Roof-mounted close-coupled thermosiphon solar water heater.

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Solar water heating (SWH) or solar hot water (SHW) systems comprise several innovations and many mature renewable energy technologies that have been well established for many years. SWH has been widely used in Greece, Cyprus, Turkey, Israel, Australia, Japan, Austria and China.

In a "close-coupled" SWH system the storage tank is horizontally mounted immediately above the solar collectors on the roof. No pumping is required as the hot water naturally rises into the tank through thermosiphon flow. In a "pump-circulated" system the storage tank is ground- or floor-mounted and is below the level of the collectors; a circulating pump moves water or heat transfer fluid between the tank and the collectors.

SWH systems are designed to deliver hot water for most of the year. However, in winter there sometimes may not be sufficient solar heat gain to deliver sufficient hot water. In this case a gas or electric booster is normally used to heat the water.

Overview[link]

Hot water heated by the sun is used in many ways. While perhaps best known in a residential setting to provide domestic hot water, solar hot water also has industrial applications, e.g. to generate electricity.^[1] Designs suitable for hot climates can be much simpler and cheaper, and can be considered an appropriate technology for these places. The global solar thermal market is dominated by China, Europe, Japan and India.

A solar hot water heater installed on a house in Belgium

In order to heat water using solar energy, a collector, often fastened to a roof or a wall facing the sun, heats working fluid that is either pumped (active system) or driven by natural convection (passive system) through it. The collector could be made of a simple glass-topped insulated box with a flat solar absorber made of sheet metal, attached to copper pipes and dark-colored, or a set of metal tubes surrounded by an evacuated (near vacuum) glass cylinder. In industrial cases a parabolic mirror can concentrate sunlight on the tube. Heat is stored in a hot water storage tank. The volume of this tank needs to be larger with solar heating systems in order to allow for bad weather^{[clarification needed]}, and because the optimum final temperature for the solar collector^{[clarification needed]} is lower than a typical immersion or combustion heater. The heat transfer fluid (HTF) for the absorber may be the hot water from the tank, but more commonly (at least in active systems) is a separate loop of fluid containing anti-freeze and a corrosion inhibitor which delivers heat to the tank through a heat exchanger (commonly a coil of copper tubing within the tank). Another lower-maintenance concept is the 'drain-back': no anti-freeze is required; instead, all the piping is sloped to cause water to drain back to the tank. The tank is not pressurized and is open to atmospheric pressure. As soon as the pump shuts off, flow reverses and the pipes are empty before freezing could occur.

Residential solar thermal installations fall into two groups: passive (sometimes called "compact") and active (sometimes called "pumped") systems. Both typically include an auxiliary energy source (electric heating element or connection to a gas or fuel oil central heating system) which is activated when the water in the tank falls below a minimum temperature setting such as 55 °C. Hence, hot water is always available. The combination of solar water heating and using the back-up heat from a wood stove chimney to heat water^[2] can enable a hot water system to work all year round in cooler climates, without the supplemental heat requirement of a solar water heating system being met with fossil fuels or electricity.

When a solar water heating and hot-water central heating system are used in conjunction, solar heat will either be concentrated in a pre-heating tank that feeds into the tank heated by the central heating, or the solar heat exchanger will replace the lower heating element and the upper element will remain in place to provide for any heating that solar cannot provide. However, the primary need for central heating is at night and in winter when solar gain is lower. Therefore, solar water heating for washing and bathing is often a better application than central heating because supply and demand are better matched. In many climates, a solar hot water system can provide up to 85% of domestic hot water energy. This can include domestic non-electric concentrating solar thermal systems. In many northern European countries, combined hot water and space heating systems (solar combisystems) are used to provide 15 to 25% of home heating energy.

History[link]

There are records of solar collectors in the United States dating back to before 1900,^[3] comprising a black-painted tank mounted on a roof. In 1896 Clarence Kemp of Baltimore, USA enclosed a tank in a wooden box, thus creating the first 'batch water heater' as they are known today. Although flat-plate collectors for solar water heating were used in Florida and Southern California in the 1920s there was a surge of interest in solar heating in North America after 1960, but especially after the 1973 oil crisis.

See Appendix 1 at the bottom of this article for a number of country-specific statistics on the "Use of solar water heating worldwide". Wikipedia also has country-specific articles about solar energy use (thermal as well as photovoltaic) in Australia, Canada, China, Germany, India, Israel, Japan, Portugal, Romania, Spain, the United Kingdom and the United States.

Mediterranean[link]

Passive (thermosiphon) solar water heaters on a rooftop in Jerusalem

Israel and Cyprus are the per capita leaders in the use of solar water heating systems with over 30%–40% of homes using them.^[4]

Flat plate solar systems were perfected and used on a very large scale in Israel. In the 1950s there was a fuel shortage in the new Israeli state, and the government forbade heating water between 10 pm and 6 am. Levi Yissar built the first prototype Israeli solar water heater and in 1953 he launched the NerYah Company, Israel's first commercial manufacturer of solar water heating.^[5] Despite the abundance of sunlight in Israel, solar water heaters were used by only 20% of the population by 1967. Following the energy crisis in the 1970s, in 1980 the Israeli Knesset passed a law requiring the installation of solar water heaters in all new homes (except high towers with insufficient roof area).^[6] As a result, Israel is now the world leader in the use of solar energy per capita with 85% of the households today using solar thermal systems (3% of the primary national energy consumption),^[7] estimated to save the country 2 million barrels (320,000 m³) of oil a year, the highest per capita use of solar energy in the world.^[8]

In 2005, Spain became the first country in the world to require the installation of photovoltaic electricity generation in new buildings, and the second (after Israel) to require the installation of solar water heating systems in 2006.^[9]

Asia-Pacific[link]

New solar hot water installations during 2007, worldwide.

The world saw a rapid growth of the use of solar warm water after 1960, with systems being marketed also in Japan and Australia^[3] Technical innovation has improved performance, life expectancy and ease of use of these systems. Installation of solar water heating has become the norm in countries with an abundance of solar radiation, like the Mediterranean,^[10] and Japan and Austria, where there Colombia developed a local solar water heating industry thanks to the designs of Las Gaviotas, directed by Paolo Lugari. Driven by a desire to reduce costs in social housing, the team of Gaviotas studied the best systems from Israel, and made adaptations as to meet the specifications set by the Banco Central Hipotecario (BCH) which prescribed that the system must be operational in cities like Bogotá where there are more than 200 days overcast. The ultimate designs were so successful that Las Gaviotas offered in 1984 a 25 year warranty on any of its installations. Over 40,000 were installed, and still function a quarter of a century later.

Australia has a variety of incentives (national and state) and regulations (state) for solar thermal introduced starting with MRET in 1997.^[11][12][13]

Solar water heating systems have become popular in China, where basic models start at around 1,500 yuan (US$190), much cheaper than in Western countries (around 80% cheaper for a given size of collector). It is said that at least 30 million Chinese households now have one, and that the popularity is due to the efficient evacuated tubes which allow the heaters to function even under gray skies and at temperatures well below freezing.^[14]

System design requirements[link]

The type, complexity, and size of a solar water heating system is mostly determined by:

• The temperature and amount of the water required from the system.
• Changes in ambient temperature and solar radiation between summer and winter.
• The changes in ambient temperature during the day-night cycle.
• The possibility of the potable water or collector fluid overheating.
• The possibility of the potable water or collector fluid freezing.

The minimum requirements of the system are typically determined by the amount or temperature of hot water required during winter, when a system's output and incoming water temperature are typically at their lowest. The maximum output of the system is determined by the need to prevent the water in the system from becoming too hot.

Freeze protection[link]

Freeze protection measures prevent damage to the system due to the expansion of freezing transfer fluid. Drainback systems drain the transfer fluid from the system when the pump stops. Many indirect systems use antifreeze (e.g. propylene glycol) in the heat transfer fluid.

In some direct systems, the collectors can be manually drained when freezing is expected. This approach is common in climates where freezing temperatures do not occur often, but is somewhat unreliable since the operator can forget to drain the system. Other direct systems use freeze-tolerant collectors made with flexible polymers such as silicone rubber.

A third type of freeze protection is freeze-tolerance, where low pressure polymer water channels made of silicone rubber simply expands on freezing. One such collector now has European Solar Keymark accreditation, following extra durability testing.

Overheat protection[link]

When no hot water has been used for a day or two, the fluid in the collectors and storage can reach very high temperatures in all systems except for those of the drainback variety. When the storage tank in a drainback system reaches its desired temperature, the pumps are shut off, putting an end to the heating process and thus preventing the storage tank from overheating.

One method of providing over heat protection is to dump the heat into a hot tub.

Some active systems deliberately cool the water in the storage tank by circulating hot water through the collector at times when there is little sunlight or at night, causing increased heat loss. This is most effective in direct or thermal store plumbing and is virtually ineffective in systems that use evacuated tube collectors, due to their superior insulation. No matter the collector type, however, they may still overheat. High pressured sealed solar thermal systems versions ultimately rely on the operation of temperature and pressure relief valves. Low pressure, open vented ones have simpler, more reliable safety controls, typically an open vent.

