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Water heating

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 kettles, cauldrons, pots 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, 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 can 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.

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 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 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 (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 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 Megaflos 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 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 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.

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.

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 L to 35 L Usage of 6 L 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 the better, since it reduces standby heat loss. Tanks are available with insulation ratings ranging from R-6 to R-24. It may be possible to add an extra insulating blanket or jacket on the outside of a poorly insulated tank to reduce heat loss. The most common type of water heater blanket is fiberglass 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 (Polyurethane 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 ANSI 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. In the UK, this is called a back boiler. Modern wetbacks may run the pipe in a more sophisticated design to assist heat-exchange.


It is important that a water heater, particularly a combustion type, be properly installed to avoid safety hazards. The area around it must be kept clear. The T&P (Temperature and Pressure) relief valve should not be tested, as it may not stop leaking if you pop it open. 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.

In the past, tanks were made out of copper and were very corrosion resistant; they could last for decades. Now tanks are made out of steel lined with glass or porcelain. A steel tank can rust and leak. To prevent this, sacrificial anode 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. Modern US water heater tanks last about ten years. 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. All water heaters might 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.

Electric water heating

For small electric boilers see Electric water boiler
In the UK, electric water heating is often done by an immersion heater fitted near the bottom of the hot water tank. The immersion heater is a metal tube containing an insulated electric resistance heater which is usually rated at 3 kilowatts.

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. See Economy 7.

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. 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. Tankless heaters are often installed throughout a household at more than one point-of-use (POU), far from the central water heater, or larger models may still be used to provide all the hot water requirements for an entire house. The chief advantages of tankless water heaters are a continuous flow of hot water and energy savings (as compared to a limited flow of continuously heating hot water from conventional tank water heaters).

Various types and their advantages

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 can outweigh 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.

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. However, if the tank water heater is located inside the building, the heat lost through standby merely heats the building, thus reducing energy consumed by the building's furnace so actual energy savings due to standby loss elimination only happens if the building's central heating is off. 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.

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.

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.

In certain parts of South America as well as Costa Rica and Puerto Rico, a point of use style water heater commonly referred to as the "Electric Shower Head" is used in many residential and some commercial installations. As the name implies, an electric heating element is incorporated into such shower heads to heat the water. However, many of these units are often poorly installed, often with exposed wiring in wet locations.

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.


Tankless heaters also have some disadvantages. Some of these are listed below.

  • Installing a tankless system comes at an increased cost, particularly in retro-fit applications. They tend to be particularly expensive in areas such as the US where they are not dominant, compared to the established tank design. 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 and the existing vent pipe may have to be replaced, possibly adding expense to the retrofit installation. Many tankless units have fully modulating gas valves that can range from as low as 10,000 to over 1,000,000 BTUs. For electrical installations (non-gas), AWG 10 or 8 wire, corresponding to 10 or 6 mm², is required for most POU (point of use) 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.
  • There is a longer wait to obtain hot water. A tankless water heater only heats water upon demand, so all idle water in the piping starts at room temperature. Thus there is a more apparent "flow delay" for hot water to reach a distant faucet.
  • There is a short delay between the time when the water begins flowing and when the heater's flow detector activates the heating elements or gas burner. In the case of continuous use applications (showers, baths, washing machine) this is not an issue. However, for intermittent use applications (for example when a hot water faucet is turned on and off repeatedly) this can result in periods of hot water, then some small amount of cold water as the heater activates, followed quickly by hot water again. The period between hot/cold/hot is the amount of water which has flowed though the heater before becoming active. This cold section of water takes some amount of time to reach the faucet and is dependent on the length of piping.
  • Since a tankless water heater is inactive when hot water is not being used, they are incompatible with passive (convection based) hot water recirculation systems. They may be incompatible with active hot water recirculation systems and will certainly use more energy to constantly heat water within the piping, defeating one of a tankless water heater's primary advantages.
  • Tankless water heaters often have minimum flow requirements before the heater is activated, and this can result in a gap between the cold water temperature, and the coolest warm water temperature that can be achieved with a hot and cold water mix.
  • Similarly, unlike with a tank heater, the hot water temperature from a tankless heater is inversely proportional to the rate of the water flow -- the faster the flow, the less time the water spends in the heating element being heated. Mixing hot and cold water to the "right" temperature from a single-lever faucet (say, when taking a shower) takes some practice. Also, when adjusting the mixture in mid-shower, the change in temperature will initially react as a tanked heater does, but this also will change the flow rate of hot water. Therefore some finite time later the temperature will change again very slightly and require readjustment. This is typically not noticeable in non-shower applications. A temperature compensating valve tends to eliminate this issue.

