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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 Vessel s known as Kettle s, ''' Cauldron s''', ''' Pot s''' or '''coppers'''. These vessels heat a batch of water but do not produce a continual supply. Appliances for providing a more-or-less constant supply of hot water are variously known as '''water heaters''', ''' Boiler s''', ''' Heat Exchanger s''', '''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'''). The most common energy sources for heating water are fossil fuels: , and sometimes Geothermal Power , may also be used as available, usually in combination with gas, oil or electricity. TYPES OF WATER HEATING APPLIANCE Water for space heating is usually 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. 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 Tank , usually in the Roof space of the building. This produces a fairly steady supply of DHW at low Pressure but usually with a good Flow . Water heating appliances in most other parts of the world do not use cold water storage tanks 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 are known in North America as tankless heaters, elsewhere as '''multipoint heaters''', '''geyser'''s or '''Ascot'''s. 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 (Domestic Hot Water) 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 tanks and may incorporate 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). Adults generally prefer shower temperatures of 40-49°C (105-120°F), requiring the water temperature to be raised about 30°C (55°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 Unit s. 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 40 amp service at 240 volts, which results in a heating time of about 30 minutes. Since 16,600 BTU is roughly 4.9 KWh , at 10 cents/kWh the electricity would cost $0.49. Energy efficiencies of tank water heaters in residential use can vary greatly, particularly based on manufacturer and model. However, electric heaters tend to be slightly more efficient 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.[http://www.gamapower.org/ Consumers' Directory of Certified Efficiency Ratings] 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/hr 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 .17 gpm at 40°C temperature elevation. TANK-TYPE 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 tank 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 rate, storing the heat for later use. Larger tanks tend to provide hot water with less temperature fluctuation at moderate flow rates. Storage water heaters in the United States and New Zealand are typically vertical, cylindrical tanks, usually standing on the floor or on a platform raised a short distance above the floor. Storage water heater tanks in Spain are typically horizontal. In india, they are mainly vertical. In apartments they can be mounted in the ceiling space over laundry-utility rooms. In western countries, where ambient temperature is colder, tiny point-of-use electric storage water heaters with capacities ranging from 8 to 32 litres (2 to 6 gallons) are made for installation in kitchen and bath cabinets or on the wall above a sink. They typically use low power heating elements, about 1 kW to 1.5 kW, and can provide hot water long enough for hand washing, or, if plumbed into an existing hot water line, until hot water arrives from a remote high capacity water heater. They are sometimes used when retrofitting a pump and recirculating plumbing in a building is too costly or impractical. Since they maintain water temperature thermostatically, they will supply hot water at extremely low flow rates, unlike tankless heaters. In tropical countries, like Singapore, India: An ideal storage water heater may vary from 10 lit to 35 lit. Usage of 6 ltr tanks is not uncommon. Smaller tanks are sufficient as ambient weather and water temperature are moderate. The inner tank of the Water heater is the single most important feature of a water heater. The best heaters have a copper container. The second most important feature may be the type of heating element. The cartridge elements score over tubular elements. Insulation and other improvements In general, the more tank ''' insulation with a Vinyl film on the outside. The insulation is wrapped around the tank and the ends are taped together. It is important that the blanket be the right size for the tank and not block air flow or cover safety and drainage valves, the controls, or block airflow through the exhaust vent, if any. In very humid locations, adding insulation to an already well-insulated tank may cause condensation problems, potentially causing rust, mold, or operational problems. Modern water heaters have PUF (Poly Urathene Foam) insulation. In countries where serviceability is very important, PUF capsules are kept between the inner tank and the outer body. Depending upon the insulation efficiency, star rating is given in India. Other improvements include check valve devices at their inlet and outlet, cycle timers, electronic Ignition in the case of fuel-using models, sealed air intake systems in the case of