Water heating

Water heating
Water heating can be done using a variety of ways

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. The temperature will vary based on the consumption rate of hot water, use more and the water becomes cooler.

Appliances for providing a more-or-less constant supply of hot water are variously known as water heaters, 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 sometimes known as domestic hot water (DHW).

In many countries the most common energy sources for heating water are fossil fuels: natural gas, liquefied petroleum gas, oil, or sometimes solid fuels. These fuels may be consumed directly or by the use of electricity (which may derive from any of the above fuels or from nuclear or renewable sources). Alternative energy such as solar energy, heat pumps, hot water heat recycling, and sometimes geothermal heating, may also be used as available, usually in combination with backup systems supplied by gas, oil or electricity.

In some countries district heating is a major source of water heating. This is especially the case in Scandinavia. District heating systems make it possible to supply all of the energy for water heating as well as space heating from waste heat from industries, power plants, incinerators, geothermal heating, and central solar heating. The actual heating of the tap water is performed in heat exchangers at the consumers' premises. Generally the consumer needs no backup system due to the very high availability of district heating systems.

Image of water heaters utilised in the past.



Types of water heating appliance

Water for space heating may be heated by fossil fuels in a boiler. Potable water may be heated in a separate appliance: this is common practice in the USA where warm-air space heating is usually employed. The most useful heating transmitter is known as the radiator.[2]

Gravity system

Where a space-heating water boiler is employed the traditional arrangement in the UK is to use boiler-heated ("primary") water to heat ("secondary") water in a cylindrical vessel (usually made of copper) containing potable water supplied from a cold water storage vessel/container, usually in the roof space of the building. This produces a fairly steady supply of DHW at low static pressure head but usually with a good flow. Water heating appliances in most other parts of the world do not use cold water storage vessel/container but heat water at pressures close to that of the incoming mains water supply.

On-demand water heaters

Stand-alone appliances for quickly heating water for DHW (Domestic Hot Water) are known in North America as tankless heaters, elsewhere as multipoint heaters, geysers or Ascots. In Australia and New Zealand they are called an "Instantaneous Hot Water Unit". 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

Another popular arrangement where higher flow rates are required (although for limited periods) is to heat water in a pressure vessel capable of withstanding a hydrostatic pressure close to that of the incoming mains supply. (A pressure reducing valve is usually employed to limit the pressure to a safe level for the vessel.[citation needed])

In North America these vessels are known as hot water tanks and may incorporate an electrical resistance heater, an air source heat pump or a gas or oil burner heating the water directly.

Where hot-water space heating boilers are used DHW cylinders are usually heated indirectly by primary water from the boiler, or by an electric immersion heater (often as backup to the boiler). In the UK these vessels are known as unvented cylinders (or commonly as Megaflos after the brand name of a widely-used model). In the US, when connected to a boiler they are known as indirect-fired water heaters.

Thermodynamics and economics

Water enters residences in the US at about 10 °C (50 °F) (varies with latitude and season). 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 ].[3] 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. 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 attempting to boil a gallon of water on a stove. For this reason, tankless on-demand water heaters need to have a very large energy source to be usable. A wall outlet, by comparison, can only source enough energy to warm a disappointingly small amount of water: about 0.17 gpm at 40 °C temperature elevation.[where?]

Water heaters

A storage water heater


Although not that popular in 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." 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.

From Britain to America The water heater is still sometimes called a geyser in the U.K. It is often called an electric water boiler, electric dispensing pot or electric water urn. Maughn's invention influenced the work of a Norwegian mechanical engineer named Edwin Ruud. The electric water heater was invented in 1889 by Ruud after he immigrated to Pittsburgh, Pennsylvania. The Ruud Manufacturing Company, still in existence today, made many advancements in water heater design and operation.[1]

In household and commercial usage, most water heaters in North America are of the tank type. Also called storage water heaters, these consist of a cylindrical vessel/container in which water is kept continuously hot and ready for use. Typical sizes for household use range from 75 to 400 litres (20 to 100 U.S. gallons). These may use electricity, natural gas, propane, heating oil, solar, or other energy sources. Natural gas heaters are most popular in the United States and most European countries, since the gas is often conveniently piped throughout cities and towns and currently is the cheapest to use. Compared to tankless heaters, storage water heaters have the advantage of using energy (gas or electricity) at a relatively slow rate, storing the heat for later use. The disadvantage is that after a while, 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 vessel/containers 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 Western countries, where ambient temperature is colder, tiny point-of-use electric storage water heaters with capacities ranging from 8 to 32 litres (2 to 6 gallons) are made for installation in kitchen and bath cabinets or on the wall above a sink. They typically use low power heating elements, about 1 kW to 1.5 kW, and can provide hot water long enough for hand washing, or, if plumbed into an existing hot water line, until hot water arrives from a remote high capacity water heater. They are sometimes used when retrofitting a pump and recirculating plumbing in a building is too costly or impractical. Since they maintain water temperature thermostatically, they will supply hot water at extremely low flow rates, unlike tankless heaters.