Types of solar water heating systems[link]

Solar water heaters can be either active or passive. An active system uses an electric pump to circulate the heat-transfer fluid; a passive system has no pump. The amount of hot water a solar water heater produces depends on the type and size of the system, the amount of sun available at the site, proper installation, and the tilt angle and orientation of the collectors.

Solar water heaters are also characterized as open loop (also called "direct") or closed loop (also called "indirect"). An open-loop system circulates household (potable) water through the collector. A closed-loop system uses a heat-transfer fluid (water or diluted antifreeze, for example) to collect heat and a heat exchanger to transfer the heat to household water.

Direct and indirect systems[link]

Direct or open loop systems circulate potable water through the collectors. They are cheaper than but can have drawbacks:

• They offer little or no overheat protection unless they have a heat export pump.
• They offer little or no freeze protection, unless the collectors are freeze-tolerant.
• Collectors accumulate scale in hard water areas, unless an ion-exchange softener is used.

Until the advent of freeze-tolerant solar collectors, they were not considered suitable for cold climates since, in the event of the collector being damaged by a freeze, pressurized water lines will force water to gush from the freeze-damaged collector until the problem is noticed and rectified.

Indirect active systems: (C) Indirect system with heat exchanger in tank; (D) Drainback system with drainback reservoir. In these schematics the controller and pump are driven by mains electricity

Indirect or closed loop systems use a heat exchanger that separates the potable water from the fluid, known as the "heat-transfer fluid" (HTF), that circulates through the collector. The two most common HTFs are water and an antifreeze/water mix that typically uses non-toxic propylene glycol. After being heated in the panels, the HTF travels to the heat exchanger, where its heat is transferred to the potable water. Though slightly more expensive, indirect systems offer freeze protection and typically offer overheat protection as well.

Passive and active systems[link]

Passive systems rely on heat-driven convection or heat pipes to circulate water or heating fluid in the system. Passive solar water heating systems cost less and have extremely low or no maintenance, but the efficiency of a passive system is significantly lower than that of an active system, and overheating and freezing are major concerns.

Active systems use one or more pumps to circulate water and/or heating fluid in the system.

Though slightly more expensive, active systems offer several advantages:

• The storage tank can be situated lower than the collectors, allowing increased freedom in system design and allowing pre-existing storage tanks to be used.
• The storage tank can always be hidden from view.
• The storage tank can be placed in conditioned or semi-conditioned space, reducing heat loss.
• Drainback tanks can be used.
• Superior efficiency.
• Increased control over the system.

Modern active solar water systems have electronic controllers that offer a wide-range of functionality, such as the modification of settings that control the system, interaction with a backup electric or gas-driven water heater, calculation and logging of the energy saved by a SWH system, safety functions, remote access, and various informative displays, such as temperature readings.

A typical programmable differential controller

The most popular pump controller is a differential controller that senses temperature differences between water leaving the solar collector and the water in the storage tank near the heat exchanger. In a typical active system, the controller turns the pump on when the water in the collector is about 8–10 °C warmer than the water in the tank, and it turns the pump off when the temperature difference approaches 3–5 °C. This ensures the water always gains heat from the collector when the pump operates and prevents the pump from cycling on and off too often. (In direct systems this "on differential" can be reduced to around 4C because there is no heat exchanger impediment.)

Some active SWH systems use energy obtained by a small photovoltaic (PV) panel to power one or more variable-speed DC pump(s). In order to ensure proper performance and longevity of the pump(s), the DC-pump and PV panel must be suitably matched. Some PV pumped solar thermal systems are of the antifreeze variety and some use freeze-tolerant solar collectors. The solar collectors will almost always be hot when the pump(s) are operating (i.e. when the sun is bright), and some do not use solar controllers. Sometimes, however, a differential controller (that can also be powered by the DC output of a PV panel) is used to prevent the operation of the pumps when there is sunlight to power the pump but the collectors are still cooler than the water in storage. One advantage of a PV-driven system is that solar hot water can still be collected during a power outage if the Sun is shining. Another advantage is that the operational carbon clawback of using mains pumped solar thermal (which typically negates up to 23% of its carbon savings) is completely avoided.

The bubble separator of a bubble-pump system

An active solar water heating system can also be equipped with a bubble pump (also known as geyser pump) instead of an electric pump. A bubble pump circulates the heat transfer fluid (HTF) between collector and storage tank using solar power and without any external energy source and is suitable for flat panel as well as vacuum tube systems. In a bubble pump system, the closed HTF circuit is under reduced pressure, which causes the liquid to boil at low temperature as it is heated by the sun. The steam bubbles form a geyser pump, causing an upward flow. The system is designed such that the bubbles are separated from the hot fluid and condensed at the highest point in the circuit, after which the fluid flows downward towards the heat exchanger caused by the difference in fluid levels.^[15][16][17] The HTF typically arrives at the heat exchanger at 70 °C and returns to the circulating pump at 50 °C. In frost prone climates the HTF is water with propylene glycol anti-freeze added, usually in the ratio of 60 to 40. Pumping typically starts at about 50 °C and increases as the sun rises until equilibrium is reached, which depends on the efficiency of the heat exchanger, the temperature of the water being heated, and the total solar energy available.

Passive direct systems[link]

An integrated collector storage (ICS) system

An integrated collector storage (ICS or Batch Heater) system uses a tank that acts as both storage and solar collector. Batch heaters are basically thin rectilinear tanks with a glass side facing the position of the sun at noon. They are simple and less costly than plate and tube collectors, but they sometimes require extra bracing if installed on a roof (since they are heavy when filled with water [400–700 lbs],) suffer from significant heat loss at night since the side facing the sun is largely uninsulated, and are only suitable in moderate climates.

A convection heat storage unit (CHS) system is similar to an ICS system, except the storage tank and collector are physically separated and transfer between the two is driven by convection. CHS systems typically use standard flat-plate type or evacuated tube collectors, and the storage tank must be located above the collectors for convection to work properly. The main benefit of a CHS systems over an ICS system is that heat loss is largely avoided since (1) the storage tank can be better insulated, and (2) since the panels are located below the storage tank, heat loss in the panels will not cause convection, as the cold water will prefer to stay at the lowest part of the system.

Active indirect systems: drainback and antifreeze[link]

Pressurized antifreeze or pressurized glycol systems use a mix of antifreeze (almost always non-toxic propylene glycol) and water mix for HTF in order to prevent freeze damage.

Though effective at preventing freeze damage, antifreeze systems have many drawbacks:

• If the HTF gets too hot (for example, when the homeowner is on vacation,) the glycol degrades into acid. After degradation, the glycol not only fails to provide freeze protection, but also begins to eat away at the solar loop's components: the collectors, the pipes, the pump, etc. Due to the acid and excessive heat, the longevity of parts within the solar loop is greatly reduced.
• Most do not feature drainback tanks, so the system must circulate the HTF – regardless of the temperature of the storage tank – in order to prevent the HTF from degrading. Excessive temperatures in the tank cause increased scale and sediment build-up, possible severe burns if a tempering valve is not installed, and, if a water heater is being used for storage, possible failure of the water heater's thermostat.
• The glycol/water HTF must be replaced every 3–8 years, depending on the temperatures it has experienced.
• Some jurisdictions require double-walled heat exchangers even though propylene glycol is non-toxic.
• Even though the HTF contains glycol to prevent freezing, it will still circulate hot water from the storage tank into the collectors at low temperatures (e.g. below 40 degrees Fahrenheit), causing substantial heat loss.

A drainback system is an indirect active system where the HTF (almost always pure water) circulates through the collector, being driven by a pump. The collector piping is not pressurized and includes an open drainback reservoir that is contained in conditioned or semi-conditioned space. If the pump is switched off, the HTF drains into the drainback reservoir and none remains in the collector. Since the system relies upon being able to drain properly, all piping above the drainback tank, including the collectors, must slope downward in the direction of the drainback tank. Installed properly, the collector cannot be damaged by freezing or overheating.^[18] Drainback systems require no maintenance other than the replacement of failed system components.

A rough comparison of solar hot water systems[link]

Comparison of SWH systems^[19]

Characteristic	ICS (Batch)	Thermosyphon	Active direct	Active indirect	Drainback	Bubble Pump
Low profile-unobtrusive			Y	Y	Y	Y
Lightweight collector			Y	Y	Y	Y
Survives freezing weather			Y	Y	Y	Y
Low maintenance	Y	Y	Y		Y	Y
Simple: no ancillary control	Y	Y				Y
Retrofit potential to existing store			Y	Y	Y	Y
Space saving: no extra storage tank	Y	Y

Collectors used in modern domestic SWH systems[link]

Solar thermal collectors capture and retain heat from the sun and transfer this heat to a liquid. Two important physical principles govern the technology of solar thermal collectors:

• Any hot object ultimately returns to thermal equilibrium with its environment, due to heat loss from the hot object. The processes that result in this heat loss are conduction, convection and radiation.^[20] The efficiency of a solar thermal collector is directly related to heat losses from the collector surface (efficiency being defined as the proportion of heat energy that can be retained for a predefined period of time). Within the context of a solar collector, convection and radiation are the most important sources of heat loss. Thermal insulation is used to slow down heat loss from a hot object to its environment. This is actually a direct manifestation of the Second law of thermodynamics but we may term this the 'equilibrium effect'.
• Heat is lost more rapidly if the temperature difference between a hot object and its environment is larger. Heat loss is predominantly governed by the thermal gradient between the temperature of the collector surface and the ambient temperature. Conduction, convection as well as radiation occur more rapidly over large thermal gradients.^[20] We may term this the 'delta-t effect'.

