water heaters, these consist of a cylindrical vessel or container that keeps water continuously hot and ready to use. Typical sizes for household use range from 75 to 400 litres (20 to 100 US gallons). These may use electricity, natural gas, propane, heating oil, solar, or other energy sources. Natural gas heaters are most popular in the US and most European countries, since the gas is often conveniently piped throughout cities and towns and currently is the cheapest to use.

Another popular arrangement where higher flow rates are required for limited periods is to heat water in a pressure vessel that can withstand a hydrostatic pressure close to that of the incoming mains supply. In North America, these vessels are called hot water tanks, and may incorporate an electrical resistance heater, an air source heat pump, or a gas or oil burner that heats water directly.

Where hot-water space heating boilers are installed, DHW cylinders are usually heated indirectly by primary water from the boiler, or by an electric immersion heater (often as backup to the boiler). In the UK these vessels are called unvented cylinders. In the US, when connected to a boiler they are called indirect-fired water heaters.

Compared to tankless heaters, storage water heaters have the advantage of using energy (gas or electricity) at a relatively slow rate, storing the heat for later use. The disadvantage is that over time, the water inside the tank will cool down causing the heating system to activate to heat the water back up. Additionally, once the tank's supply of hot water has been exhausted, there is a significant delay before hot water is available again. Larger tanks tend to provide hot water with less temperature fluctuation at moderate flow rates.

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Water heating - Wikipedia, the free encyclopedia