Water heating is a thermodynamic process using an energy source to heat water above its initial temperature. Typical domestic uses of hot water are for cooking, cleaning, bathing, and space heating. In industry, both 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 heat a batch of water but do not produce a continual supply of heated water at a preset temperture. The temperature will vary based on the consumption rate of hot water, use more and the water becomes cooler.
Appliances for providing a more-or-less constant supply of hot water are variously known as water heaters , boilers , heat exchangers , calorifiers , or geysers depending on whether they are heating potable or non-potable water, in domestic or industrial use, their energy source, and in which part of the world they are found. In domestic installations, potable water heated for uses other than space heating is sometimes known as domestic hot water ( DHW ).
In many countries the most common energy sources for heating water are fossil fuels: natural gas, liquefied petroleum gas, oil, or sometimes solid fuels. These fuels may be consumed directly or by the use of electricity (which may derive from any of the above fuels or from nuclear or renewable sources). Alternative energy such as solar energy, heat pumps, hot water heat recycling, and sometimes geothermal heating, may also be used as available, usually in combination with backup systems supplied by gas, oil or electricity.
In some countries district heating is a major source of water heating. This is especially the case in Scandinavia. District heating systems make it possible to supply all of the energy for water heating as well as space heating from waste heat from industries, power plants, incinerators, geothermal heating, and central solar heating. The actual heating of the tap water is performed in heat exchangers at the consumers premises. Generally the consumer needs no backup system due to the very high availability of district heating systems.
Types of water heating appliance
Water for space heating may be heated by fossil fuels in a boiler. Potable water may be heated in a separate appliance: this is common practice in the USA where warm-air space heating is usually employed. The most useful heating transmitter is known as the radiator.
Gravity system
Where a space-heating water boiler is employed the traditional arrangement in the UK is to use boiler-heated (" primary ") water to heat (" secondary ") water in a cylindrical vessel (usually made of copper) containing potable water supplied from a cold water storage vessel/container, usually in the roof space of the building. This produces a fairly steady supply of DHW at low static pressure head but usually with a good flow. Water heating appliances in most other parts of the world do not use cold water storage vessel/container but heat water at pressures close to that of the incoming mains water supply.
Instantaneous water heaters
Stand-alone appliances for instantaneously heating water for DHW (Domestic Hot Water) are known in North America as tankless heaters, elsewhere as multipoint heaters , geysers or Ascots . In Australia and New Zealand there was a similar wood fired appliance known as the chip heater.
A common arrangement where hot-water space heating is employed is for the boiler to also heat potable water giving a continuous supply of DHW without any extra equipment required. Appliances capable of supplying both space-heating and DHW are known as combination (or " combi ") boilers.
Although instantaneous heaters can give a continuous supply of DHW the rate at which they can produce it is limited by the thermodynamics of heating water from the available fuel supplies.
Storage systems
Another popular arrangement where higher flow rates are required (although for limited periods) is to heat water in a pressure vessel capable of withstanding a hydrostatic pressure close to that of the incoming mains supply. (A pressure reducing valve is usually employed to limit the pressure to a safe level for the vessel.)
In North America these vessels are known as hot water tanks and may incorporate an electrical resistance heater, an air source heat pump or a gas or oil burner heating the water directly.
Where hot-water space heating boilers are used 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 known as unvented cylinders (or commonly as Megaflo s after the brand name of a widely-used model). In the US, when connected to a boiler they are known as indirect-fired water heaters .
Thermodynamics and economics
Water enters residences in the US at about 10 °C (50 °F) (varies with latitude and season). Hot water temperatures of 40–49 °C (105–120 °F) are preferred for dish-washing, laundry and showering; requiring the water temperature to be raised about 30 °C (54 °F) or more, if the hot water is later mixed with cold water. The Uniform Plumbing Code reference shower flow rate is 2.5 gpm (gallons per minute); sink and dishwasher usages range from 1–3 gpm.
Natural gas in the U.S. is measured in CCF (100 cubic feet), which is converted to a standardized heat content unit called the therm, equal to 100,000 British thermal units. A BTU is the energy required to raise one pound of water by one degree Fahrenheit. A U.S. gallon of water weighs 8.3 pounds. So, to raise a 40-gallon tank of 55 °F water up to 105 °F would require 40 x 8.3 x (105 − 55) / 100,000 BTU, or approximately 0.17 CCF, at 100% efficiency. A 40,000 BTU (per hour) heater would take 25 minutes to do this, at 100% efficiency. At $1 per therm, the cost of the gas would be about 17 cents.
In comparison, a typical electric water heater has a 4500 watt heating element, which if 100% efficient results in a heating time of about 1.1 hours. Since 16,600 BTU is roughly 4.9 kWh, at 10 cents/kWh the electricity would cost $0.49. Operating a shower at 2.5 gpm and 104 degrees Fahrenheit is equivalent to operating a 13.2 kW appliance. In the UK, domestic electric immersion heaters are usually rated at 3 kilowatts.
Energy efficiencies of water heaters in residential use can vary greatly, particularly based on manufacturer and model. However, electric heaters tend to be slightly more efficient (if one omits the power station losses) with recovery efficiency (how efficient energy is transferred to the water) reaching about 98%. Gas fired heaters have maximum recovery efficiencies of only about 86% (the remaining heat is lost with the flue gasses). Overall energy factors can be as low as 80% for electric and 50% for gas systems. However, because electricity production itself today has efficiency levels ranging from only 15% to slightly over 40%, with a natural limit being set by the Carnot cycle, electric water heating actually is the least efficient option in terms of use of natural resources and energy.
A tankless water heater operating at those same power levels (at 100% efficiency) would be able to supply 1.6 gpm continuously, raising the temperature by 50 °F. The same unit could supply 1.3 gpm while raising the temperature by 60 °F. To be able to handle a full house load of multiple uses (at least 5 gpm) with a centralized tankless water heater would require three to four times this power level — somewhat difficult to achieve with natural gas, and very difficult to achieve with electricity. Many tankless water heaters can use over 100,000 BTU/h during high flow, and so require especially large power supplies.
Unfortunately, it takes a great deal of energy to heat water, as one may experience when attempting to boil a gallon of water on a stove. For this reason, tankless on-demand water heaters need to have a very large energy source to be usable. A wall outlet, by comparison, can only source enough energy to warm a disappointingly small amount of water: about 0.17 gpm at 40 °C temperature elevation.
Water heaters
In household and commercial usage, most water heaters in North America are of the tank type. Also called storage water heaters , these consist of a cylindrical vessel/container in which water is kept continuously hot and ready for use. Typical sizes for household use range from 75 to 400 litres (20 to 100 U.S. gallons). These may use electricity, natural gas, propane, heating oil, solar, or other energy sources. Natural gas heaters are most popular in the United States and most European countries, since the gas is often conveniently piped throughout cities and towns and currently is the cheapest to use. Compared to tankless heaters, storage water heaters have the advantage of using energy (gas or electricity) at a relatively slow
Water heating - Wikipedia, the free encyclopedia
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