The most simple approach to solar heating of water is to simply mount a metal tank filled with water in a sunny place. The heat from the sun would then heat the metal tank and the water inside. Indeed, this was how the very first SWH systems worked more than a century ago.^[3] However, this setup would be inefficient due to an oversight of the equilibrium effect, above: as soon as heating of the tank and water begins, the heat gained starts to be lost back into the environment, and this continues until the water in the tank reaches the ambient temperature. The challenge is therefore to limit the heat loss from the tank, thus delaying the time when thermal equilibrium is regained.

ICS or batch collectors reduce heat loss by placing the water tank in a thermally insulated box.^[21][22] This is achieved by encasing the water tank in a glass-topped box that allows heat from the sun to reach the water tank.^[23] However, the other walls of the box are thermally insulated, reducing convection as well as radiation to the environment.^[24] In addition, the box can also have a reflective surface on the inside. This reflects heat lost from the tank back towards the tank. In a simple way one could consider an ICS solar water heater as a water tank that has been enclosed in a type of 'oven' that retains heat from the sun as well as heat of the water in the tank. Using a box does not eliminate heat loss from the tank to the environment, but it largely reduces this loss.

Standard ICS collectors have a characteristic that strongly limits the efficiency of the collector: a small surface-to-volume ratio.^[25] Since the amount of heat that a tank can absorb from the sun is largely dependent on the surface of the tank directly exposed to the sun, it follows that a small surface would limit the degree to which the water can be heated by the sun. Cylindrical objects such as the tank in an ICS collector inherently have a small surface-to-volume ratio and most modern collectors attempt to increase this ratio for efficient warming of the water in the tank. There are many variations on this basic design, with some ICS collectors comprising several smaller water containers and even including evacuated glass tube technology, a type of ICS system known as an Evacuated Tube Batch (ETB) collector.^[21]

Flat-plate solar thermal collector, viewed from roof-level

Flat plate collectors are an extension of the basic idea to place a collector in an 'oven'-like box with glass in the direction of the Sun.^[21] Most flat plate collectors have two horizontal pipes at the top and bottom, called headers, and many smaller vertical pipes connecting them, called risers. The risers are welded (or similarly connected) to thin absorber fins. Heat-transfer fluid (water or water/antifreeze mix) is pumped from the hot water storage tank (direct system) or heat exchanger (indirect system) into the collectors' bottom header, and it travels up the risers, collecting heat from the absorber fins, and then exits the collector out of the top header. Serpentine flat plate collectors differ slightly from this "harp" design, and instead use a single pipe that travels up and down the collector. However, since they cannot be properly drained of water, serpentine flat plate collectors cannot be used in drainback systems.

The type of glass used in flat plate collectors is almost always low-iron, tempered glass. Being tempered, the glass can withstand significant hail without breaking, which is one of the reasons that flat-plate collectors are considered the most durable collector type.

Unglazed or formed collectors are similar to flat-plate collectors, except they are not thermally insulated nor physically protected by a glass panel. Consequently these types of collectors are much less efficient for domestic water heating. For pool heating applications, however, the water being heated is often colder than the ambient roof temperature, at which point the lack of thermal insulation allows additional heat to be drawn from the surrounding environment.^[26]

Evacuated tube collectors (ETC) are a way in which heat loss to the environment,^[21] inherent in flat plates, has been reduced. Since heat loss due to convection cannot cross a vacuum, it forms an efficient isolation mechanism to keep heat inside the collector pipes.^[27] Since two flat sheets of glass are normally not strong enough to withstand a vacuum, the vacuum is rather created between two concentric tubes. Typically, the water piping in an ETC is therefore surrounded by two concentric tubes of glass with a vacuum in between that admits heat from the sun (to heat the pipe) but which limits heat loss back to the environment. The inner tube is coated with a thermal absorbent.^[28] Life of the vacuum varies from collector to collector, anywhere from 5 years to 15 years.

Flat plate collectors are generally more efficient than ETC in full sunshine conditions. However, the energy output of flat plate collectors is reduced slightly more than evacuated tube collectors in cloudy or extremely cold conditions.^[21] Most ETCs are made out of annealed glass, which is susceptible to hail, breaking in roughly golf ball -sized hail. ETCs made from "coke glass," which has a green tint, are stronger and less likely to lose their vacuum, but efficiency is slightly reduced due to reduced transparency.

Heating of swimming pools[link]

Both pool covering systems floating atop the water and separate solar thermal collectors may be used for pool heating.

Pool covering systems, whether solid sheets or floating disks, act as solar collectors and provide pool heating benefits which, depending on climate, may either supplement the solar thermal collectors discussed below or make them unnecessary. See Swimming Pool Covers for a detailed discussion.

Solar thermal collectors for nonpotable pool water use are often made of plastic. Pool water, mildly corrosive due to chlorine, is circulated through the panels using the existing pool filter or supplemental pump. In mild environments, unglazed plastic collectors are more efficient as a direct system. In cold or windy environments evacuated tubes or flat plates in an indirect configuration do not have pool water pumped through them, they are used in conjunction with a heat exchanger that transfers the heat to pool water. This causes less corrosion. A fairly simple differential temperature controller is used to direct the water to the panels or heat exchanger either by turning a valve or operating the pump.^[29] Once the pool water has reached the required temperature, a diverter valve is used to return pool water directly to the pool without heating.^[30] Many systems are configured as drainback systems where the water drains into the pool when the water pump is switched off.

The collector panels are usually mounted on a nearby roof, or ground-mounted on a tilted rack. Due to the low temperature difference between the air and the water, the panels are often formed collectors or unglazed flat plate collectors. A simple rule-of-thumb for the required panel area needed is 50% of the pool's surface area.^[30] This is for areas where pools are used in the summer season only, not year 'round. Adding solar collectors to a conventional outdoor pool, in a cold climate, can typically extend the pool's comfortable usage by some months or more if an insulating pool cover is also used.^[31] An active solar energy system analysis program may be used to optimize the solar pool heating system before it is built.

Economics, energy, environment, and system costs[link]

A laundromat in California with panels on the roof providing hot washing water.

Energy production[link]

The amount of heat delivered by a solar water heating system depends primarily on the amount of heat delivered by the sun at a particular place (the insolation). In tropical places the insolation can be relatively high, e.g. 7 kW.h/m2 per day, whereas the insolation can be much lower in temperate areas where the days are shorter in winter, e.g. 3.2 kW.h/m2 per day. Even at the same latitude the average insolation can vary a great deal from location to location due to differences in local weather patterns and the amount of overcast. Useful calculators for estimating insolation at a site can be found with the Joint Research Laboratory of the European Commission^[32] and the American National Renewable Energy Laboratory.^[33][34]

Below is a table that gives a rough indication of the specifications and energy that could be expected from a solar water heating system involving some 2 m² of absorber area of the collector, demonstrating two evacuated tube and three flat plate solar water heating systems. Certification information or figures calculated from those data are used. The bottom two rows give estimates for daily energy production (kW.h/day) for a tropical and a temperate scenario. These estimates are for heating water to 50 °C above ambient temperature.

With most solar water heating systems, the energy output scales linearly with the surface area of the absorbers. Therefore, when comparing figures, take into account the absorber area of the collector because collectors with less absorber area yield less heat, even within the 2 m² range. Specifications for many complete solar water heating systems and separate solar collectors can be found at Internet site of the SRCC.^[35]

The figures are fairly similar between the above collectors, yielding some 4 kW.h/day in a temperate climate and some 8 kW.h/day in a more tropical climate when using a collector with an absorber area of about 2m² in size. In the temperate scenario this is sufficient to heat 200 litres of water by some 17 °C. In the tropical scenario the equivalent heating would be by some 33 °C. Many thermosiphon systems are quite efficient and have comparable energy output to equivalent active systems. The efficiency of evacuated tube collectors is somewhat lower than for flat plate collectors because the absorbers are narrower than the tubes and the tubes have space between them, resulting in a significantly larger percentage of inactive overall collector area. Some methods of comparison^[41] calculate the efficiency of evacuated tube collectors based on the actual absorber area and not on the 'roof area' of the system as has been done in the above table. The efficiency of the collectors becomes lower if one demands water with a very high temperature.