fuel-using models, and pipe insulation. The sealed air-intake system types are sometimes called "band- Joist " intake units. "High efficiency" condensing units can convert up to 98% of the energy in the fuel to heating the water. The exhaust gases of combustion are cooled and are mechanically Ventilated either through the roof or through an exterior wall. At high Combustion efficiencies a drain must be supplied to handle the water condensed out of the combustion products which are primarily carbon dioxide and water vapor. In traditional Plumbing in the United Kingdom the space-heating boiler is set up to heat a separate ''hot water cylinder'' or ''hot water tank'' for potable hot water. Such tanks are often fitted with an auxiliary electrical ''immersion heater'' for a quick temperature boost. Heat from the space-heating boiler is transferred to the potable water tank by means of a heat exchanger, and the boiler operates at a higher temperature than the potable hot water supply. Most potable water heaters in the United States are completely separate from the space heating units. Residential combustion water heaters manufactured since 2003 in the United States have been redesigned to resist ignition of flammable vapors and incorporate a thermal cutoff switch, per ANS Z21.10.1 . The first feature attempts to prevent vapors from flammable liquids and gasses in the vicinity of the heater from being ignited and thus causing a house fire or explosion. The second feature prevents tank overheating due to unusual combustion conditions. These safety requirements were made based on homeowners placing, and sometimes spilling, Gasoline and other flammable gases near their water heaters and causing fires. Since most of the new designs incorporate some type of flame arrestor screen, they require monitoring to make sure they don't become clogged with lint or dust, reducing the availability of air for combustion. If the flame arrestor becomes clogged, the thermal cutoff may act to shut down the heater. A wetback stove or '''wetback heater''' is the name (used in New Zealand at least) for a simple household secondary Water-heater using incidental heat. It typically consists of a hot water pipe running behind a Fireplace or Stove (rather than Hot Water Storage ), and has no facility to limit the heating. Modern wetbacks may run the pipe in a more sophisticated design to assist Heat-exchange . Maintenance It is important that a water heater, particularly a combustion type, be properly installed to avoid safety hazards.[http://www.waterheaterrescue.com/pages/WHRpages/English/Longevity/hot-water-heater-safety.html] The area around it must be kept clear. The T&P (Temperature and Pressure) relief valve should be tested at least yearly. Sediment that accumulates in the bottom of the tank should be drained out at least yearly, to maintain heating efficiency and to make the tank last longer.[http://www.waterheaterrescue.com/pages/WHRpages/English/Longevity/sediment-in-hot-water-heaters.html] {Link without Title} {Link without Title} {Link without Title} In the past, tanks were made out of rods of Magnesium or Aluminum Alloy themselves undergo Galvanic Corrosion to reduce corrosion of the steel tank. Tanks with longer guarantees have increased Anode material — when the anode is used up, the tank will rust. It may be possible to check the anode condition, and replace it if it is wearing out.[http://www.waterheaterrescue.com/pages/WHRpages/English/Longevity/water-heater-preventive-maintenance.html] Modern US water heater tanks last about ten years.[http://www.freepatentsonline.com/6903540.html] The tank should have a tray underneath to contain leaks and channel them into a drain. Simple, battery-powered alarms are available for $10-$30 that alert the homeowner to leaks.[http://www.homedepot.com/prel80/HDUS/EN_US/jsearch/product.jsp?pn=100038838] All water heaters will eventually leak, so it is important to know where the water supply shutoff valve is located! Electric water heaters made with plastic tanks (e.g, polybutylene) are available. Designed with corrosion-free and corrosion-resistant materials, they can conceivably last the life of a dwelling. The only maintenance requirement would be to periodically clean sediments from the bottom of the tank and clean mineral scale off the heating elements. {Link without Title} Electric water heating power load shifting Because tank-type water heaters store Heat , electrical water heaters can be a good match for an intelligent electrical power distribution system, heating when the electrical grid load is low and turning off when the load is high. This could be implemented by allowing the power supplier to send load-shedding requests, or by the use of real-time energy pricing. TANKLESS HEATERS Tankless water heaters, also called combination or "combi" boilers, instantaneous, continuous flow, inline, flash, on-demand or instant-on water heaters, are also available and gaining in popularity. The chief advantages of tankless water heaters is a continuous flow of hot water and by saving energy (as compared to continuously heating tank water heaters). These water heaters heat the water as the water flows through the device, and do not retain any water internally except for what is in the heat exchanger coil. Point-of-use tankless (POU) heaters may be installed at the Faucet or in the Bathroom , far from the central water heater, or larger models can be used to provide all the hot water requirements for an entire house. The high efficiency condensing combination boiler provides both space heating and water heating, an increasingly popular choice in UK houses. In fact, combination boilers now account for over half of all the new domestic boilers installed in Britain.