Solar water heaters

In some locales, solar powered water heaters are used. Their solar collectors are installed outside dwellings, typically on the roof or nearby. Nearly all models are the direct-gain type, consisting of flat panels in which water circulates. Other types may use dish or trough mirrors to concentrate sunlight on a collector tube filled with water, brine or other heat transfer fluid. A storage tank is placed indoors or out. Circulation is caused by natural convection or by a small electric pump. At night, or when insufficient sunlight is present, circulation through the panel can be stopped by closing a valve and/or stopping the circulating pump, to keep hot water in the storage tank from cooling. Depending on the local climate, freeze protection, as well as prevention of overheating, must be addressed in their design, installation, and operation.

Another type of solar water heater is the evacuated tube collector. It is usually mounted on a roof, and has a row of glass tubes containing heat conducting rods, typically copper. The rods act as heating elements in a circulating loop of antifreeze. The captured heat is transferred into the domestic hot water system by a heat exchanger. This design is smaller and more efficient than traditional flat plate collectors, and works well in very cold climates. The evacuated description refers to air having been removed from the glass tubes to create a vacuum. This results in very low heat loss, once the inside coating has absorbed solar radiation.

Geothermal heating

In countries like Iceland and New Zealand, and other volcanic regions, water heating may be done using geothermal heating, rather than combustion.

Water heater safety

Water heaters potentially can explode and cause significant damage, injury, or death if certain safety devices are not installed. When the water temperature exceeds 100 °C (212 °F), the water will remain a liquid inside the tank, but when the pressure is released as the water comes out the tap the water will boil, potentially inflicting steam burns. Water above about 88°C (190 °F) will cause burns on contact. A safety device called a temperature and pressure relief (T&P or TPR) valve, is normally fitted on the top of the water heater to dump water if the temperature or pressure becomes too high. Most plumbing codes require that a discharge pipe be connected to the valve to direct the flow of discharged hot water to a drain, typically a nearby floor drain, or outside the living space. Some building codes will allow for the discharge pipe to terminate in the garage.

If a water heater is installed in a garage, it is recommended, and many codes require, that it be elevated at least 18 inches (0.46 m) above the floor to reduce the potential for fire or explosion due to spillage or leakage of combustible liquids in the garage. Furthermore, some local codes mandate that tank-type heaters in new and retrofit installations be braced to an adjacent wall with a strap to prevent them from tipping over and breaking the water and gas pipes in the event of an earthquake.

For older houses where the water heater is part of the space heating boiler, and plumbing codes allow, some plumbers will install a "Watts 210" device in place of a TPR valve. When the device senses that the temperature reaches 99 °C (210 °F), it will shut off the gas supply and prevent further heating. In addition, an expansion tank or exterior pressure relief valve must be installed to prevent pressure buildup in the plumbing from rupturing pipes, valves, or the water heater.

Scalding is a serious concern with any water heater. Human skin burns quickly at high temperature, e.g., only 60 °C (140 °F), but also at lower temperatures, e.g., 50 °C (120 °F), if the exposure times are sufficient. Older people and children often receive the most serious scalds due to disabilities or slow reaction times. In Australia and elsewhere it is common practice to put a tempering valve on the outlet of the water heater. A tempering valve mixes enough cold water with the hot from the heater to keep the outgoing water temperature fixed, often set to 50 °C. Without a tempering valve, reduction of the water heater's setpoint temperature is the most direct way to reduce scalding. However, for sanitation, hot water is needed. Most residential dishwashing machines, for example, include an electric heating element for increasing the water temperature above that provided by water heaters.

There are two seemingly conflicting safety issues around water heater temperature — the risk of scalding from excessively hot water, 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 at least 50 °C (120 °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 such that a temperature of at least 50°C and preferably 55°C is achieved within one minute at outlets. If there is a dishwasher without a booster heater, it may require a water temperature within a range of 57 °C (130 °F) to 60 °C (140 °F) for optimum cleaning, in which case tempering valves set to no more than 55°C can be applied to faucets to avoid scalding. (Note: Tank temperatures above 60°C may produce calcium deposits, which could later harbor bacteria, in the water tank. Temperatures above 60°C may also cause gradual erosion of glassware in a dishwasher.)

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