System cost[link]

In sunny, warm locations, where freeze protection is not necessary, an ICS (batch type) solar water heater can be extremely cost effective.^[42] In higher latitudes, there are often additional design requirements for cold weather, which add to system complexity. This has the effect of increasing the initial cost (but not the life-cycle cost) of a solar water heating system, to a level much higher than a comparable hot water heater of the conventional type. The biggest single consideration is therefore the large initial financial outlay of solar water heating systems.^[43] Offsetting this expense can take several years^[44] and the payback period is longer in temperate environments where the insolation is less intense.^[45] When calculating the total cost to own and operate, a proper analysis will consider that solar energy is free, thus greatly reducing the operating costs, whereas other energy sources, such as gas and electricity, can be quite expensive over time. Thus, when the initial costs of a solar system are properly financed and compared with energy costs, then in many cases the total monthly cost of solar heat can be less than other more conventional types of hot water heaters (also in conjunction with an existing hot water heater). At higher latitudes, solar heaters may be less effective due to lower solar energy, possibly requiring larger and/or dual-heating systems.^[46] In addition, federal and local incentives can be significant.

The calculation of long term cost and payback period for a household SWH system depends on a number of factors. Some of these are:

• Price of purchasing solar water heater (more complex systems are more expensive)
• Efficiency of SWH system purchased
• Installation cost
• Price of electricity use for mains pumping (if this is used)
• Price of water heating fuel (e.g. gas or electricity) saved per kW.h
• Amount of water heating fuel used per month by a household
• Upfront state or government subsidy for installation of a solar water heater
• Recurrent or annual tax rebates or subsidy for operating renewable energy
• Annual maintenance cost of SWH system (e.g. antifreeze or pump replacements)
• Savings in annual maintenenance of conventional (electric/gas/oil) water heating system

The following table gives some idea of the cost and payback period to recover the costs. It does not take into account annual maintenance costs, annual tax rebates and installation costs. However, the table does give an indication of the total cost and the order of magnitude of the payback period. The table assumes an energy savings of 200 kW.h per month (about 6.57 kW.h/day) due to SWH. Unfortunately payback times can vary greatly due to regional sun, extra cost due to frost protection needs of collectors, household hot water use etc. so more information may be needed to get accurate estimates for individual households and regions. For instance in central and southern Florida the payback period could easily be 7 years or less rather than the 12.6 years indicated on the chart for the US.^[47]

Costs and payback periods for residential SWH systems with savings of 200 kW.h/month (using 2010 data)
Country	Currency	System cost	Subsidy(%)	Effective cost	Electricity cost/kW.h	Electricity savings/month	Payback period(y)
Brazil	Real	2500[48]	0	2500	0.25	50	4.2
South Africa	ZA Rand	14000	15[49]	11900	0.9	180	5.5
Australia	$Aus	5000[50]	40[51]	3000	0.18[52]	36	6.9
Belgium	Euro	4000[53]	50[54]	2000	0.1[55]	20	8.3
USA	US$	5000[56]	30[57]	3500	$0.1158[58]	$23.16	12.6
UK	UK Pound	4800[59]	0	4800	0.11[60]	22	18.2

Two points are clear from the above table. Firstly, the payback period is shorter in countries with a large amount of insolation and even in parts of the same country with more insolation. This is evident from the payback period less than 10 years in most southern hemisphere countries, listed above. This is partly because of good sunshine, allowing users in those countries to need smaller systems than in temperate areas. Secondly, even in the northern hemisphere countries where payback periods are often longer than 10 years, solar water heating is financially extremely efficient. This is partly because the SWH technology is efficient in capturing irradiation. The payback period for photovoltaic systems is much longer.^[61] In many cases the payback period for a SWH system is shortened if it supplies all or nearly all of the warm water requirements used by a household. Many SWH systems supply only a fraction of warm water needs and are augmented by gas or electric heating on a daily basis,^[44] thus extending the payback period of such a system.

Solar leasing is now available in Spain for solar water heating systems from Pretasol^[62] with a typical system costing around 59 euros and rising to 99 euros per month for a system that would provide sufficient hot water for a typical family home of six persons. The payback period would be five years.

Australia has instituted a system of Renewable Energy Credits, based on national renewable energy targets. This expands an older system based only on rebates.^[51]

Operational carbon/energy footprint and life cycle assessment[link]

Terminology[link]

• Operational energy footprint (OEF) is also called energy parasitics ratio (EPR) or coefficient of performance (CoP).
• Operational carbon footprint (OCF) is also called carbon clawback ratio (CCR).
• Life cycle assessment is usually referred to as LCA.

Carbon/energy footprint[link]

The source of electricity in an active SWH system determines the extent to which a system contributes to atmospheric carbon during operation. Active solar thermal systems that use mains electricity to pump the fluid through the panels are called 'low carbon solar'. In most systems the pumping cancels the energy savings by about 8% and the carbon savings of the solar by about 20%.^[63] However, some new low power pumps will start operation with 1W and use a maximum of 20W.^[64][65] Assuming a solar collector panel delivering 4 kW.h/day and a pump running intermittently from mains electricity for a total of 6 hours during a 12-hour sunny day, the potentially negative effect of such a pump can be reduced to about 3% of the total power produced.

The carbon footprint of such household systems varies substantially, depending on whether electricity or other fuels such as natural gas are being displaced by the use of solar. Except where a high proportion of electricity is already generated by non-fossil fuel means, natural gas, a common water heating fuel, in many countries, has typically only about 40% of the carbon intensity of mains electricity per unit of energy delivered. Therefore the 3% or 8% energy clawback in a gas home referred to above could therefore be considered 8% to 20% carbon clawback, a very low figure compared to technologies such as heat pumps.

However, PV-powered active solar thermal systems typically use a 5–30 W PV panel which faces in the same direction as the main solar heating panel and a small, low power diaphragm pump or centrifugal pump to circulate the water. This reduces the operational carbon and energy footprint: a growing design goal for solar thermal systems.

Work is also taking place in a number of parts of the world on developing alternative non-electrical pumping systems. These are generally based on thermal expansion and phase changes of liquids and gases, a variety of which are under development.

Life cycle carbon/energy assessment[link]

Now looking at a wider picture than just the operational environmental impacts, recognised standards can be used to deliver robust and quantitative life cycle assessment (LCA). LCA takes into account the total environmental cost of acquisition of raw materials, manufacturing, transport, using, servicing and disposing of the equipment. There are several aspects to such an assessment, including:

• The financial costs and gains incurred during the life of the equipment.
• The energy used during each of the above stages.
• The CO₂ emissions due to each of the above stages.

Each of these aspects may present different trends with respect to a specific SWH device.

Financial assessment. The table in the previous section as well as several other studies suggest that the cost of production is gained during the first 5–12 years of use of the equipment, depending on the insolation, with cost efficiency increasing as the insolation does.^[66]

In terms of energy, some 60% of the materials of a SWH system goes into the tank, with some 30% towards the collector^[67] (thermosiphon flat plate in this case) (Tsiligiridis et al.). In Italy,^[68] some 11 GJ of electricity are used in producing the equipment, with about 35% of the energy going towards the manufacturing the tank, with another 35% towards the collector and the main energy-related impact being emissions. The energy used in manufacturing is recovered within the first two to three years of use of the SWH system through heat captured by the equipment a this southern European study.

Moving further north into colder, less sunny climates, the energy payback time of a solar water heating system in a UK climate is reported as only 2 years.^[69] This figure was derived from the studied solar water heating system being: direct, retrofitted to an existing water store, PV pumped, freeze tolerant and of 2.8 sqm aperture. For comparison, a solar electric (PV) installation took around 5 years to reach energy payback, according to the same comparative study.

In terms of CO₂ emissions, a large degree of the emissions-saving traits of a SWH system is dependent on the degree to which water heating by gas or electricity is used to supplement solar heating of water. Using the Eco-indicator 99 points system as a yardstick (i.e. the yearly environmental load of an average European inhabitant) in Greece,^[67] a purely gas-driven system may be cheaper in terms of emissions than a solar system. This calculation assumes that the solar system produces about half of the hot water requirements of a household. The production of a test SWH system in Italy^[68] produced about 700 kg of CO₂, with all the components of manufacture, use and disposal contributing small parts towards this. Maintenance was identified as an emissions-costly activity when the heat transfer fluid (Glycol-based) was periodically replaced. However, the emissions cost was recovered within about two years of use of the equipment through the emissions saved by solar water heating. In Australia,^[70] the life cycle emissions of a SWH system are also recovered fairly rapidly, where a SWH system has about 20% of the impact of an electrical water heater and half of the emissions impact of a gas water heater.

Analysing their lower impact retrofit freeze-tolerant solar water heating system, Allen et al. (qv) report a production CO₂ impact of 337 kg, which is around half the environmental impact reported in the Ardente et al. (qv) study.

Where information based on established standards are available, the environmental transparency afforded by life cycle analysis allows consumers (of all products) to make increasingly well-informed product selection decisions. As for identifying sectors where this information is likely to appear first, environmental technology suppliers in the microgeneration and renewable energy technology arena are increasingly being pressed by consumers to report typical CoP and LCA figures for their products.