[http://www.worcester-bosch.co.uk/index.php?fuseaction=site.articleDetail2&con_id=130817 Tankless water heaters can be divided into two categories: "full on/full off" and "modulated". Full on/full off units do not have a variable Power output level; the unit is either on or off. Modulated tankless water heaters base the heat output on the flow of water running through the unit. This is usually done through the use of a 'flow sensor', modulating gas valve, inlet water temperature sensor and an outlet water temperature sensor-choke valve and means that the occupants should receive the same output Temperature of water at differing velocities, usually within a close range of ±2°C. Tankless heaters can ideally be somewhat more efficient than storage water heaters. In both kinds of installation (centralized and POU) the absence of a tank saves Energy as conventional water heaters have to reheat the water in the tank as it cools off, called standby loss. With a central water heater of any type, water is wasted waiting for water to heat up because of the cold water in the pipes between the Faucet and the water heater. Point of use tankless water heaters are located right where the water is being used, so the water is almost instantly hot, which saves water. They also save even more energy than centrally installed tankless water heaters because no hot water is left in the pipes after the water is shut off. However, point of use tankless water heaters are usually used in combination with a central water heater since they are usually limited to under 6 litres/minute (1.5 U.S. gallons/minute), as the expense of buying a heater for every Kitchen , Laundry Room , bathroom or sink, often outweighs the money saved in water and energy bills. In addition, point of use water heaters until recently were almost always electrical, and electricity is often substantially more expensive than Natural Gas . Under current North American conditions, the most cost effective configuration from an operating viewpoint is usually to use a central tankless water heater for most of the house, and install a point of use tankless water heater at any distant faucets or bathrooms. However, this may vary according to how much electricity, gas and water costs in the area, the layout of the house, and how much hot water is used. Only electric tankless water heaters were available at first and they are still used for almost all point of use heaters, but natural gas and propane heaters are now common. When consumers are considering a whole house gas tankless unit, they are advised to look at how the unit functions when raising the water temperature by about 42°C (75–77°F). Thus, if they live in a cold weather climate, they are advised to look at the unit's capacity with 3-10°C (38–50°F) inlet water temperatures, and find a size that produces approximately 15 litres/minute (4 gpm) even in winter if they have a typical-sized house and desire what is called a 2-appliance heater. This same unit may produce 25-30 litres/minute (6.3–6.9 gpm) in summer with higher inlet temperatures, but there is greater interest in year round production and usability. Since the water must be heated instantly, tankless water heaters use a very large amount of electricity or gas while they are on. If a storage water heater is being replaced with a tankless one, the size of the Electrical Wiring or gas pipeline may have to be increased to handle the load, adding significant expense to the installation. Gas units are efficient but require a large volume of gas when operating at high firing rates. Many tankless units have fully modulating gas valves that can range from as low as 10,000 to over 1,000,000 BTU s. For electrical installations, AWG 10 or 8 wire, corresponding to 10 or 6 mm&2, is required for most POU heaters at North American voltages. Larger whole house electric units may require up to AWG 2 wire. In gas appliances both pressure and volume requirements must be met for optimum operation. The two main advantages of tankless water heater (continuous hot water and energy conservation) are considered to far outweigh the few disadvantages as listed below; 1) A tank water heater tends to heat the water in the surrounding piping by virtue of Convection . On the other hand a tankless water heater only heats water upon demand and all water in the piping is cold. Thus there is a more apparent "flow delay" for hot water to reach a distant faucet as a greater amount of cold water must be flushed from the piping first.
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