In summary, the energy and emissions cost of a SWH system forms a small part of the life cycle cost and can be recovered fairly rapidly during use of the equipment. Their environmental impacts can be reduced further by sustainable materials sourcing, using non-mains circulation, by reusing existing hot water stores and, in cold climates, by eliminating antifreeze replacement visits.

Do-it-yourself (DIY) systems[link]

People have begun building their own (small-scale) solar water heating systems from scratch or buying kits. Plans for solar water heating systems are available on the Internet.^[71] and people have set about building them for their own domestic requirements. DIY SWH systems are usually cheaper than commercial ones, and they are used both in the developed and developing world.^[72]

System specification and installation[link]

• Except in rare instances it will be insufficient to install a SWH system with no electrical or gas or other fuel backup. Many SWH systems have a back-up electric heating element in the integrated tank, the operation of which may be necessary on cloudy days to ensure a reliable supply of hot water.
• The temperature stability of a system is dependent on the ratio of the volume of warm water used per day as a fraction of the size of the water reservoir/tank that stores the hot water. If a large proportion of hot water in the reservoir is used each day, a large fraction of the water in the reservoir needs to be heated. This brings about significant fluctuations in water temperature every day, with possible risks of overheating or underheating, depending on the design of the system. Since the amount of heating that needs to take place every day is proportional to hot water usage and not to the size of the reservoir, it is desirable to have a fairly large reservoir (i.e. equal to or greater than daily usage,) which will help prevent fluctuations in water temperature.
• If ample storage is pre-existing or can otherwise be reasonably acquired, a large SWH system is more efficient economically than a small system.^[67] This is because the price of a system is not linearly proportional to the size of the collector array, so the price per square meter of collector is cheaper in a larger system. If this is the case, it pays to use a system that covers nearly all of the domestic hot water needs, and not only a small fraction of the needs. This facilitates more rapid cost recovery.
• Not all installations require new replacement solar hot water stores. Existing stores may be large enough and in suitable condition. Direct systems can be retrofitted to existing stores while indirect systems can be also sometimes be retrofitted using internal and external heat exchangers.
• The installation of a SWH system needs to be complemented with efficient insulation of all the water pipes connecting the collector and the water storage tank, as well as the storage tank (or "geyser") and the most important warm water outlets. The installation of efficient lagging significantly reduces the heat loss from the hot water system. The installation of lagging on at least two meters of pipe on the cold water inlet of the storage tank reduces heat loss, as does the installation of a "geyser blanket" around the storage tank (if inside a roof). In cold climates the installation of lagging and insulation is often performed even in the absence of a SWH system.
• The most efficient PV pumps are designed to start very slowly in very low light levels, so if connected uncontrolled, they may cause a small amount of unwanted circulation early in the morning – for example when there is enough light to drive the pump but while the collector is still cold. To eliminate the risk of hot water in the storage tank from being cooled that way this is very important. solar controller may be required.
• The modularity of an evacuated tube collector array allows the adjustment of the collector size by removing some tubes or their heat pipes. Budgeting for a larger than required array of tubes therefore allows for the customisation of collector size to the needs of a particular application, especially in warmer climates.
• Particularly in locations further towards the poles than 45 degrees from the equator, roof mounted sun facing collectors tend to outperform wall mounted collectors in terms of total energy output. However, it is total useful energy output which usually matters most to consumers. So arrays of sunny wall mounted steep collectors can sometimes produce more useful energy because there can be a small increase in winter gain at the expense of a large unused summer surplus.

Standards[link]

Europe[link]

• EN 806: Specifications for installations inside buildings conveying water for human consumption. General.
• EN 1717: Protection against pollution of potable water in water installations and general requerements of devices to prevent pollution by backflow.
• EN 60335: Specification for safety of household and similar electrical appliances. (2–21)
• UNE 94002:2005 Thermal solar systems for domestic hot water production. Calculation method for heat demand.

APPENDIX 1. Worldwide use[link]

Top countries worldwide[link]

Solar hot water system installed on low cost housing in the Kouga Local Municipality, South Africa

Top countries using solar thermal power, worldwide: GW_th^{[73][74][75][76][77]}

#	Country	2005	2006	2007	2008	2009
1	People's Republic of China	55.5	67.9	84.0	105.0	101.5
–	EU	11.2	13.5	15.5	20.0	22.8
2	United States	1.6	1.8	1.7	2.0	14.4
3	Germany	–	–	–	7.8	8.9
4	Turkey	5.7	6.6	7.1	7.5	8.4
5	Australia	1.2	1.3	1.2	1.3	5.
6	Japan	5.0	4.7	4.9	4.1	4.3
7	Brazil	1.6	2.2	2.5	2.4	3.7
8	Austria	–	–	–	2.5	3.0
9	Greece	–	–	–	2.7	2.9
10	Israel	3.3	3.8	3.5	2.6	2.8
	World (GWth)	88	105	126	149	172

Solar heating in European Union + CH[link]

Solar thermal heating in European Union (MW_th)^[78][79][80]

#	Country	2008	2009	2010[76]
1	Germany	7,766	8,896	9,677
2	Greece	2,708	2, 852	2,859
3	Austria	2,268	2,518	2,686
4	Italy	1,124	1,404	1,870
5	Spain	988	1,262	1,475
6	France	1,137	1,371F	1,102
7	Switzerland	416	538	627
8	Cyprus	485	515	501
9	Portugal	223	345	471
10	Poland	256	357	459
11	UK	270	333	401
12	Denmark	293	331	368
13	Netherlands	254	285	313
14	Belgium	188	204	230
15	Sweden	202	217	227
16	Czech Republic	116	148	216
17	Slovenia	96	112	123
18	Hungary	18	58	105
19	Ireland	52	75	92
20	Slovakia	67	76	85
21	Bulgaria *	22	90	74
22	Romania *	66	80	73
23	Malta*	25	29	32
24	Finland *	18	19	23
25	Luxembourg *	16	19	22
26	Estonia*	1	2	2
27	Lithuania *	3	2	2
28	Latvia *	5	1	1
Total	EU27+Sw (MWth)	19,083	22,137	24,114
* = estimation, F = France as a whole

By country[link]

• Australia: Solar hot water in Australia

See also[link]

	Renewable energy portal
	Energy portal

Wikimedia Commons has media related to: Solar hot water panels

• Solar thermal collector
• Solar air heating
• Solar air conditioning
• Concentrating solar power
• Passive solar
• Renewable heat
• Seasonal thermal store
• Solar combisystem
• Solar energy
• Solar pool panels
• Solar thermal energy
• Sustainable design

References[link]

External links[link]

• Parts of a solar heating system

v t e Renewable energy by country Wind by region Asia · Europe (EU) by country Australia · Austria · Belgium · Brazil · Canada · China · Croatia · Denmark · Estonia · Finland · France · Germany · Greece · India · Iran · Ireland · Italy · Japan · Malta  · Morocco  · New Zealand · Pakistan · Portugal · Romania · Russia · Serbia  · Spain · Sweden · Turkey · United Kingdom · United States Solar by region Asia · Europe (EU) by country Australia · Canada · China · Czech · Germany · Greece · India · Israel · Italy · Japan · Mexico · Morocco · Pakistan · Portugal · Romania · South Africa · Spain · Turkey · Ukraine · United Kingdom · United States Geothermal by country Australia · Canada · Chile · China · Denmark · Germany · Iceland · Indonesia · Italy · Japan · Kenya · Mexico · New Zealand · Philippines · Portugal · Romania · Russia · Turkey · United Kingdom · United States · West Indies Portals: Energy · Renewable energy · Sustainable development

This article includes a list of references, but its sources remain unclear because it has insufficient inline citations. Please help to improve this article by introducing more precise citations. (April 2009)

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 temperature. Rarely, hot water will be naturally occurring, usually from natural hot springs. The temperature will vary based on the consumption rate of hot water; the water becomes cooler as flow is increased.

Appliances for providing a more-or-less constant supply of hot water are variously known as water heaters, hot water heaters, hot water tanks, 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 also called 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 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 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 in densely-populated urban areas. 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 has no in-building backup system, due to the very high expected availability of district heating systems.

Types of water heating appliance[link]

Electric tank-type storage water heater (US)

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]

Gravity-fed system[link]

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 or container, usually in the roof space of the building. This produces a fairly steady supply of DHW (Domestic Hot Water) at low static pressure head but usually with a good flow. In most other parts of the world, water heating appliances do not use a cold water storage vessel or container, but heat water at pressures close to that of the incoming mains water supply.

On-demand water heaters (tankless)[link]

Stand-alone appliances for quickly heating water for DHW are known in North America as tankless water heaters, elsewhere as multipoint heaters, geysers or Ascots. In Australia and New Zealand they are called Instantaneous Hot Water Units. 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 on-demand 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 (tank-type)[link]

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. 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 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 known as unvented cylinders. In the US, when connected to a boiler they are known as indirect-fired water heaters.

Point-of-use (POU) vs. Centralized hot water[link]

A locational design decision may be made between point-of-use and centralized hot water heaters. Centralized water heaters are more traditional, and are still a good choice for small buildings. For larger buildings with intermittent or occasional hot water use, multiple POU water heaters may be a better choice, since they can reduce long waits for hot water to arrive from a remote heater. The decision where to locate the water heater(s) is only partially independent of the decisions to use a tanked vs. tankless water heater, or the choice of energy source for the heat.

History[link]

Display of water heaters used in the past.

Although not very popular in North America, 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 which did not utilize solid fuel".^[2] Named "the geyser" after an Icelandic gushing hot spring, Maughan's invention had cold water at the top flowing through wires which were heated by hot gases from a burner at the bottom. Hot water then flowed into the 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. The terms electric water boiler, electric dispensing pot or electric water urn are also commonly used there.

Maughn's invention influenced the work of a Norwegian mechanical engineer named Edwin Ruud. The first automatic, storage tank-type gas water was invented around 1889 by Ruud after he immigrated to Pittsburgh, Pennsylvania. The Ruud Manufacturing Company, still in existence today, made many advancements in tank-type and tankless water heater design and operation.^[2][3]

Thermodynamics and economics[link]

Gas-fired condensing boiler with indirect tankless water heater (US)

Water enters residences in the US at about 10 °C (50 °F) (varies with latitude and season). Hot water temperatures of 40–49 °C (104–120 °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 US gallons (9.5 L) per minute; sink and dishwasher usages range from 1–3 US gallons (3.8–11 L) per minute.

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 (BTU). 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 (3.8 kg). So, to raise a 40-gallon tank of 55 °F (13 °C) water up to 105 °F (41 °C) would require (40 × 8.3 × (105 − 55) / 100,000) BTU, or approximately 0.17 CCF, at 100% efficiency. A 40,000 BTU/h 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 °F (40 °C) is equivalent to operating a 19.8 kW appliance [ ref. w computes 13.2 kW, but that is for 20 degree C increase instead of 30 ].^[4] 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 efficiently 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. Natural gas and propane tank water heaters with energy factors of 62% or greater, as well as electric tank water heaters with energy factors of 93% or greater, are considered high-efficiency units. Energy Star-qualified natural gas and propane tank water heaters have energy factors of 67% or higher; direct electric tank water heaters are not included in the Energy Star program, however, the Energy Star program does include electric heat pump units with energy factors of 200% or higher. Tankless gas water heaters must have an energy factor of 82% or higher for Energy Star qualification. Since electricity production itself today has efficiency levels ranging from only 15% to slightly over 55% (combined cycle gas turbine), with around 40% typical for thermal power stations, direct electric water heating is typically the least energy efficient option. However, use of a heat pump can make electric water heaters much more energy efficient and lead to a decrease in carbon dioxide emissions, even more so if a renewable source of electricity is used.

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 30 °C (54.0 °F). The same unit could supply 1.3 gpm while raising the temperature by 33 °C (59.4 °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 waiting to boil a gallon of water on a stove. For this reason, tankless on-demand water heaters need to have a very powerful source to be usable. A standard 15-ampere rated wall electric outlet, by comparison, can only source enough power to warm a disappointingly small amount of water: about 0.17 gpm at 40 °C temperature elevation.

Storage water heaters (tank-type)[link]

Gas furnace (top) and storage water heater (bottom) (Germany)

In household and commercial usage, most water heaters in North America have traditionally been of the tank type. Also called storage water heaters, these consist of a cylindrical vessel or 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 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.

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.

Volume 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. Volume storage water heaters 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 Australia, gas and electric outdoor tank heaters have mainly been used (with high temperatures to increase effective capacity), but solar roof tanks are becoming fashionable.

In temperate zone countries, where ambient temperature are seasonally colder, tiny point-of-use (POU) 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 may be used when retrofitting a pump and recirculating plumbing in a building is too costly or impractical. Since they maintain water temperature thermostatically, they can only supply a continuous flow of hot water at extremely low flow rates, unlike high-capacity tankless heaters.

In tropical countries, like Singapore and India, a storage water heater may vary from 10 L to 35 L. Smaller water heaters are sufficient, as ambient weather temperatures and incoming water temperature are moderate.

Water tank leakage[link]

Water heater tanks may be made of vitreous enamel-lined carbon steel, stainless steel, or copper. Vitreous-lined tanks are much lower in initial cost, and often include one or more sacrificial anode rods designed to protect the tank from perforation caused by corrosion.^[5] Such protection is necessary since chlorinated hot water is very corrosive to carbon steel, and it is nearly impossible to apply any protective coating perfectly, without small cracks or pinhole defects in the protective layer.^[6] Manufacturers may recommend checking the sacrificial anode rod periodically and replacing it when necessary; some manufacturers offer an extended warranty kit that includes an additional anode rod. Common practice in the US is to neglect this maintenance until the tank develops a leak, and then to replace the entire appliance.^[7] Even when neglected, carbon steel tanks tend to last for a few years more than their manufacturer's warranty, which is typically 3 to 12 years in the US.

Because conventional hot water storage tanks can be expected to leak every 5 to 15 years, high-quality installations will include, and most US building/plumbing codes now require, a shallow metal or plastic pan to collect the inevitable seepage when it occurs. Such a pan should be connected to a drain, or plumbed to discharge directly outdoors. When this is difficult or impossible, an alternative precaution to reduce flood damage is to place some sort of water alarm on the floor near the water tank, possibly connected to an automatic water shutoff valve.

Water heater tanks should be installed with physical access in mind, both for inspection and replacement of the long anode rod, and for the eventual replacement of the entire tank.^[8]

Insulation and other improvements[link]

In general, the more thermal insulation the better, since it reduces standby heat loss. Water heaters 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 water heater 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 polyurethane foam (PUF) insulation. In countries where serviceability is very important, PUF capsules are kept between the inner tank and the outer body.^{[clarification needed]}

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 UK, the space-heating boiler is set up to heat a separate hot water cylinder or water heater for potable hot water. Such water heaters are often fitted with an auxiliary electrical immersion heater for use if the boiler is out of action for a time. Heat from the space-heating boiler is transferred to the water heater vessel/container 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 North America are completely separate from the space heating units, due to the popularity of HVAC/forced air systems in North America.

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 storing, or spilling, gasoline or other flammable liquids 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 (NZ), wetback heater (NZ), or back boiler (UK), is 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. These designs are being forced out by government efficiency regulations which do not count the energy used to heat water as being 'efficiently' used.^[9]

Electric water heating[link]

Electric tankless water heater (Japan)

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. Electric heaters may be tubular or cartridge type.^{[further explanation needed]}

Water heaters that have residual hot water storage in a vessel/container 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[link]

The inside of a hydraulically-operated two-stage tankless heater, heated by single-phase electric power. The copper tank contains heating elements with 18kW maximum power.

Tankless water heaters, also called instantaneous, continuous flow, inline, flash, on-demand, or instant-on water heaters, are also available and gaining in popularity. These high-power water heaters instantly heat water as it flows through the device, and do not retain any water internally except for what is in the heat exchanger coil.

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

How tankless water heaters work[link]

When there is a demand for hot water (e.g. a hot water tap is opened for a sink, shower, tub, or washing machine) the tankless water heater's water flow turbine senses the flow and starts the heating process. The water flow turbine sends a signal to the control board which looks at multiple factors: incoming water temperature, desired water temperature as set on the temperature controller, and the calculated difference between the two temperatures. Depending on the calculated incoming and desired water temperatures, the gas or electric flow into the burner assembly is modulated and the electronic ignition sequence begins. Water is heated to the desired temperature as it circulates through the copper heat exchanger providing continuous hot water. When the hot water tap is turned off, the tankless water heater shuts down and is placed in a standby mode pending the next call for hot water.^[10][11]

Combination boilers[link]

Combination or combi boilers, combine the central heating (CH) with (tankless) domestic hot water (DHW) in one box. They are not merely infinitely continuous water heaters having the ability to heat a hydronic heating system in a large house. When DHW is run off, the combi stops pumping water to the hydronic circuit and diverts all the boiler's power to instantly heating DHW. Some combis have small internal water storage vessels combining the energy of the stored water and the gas or oil burner to give faster DHW at the taps or increase the DHW flowrate.

Combi boilers are rated by the DHW flowrate. The kW ratings for domestic units are 24 kW to 54 kW, giving approximate flowrates of 9 to 23 litres (2.4 to 6.1 US gal) per minute. There are larger commercial units available. High flowrate models will simultaneously supply two showers.

A further advantage is that more than one combi unit may be used to supply separate heating zones, giving greater time and temperature control, and multiple bathrooms. An example is one combi supplying the downstairs heating system and another the upstairs. One unit may supply one bathroom and one another. Having two units gives backup in case one combi is down, provided the 2 systems are connected with valves that are normally closed.

Installation cost is significantly lower and less space is required as water tanks and associated pipes and controls are not required.

Combi boilers are highly popular in Europe, where in some countries market share is 70%.^{[citation needed]}

Combination boilers have disadvantages. The water flow rate is likely to be less than from a storage cylinder, particularly in winter. The power rating needs to be matched to heating requirements; heating water ‘on demand’ improves energy efficiency but limits the volume of water available at any moment. The water supply pressure must not be too low. It has been proposed that a flow regulator valve can control the amount of water used. Additionally, a combination boiler has more moving parts that can break down, so can be less reliable than a tank system.^[12]

Point-of-use (POU) tankless types[link]

Electric point-of-use (POU) tankless water heater (Britain)

Point-of-use (POU) tankless water heaters are located immediately where the water is being used, so the water is almost instantly hot, which reduces water wastage. POU tankless heaters also can save more energy than centrally installed tankless water heaters, because no hot water is left in lengthy supply pipes after the flow is shut off. However, POU tankless water heaters are often installed in combination with a central water heater, since the former type have usually been limited to under 6 litres/minute (1.5 US gallons/minute), which is sufficient for only light usage. In many situations, the initial expense of buying and installing a separate POU heater for every kitchen, laundry room, bathroom, and sink can outweigh the money saved in water and energy bills. In the US, POU water heaters until recently were almost always electrical^{[citation needed]}, and electricity is often substantially more expensive than natural gas or propane (when the latter fuels are available).

In recent years, higher-capacity tankless heaters have become more widely available, but their feasibility may still be limited by the infrastructure's ability to furnish energy (maximum electrical amperage or gas flow rate) fast enough to meet peak hot water demand. In the past, tank-type water heaters have been used to compensate for lower energy delivery capacities, and they are still useful when the energy infrastructure may have a limited capacity, often reflected in peak demand energy surcharges.

In theory, tankless heaters can always be somewhat more efficient than storage tank water heaters. In both kinds of installation (centralized and POU), the absence of a tank saves energy compared to conventional tank-type water heaters, which have to reheat the water in the tank as it cools off while waiting for use (this is called "standby loss"). In some installations, the energy lost by a tanked heater located inside a building merely helps to heat the occupied space. This is true for an electric unit, but for a gas unit some of this lost energy leaves through the exhaust vent. However, if at any time the building needs to be cooled to maintain comfortable temperatures, the heat lost from a hot water tank located in the conditioned space must be removed by the air conditioning system, thus requiring larger cooling capacity and energy usage.

With a central water heater of any type, any cold water standing in the pipes between the heater and the point-of-use is dumped down the drain as hot water travels from the heater. This water wastage can be avoided if a recirculator pump is installed, but at the cost of the energy to run the pump, plus the energy to reheat the water recirculated through the pipes. Some recirculating systems reduce standby loss by operating only when hot water is expected to be needed, at the expense of greater system complexity.

Tankless heater controls[link]

Electric point-of-use (POU) tankless water heater, wall-mounted under a sink (Germany)

Tankless water heaters can be further divided into two categories according to their heating capability: "full on/full off" versus "modulated". Full on/full off units do not have a variable power output level; the unit is either fully on or completely off. This can cause an annoying and possibly hazardous variation of hot water temperature, as the flow of water through the heater varies. Modulated tankless water heaters change their heat output in response to the flow rate of water running through the unit. This is usually done by using a flow sensor, a modulating gas valve, an inlet water temperature sensor, and an outlet water temperature sensor-choke valve.^{[clarification needed]} A properly-configured modulating heater can supply the same output temperature of water at differing water flow rates within their rated capacity, usually maintaining a close range of ±2°C.

A high-efficiency condensing combination boiler^[13] provides both space heating and water heating, and is 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.

Under current North American conditions, the most cost-effective configuration from an operating viewpoint often is to install a central (tank-type or tankless) water heater for most of the house, and to install a POU tankless water heater at any distant faucets or bathrooms. However, the most economic design may vary according to the relative electricity, gas and water prices in the locality, the layout of the building, and how much (and when) hot water is used. Only electric tankless water heaters were widely available for many years. and they are still used for low-initial-cost POU heaters, but natural gas and propane POU heaters have now become available for consideration.

Tankless advantages[link]

Mobile water heater in a train car (Russia)

• Long term energy savings: Although a tankless water heater might cost more initially it may result in both energy and cost savings in the long term. As water is heated only when it is needed, there is no storage of hot water. With a tank, water is kept warm all day even if it never gets used and heat loss through the tank walls will result in a continual energy drain. Even in homes or buildings with a high demand for hot water, a tankless water heater may provide some level of savings. In a typical home these savings are quite substantial. If instant hot water at the taps at limited hours is a priority, a recirculation system similar to those in the tank-type systems can be accommodated by using an aquastat and timer in order to decrease the added heat loss from the recirculation system. It has to be said though that if the storage tank is highly-insulated—a few tanks are available with excellent levels such as 100 mm or more polyurethane foam—the savings become minimal. For one consumer-grade electric storage water heater, the surface temperature was less than 1 °C higher than the air temperature.^{[citation needed]}
• Unlimited hot water: As water is heated while passing through the system an unlimited supply of hot water is available with a tankless water heater. Although flow rate will determine the amount of hot water that can be generated at one time it can be generated indefinitely. However, this can also be a disadvantage as running out of hot water self-limits use while a tankless heater has no such limit.
• Less physical space: Most tankless water heaters can be mounted on a wall or even internally in a building's structure. This means less physical space has to be dedicated to heating water. Even systems that can't be mounted on walls take up less space than a tank-type water heater.
• Reduced risk of water damage: No stored water means there is no risk of water damage from a tank failure or rupture, although the risk of water damage from a pipe or fitting failure remains. Improper piping in either the hot or cold water lines to the tankless water heater can result in water damage though.^{[citation needed]}
• Temperature compensation A temperature compensating valve tends to eliminate the issue where the temperature and pressure from tankless heaters decrease during continuous use. Most new generation tankless water heaters stabilize water pressure and temperature by a bypass valve and a mixing valve which is incorporated in the unit. Modern Tankless are not inversely proportional, because they will regulate the amount of water that is created and discharged, therefore stabilizing water temperature by utilizing a flow control valve. Flow speed is not the issue, but change in temperature is the important issue to address. The wider the temperature rise, the less flow you receive from the unit. The smaller the temperature rise, the more flow you receive. The flow control valve in conjunction with thermistors, maintains a stable temperature throughout the use of the unit.
• Safety Tankless Water Heaters can precisely control the temperature of the treated water, which means dangerous temperature levels and spikes are no longer a problem.^[14]

Tankless disadvantages[link]

Gas tankless water heater (Japan)

• Startup costs: Beyond the 2x-4x larger initial purchase price (as compared to a tanked water heater), installing a tankless system comes at an increased cost, particularly in retrofit 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, 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.
• Start-up delay: There is a longer wait to obtain hot water. A tankless water heater only heats water upon demand, which is one of its chief advantages, 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 (in non-point-of-use systems). Many models sold in the UK have introduced a small heat store within the combi. to address this issue. This "keep hot" facility considerably improves the standard of hot water service, which some people otherwise find unacceptably poor with a combi., but it uses considerably more fuel especially in summer.
• Intermittent-use: There is a short delay (1–3 seconds) 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 (i.e. showers, baths, washing machine) this is not an issue as the heater never stops heating. However, for intermittent-use applications (i.e. turning off/on a hot water faucet at a sink) this can result in initially hot water flowing, followed by a small amount of cold water as the heater re-activates, followed again by hot water. The user experience is that after initially getting hot water flowing, the user turns off the valve and then a short time later turns the valve back on again. Hot water starts flowing once again at the valve from the hot water already in the piping, but at the same time the heater has let some amount of cold water into the piping during its reactivation time. Some time later (depending on the length of piping from the tank to the valve) this cold section of water arrives at the sink, followed shortly thereafter by hot water again. The initial thought of the user can be that the heater is failing intermitantly.
• Heat source flexibility Tankless heaters use either gas and electricity for heat. Their design may not conveniently allow other heat sources, including certain renewable energy options. One exception is solar water heating, which can be used in conjuncion with tankless water heaters. Tank-type systems can use many heat sources such as district heating, central heating, geothermal heating, micro CHP and ground-coupled heat exchangers.
• Recirculation systems: 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. On-demand recirculating pumps are often used to minimize hot water wait times from tankless water heaters and save water being wasted down the drain. On-demand recirculating pumps are activated by push-button or other sensor. A water contacting temperature probe installed at the hot water usage point signals the pump to stop. Single-cycle pumping events only occur when hot water is needed thereby preventing the energy waste associated with constantly heating water within piping.
• Achieving cooler temperatures: 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.
• Maintaining constant shower temperature: Similarly, unlike with a tank heater, the hot water temperature from a non-modulated 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.
• Operation with low supply pressure: Tankless systems are reliant on the water pressure that is delivered to the property. In other words, if a tankless system is used to deliver water to a shower or water faucet, the pressure is the same as the pressure delivered to the property and cannot be increased, whereas in tanked systems the tanks can be positioned above the water outlets (in the loft/attic space for example) so the force of gravity can assist in delivering the water, and pumps can be added into the system to increase pressure. Power showers, for example, cannot be used with tankless systems because the tankless systems cannot deliver the hot water at a fast enough flow rate required by the pump.
• Time-of-use metering and peak electrical loads: Tankless electric heaters, if installed in a large percentage of homes within an area, can create demand management problems for electrical utilities. Because these are high-current devices, and hot water use tends to peak at certain times of the day, their use can cause short spikes in electricity demand, including during the daily peak electrical load periods, which increases utility operating costs. For households using time-of-use metering (where electricity costs more during peak periods such as daytime, and is cheaper at night), a tankless electric heater may actually increase operating costs if the hot water is used during peak times.^[15] Instantaneous-type heaters are also problematic if they are connected to district heating systems, as they raise peak demands, and most utilities prefer all buildings to have hot water storage.

Electric shower heads[link]

A poorly-installed electric shower head can pose an electrical shock hazard (Guatemala)

As the name implies, an electric heating element is incorporated into such shower heads to instantly heat the water as it flows through. These self-heating shower heads are specialized point-of-use (POU) tankless water heaters, and are widely used in some countries.

Invented in Brazil in the 1930s and used frequently since the 40s, the electric shower is a home appliance often seen in South American countries due to the higher costs with gas distribution. Earlier models were made of chromed copper or brass, which were expensive, but since 1970, units made of injected plastics are popular due to low prices similar to that of a hair dryer. Electric showers have a simple electric system, working like a coffee maker, but with a larger water flow. A flow switch turns on the device when water flows through it. Once the water is stopped, the device turns off automatically. An ordinary electric shower often has three heat settings: low (2.5 kW), high (5.5 kW) or cold (0 W) to use when a central heater system is available or in hot seasons.^{[citation needed]}

Energy usage[link]

The power consumption of electric showers in the maximum heating setting is about 5.5 kW for 120 V and 7.5 kW for 220 V. The lower costs with electric showers compared to the higher costs with boilers is due to the time of use: an electric shower uses energy only during the water flow, while a boiler works many times a day to keep a larger quantity of standing water hot for use throughout the day and night. So electric showers may save energy compared to gas central heaters. A 20 minute bath by an electric shower can cost about US$0.10, but the same bath using water from a gas heater can cost three times as much. This difference can be larger where the electricity is cheaper than the gas supply, or in tropical countries where the maximum power consumption is required only during the cold seasons.

Safety[link]

There is a wide range of electric showers all with various amounts of heating controls. The heating element of an electric shower is made from a coil made of nickel or an alloy of nickel and chromium or can even be made of sheathed heater element, like the ones used in oil heaters, radiators or irons - they provide more safety as there is insulation between the electric parts and the water. Due to electrical safety standards, modern electric showers are made of plastic instead of using metallic casings like in the past. As an electrical appliance which works with higher electrical currents than a washer or a dryer machine, the installation of electric showers needs careful planning and must be made directly from the electrical distribution box, with exclusive 6 mm wires, electric connectors for 50 A and a ground system. A poorly installed system with old aluminum wires or bad connections is very dangerous, as the wires can overheat.

Some changes in electrical distribution utilities were required before widespread use of electric showers could became practical. Electric utility transformers with higher KVA capacity are required due to increase of peak electrical demand. In countries where almost all houses use electric showers like Brazil, an ordinary street transformer per square has 112.5 to 150 kVA of capacity, and buildings must have their own transformers to support the electrical domestic demand without overloads in the electric distribution.^{[clarification needed]}

Hybrid water heaters[link]

It has been suggested that Hybrid water heater be merged into this article or section. (Discuss) Proposed since February 2012.

A hybrid water heater is a water heating system that integrates technology traits from both the tank-type water heaters and the tankless water heaters.^[16]

The hybrid water heater can maintain water pressure and consistent supply of hot water across multiple hot water applications, and like its tankless cousins, the hybrid is efficient and can supply a continuous flow of hot water on demand.^[17]

Solar water heaters[link]

Direct-gain solar heater panels with integrated storage tank

Flat-plate solar thermal collector, viewed from roof-level

Increasingly, solar powered water heaters are being used. Their solar collectors are installed outside dwellings, typically on the roof or walls or nearby, and the potable hot water storage tank is typically a pre-existing or new conventional water heater, or a water heater specifically designed for solar thermal.

The most basic solar thermal models are the direct-gain type, in which the potable water is directly sent into the collector. Many such systems are said to use integrated collector storage (ICS), as direct-gain systems typically have storage integrated within the collector. Heating water directly is inherently more efficient than heating it indirectly via heat exchangers, but such systems offer very limited freeze protection (if any), can easily heat water to temperatures unsafe for domestic use, and ICS systems suffer from severe heat loss on cold nights and cold, cloudy days.

By contrast, indirect or closed-loop systems do not allow potable water through the panels, but rather pump a heat transfer fluid (either water or a water/antifreeze mix) through the panels. After collecting heat in the panels, the heat transfer fluid flows through a heat exchanger, transferring its heat to the potable hot water. When the panels are cooler than the storage tank or when the storage tank has already reached its maximum temperature, the controller in closed-loop systems will stop the circulation pumps. In a drainback system, the water drains into a storage tank contained in conditioned or semi-conditioned space, protected from freezing temperatures. With antifreeze systems, however, the pump must be run if the panel temperature gets too hot (to prevent degradation of the antifreeze) or too cold (to prevent the water/antifreeze mixture from freezing.)

Flat panel collectors are typically used in closed-loop systems. Flat panels, which often resemble skylights, are the most durable type of collector, and they also have the best performance for systems designed for temperatures within 100 degrees Fahrenheit of ambient temperature. Flat panels are regularly used in both pure water and antifreeze systems.

Another type of solar collector is the evacuated tube collector, which are intended for cold climates that do not experience severe hail and/or applications where high temperatures are needed (i.e. over 200 degrees Fahrenheit.) Placed in a rack, evacuated tube collectors form a row of glass tubes, each containing absorption fins attached to a central heat-conducting rod (copper or condensation-driven.) The evacuated description refers to the vacuum which is created within the glass tubes during the manufacturing process, which results in very low heat loss and allows evacuated tube systems to achieve extreme temperatures, far in excess of water's boiling point.

Geothermal heating[link]

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[link]

Explosion hazard[link]

Temperature/pressure safety valve installed atop a tank-type water heater (US)

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.^[18]

If a gas or propane fired 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, certain local codes mandate that tank-type heaters in new and retrofit installations must be secured to an adjacent wall by a strap or anchor to prevent tipping over and breaking the water and gas pipes in the event of an earthquake.^[19]

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.

Thermal burns (Scalding)[link]

Scalding injury to right hand

Scalding is a serious concern with any water heater. Human skin burns quickly at high temperature, in about 5 seconds at 60 °C/140 °F, but much slower at 53 °C/127 °F — it takes a full minute for a first degree burn. Older people and children often receive serious scalds due to disabilities or slow reaction times. In the United States 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 water 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 internal electric heating element for increasing the water temperature above that provided by water heaters. The result of mixing hot and cold water via a tempering valve is referred to as "tempered water".^[20]

Bacterial contamination[link]

Bacterial colonies of Legionella pneumophila (indicated by arrows)

There are two seemingly conflicting safety issues around water heater temperature—the risk of scalding from excessively hot water greater than 55 °C (131 °F), 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 54.4 °C (130 °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.^[21] If there is a dishwasher without a booster heater, it may require a water temperature within a range of 57 °C (134.6 °F) to 60 °C (140 °F) for optimum cleaning,^[22] 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 limescale deposits, which could later harbor bacteria, in the water tank. Temperatures above 60 °C may also cause gradual erosion of glassware in a dishwasher.)

In the renewable energy industry (solar and heat pumps, in particular) the conflict between daily thermal Legionella control and high temperatures, which may drop system performance, is subject to heated debate. In a paper seeking a green exemption from normal Legionellosis safety standards, Europe's top CEN solar thermal technical committee TC 312 asserts that a 50% fall in performance would occur if solar water heating systems were heated to the base daily. However some solar simulator analysis work using Polysun 5 suggests that an 11% energy penalty is a more likely figure. Whatever the context, both energy efficiency and scalding safety requirements push in the direction of considerably lower water temperatures than the legionella pasteurisation temperature of around 60°C.

However, legionella can be safely and easily controlled with good design and engineering protocols. For instance raising the temperature of water heaters once a day or even once every few days to 55 °C (131 °F) at the coldest part of the water heater for 30 minutes will effectively control legionella. In all cases and in particular energy efficient applications, Legionnaires' disease is more often than not the result of engineering design issues that do not take into consideration the impact of stratification or low flow.

See also[link]

• Aquastat
• Architectural engineering
• Condensing boiler
• Electric water boiler
• Energy conservation
• Heat Traps
• Hot water heat recycling
• Solar hot water
• Thermal immersion circulator

References[link]

External links[link]

• U.S. Department of Energy Water Heating Guidance
• How It Works - Water Heater
• Tankless Water Heaters are Brutal on the Grid When Popularized
• Showing Different Types of Immersion Heaters
• Estimate DHW Usage
• [1] Article on water saving