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

Water heater tanks may be made of copper, stainless steel, or vitreous enamel-lined carbon steel. Vitreous-lined tanks often include a sacrificial anode designed to protect the tank from corrosion perforation. Electric heaters may be tubular or cartridge type.

Insulation and other improvements

In general, the more 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. Depending upon the insulation efficiency, star rating is given in India.

Other improvements include check valve devices at their inlet and outlet, cycle timers, electronic ignition in the case of fuel-using models, sealed air intake systems in the case of fuel-using models, and pipe insulation. The sealed air-intake system types are sometimes called "band-joist" intake units. "High-efficiency" condensing units can convert up to 98% of the energy in the fuel to heating the water. The exhaust gases of combustion are cooled and are mechanically ventilated either through the roof or through an exterior wall. At high combustion efficiencies a drain must be supplied to handle the water condensed out of the combustion products, which are primarily carbon dioxide and water vapor.

In traditional plumbing in the United Kingdom the space-heating boiler is set up to heat a separate hot water cylinder or 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 (cf. 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, and sometimes 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.[4]

Electric water heating

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.

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

Tankless heater
inside of a hydraulical controlled two-stage tankless heater, running with single-phase electric power. The copper tank contains the heaters with 18 kilowatts 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 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 are often installed throughout a household at more than one point-of-use (POU), far from the central water heater, or larger models may still be used to provide all the hot water requirements for an entire house. The main advantages of tankless water heaters are a continuous flow of hot water and energy savings (as compared to a limited flow of continuously heating hot water from conventional tank water heaters).

How tankless water heaters work

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.[5]

Combination boilers

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%.

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.


Electric shower head

As the name implies, an electric heating element is incorporated into such shower head to heat the water as it flows. Invented in Brazil in the 1930s and widely used since the 40s, the electric shower is a home appliance very commonly used in South American countries due to the higher costs with gas canalization. At one time, an electric shower cost less than a hair dryer. Electric Showers work like a coffee maker, but with a larger water flow. When the water flows inside, the pressure inflates a diaphragm which closes the electrical contacts of the heater coil with the live contacts, turning on the device. Once the water is stopped, the device turns off automatically. An ordinary electric shower used to have 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.

The power consumption of electric showers in the maximum heating level 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 bath, while a boiler works many times a day to keep a larger quantity of water hot for use throughout the day. So electric showers can 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 consupmtion is required only during the cold seasons.

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 the 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 aluminium wires or bad connections are very dangerous as the wires can overheat. Some changes in the electrical public distribution were important before a wide use of electric showers at first. Electric public transformers with higher KVA capacity are required due to the increase of the electrical demand. In countries where almost all houses use electric showers like Brazil, an ordinary street transformer per square have 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.

Various types and their advantages

Point-of-use tankless water heaters are located right where the water is being used, so the water is almost instantly hot, which saves water. They also save even more energy than centrally installed tankless water heaters because no hot water is left in the pipes after the water is shut off. However, point-of-use tankless water heaters are usually used in combination with a central water heater since they are usually limited to under 6 litres/minute (1.5 U.S. gallons/minute), as the expense of buying a heater for every kitchen, laundry room, bathroom, or sink can outweigh the money saved in water and energy bills. In addition, in the USA point of use water heaters until recently were almost always electrical[citation needed], and electricity is often substantially more expensive than natural gas or propane. Tankless heaters can ideally be somewhat more efficient than storage water heaters. In both kinds of installation (centralized and POU) the absence of a tank saves energy as conventional water heaters have to reheat the water in the tank as it cools off, called standby loss. There is a misconception that the energy lost by a tanked heater stored inside a home merely helps to heat the home. This is true of an electric unit, but for a gas unit some of this wasted energy leaves through the exhaust vent. However, if the building needs to be cooled to maintain normal temperatures this results in a loss in efficiency. With a central water heater of any type, water is wasted waiting for water to heat up because of the cold water in the pipes between the faucet and the water heater. This water waste can be avoided if a recirculating pump is installed, but at the cost of electricity to run the pump and wasted energy to heat the water circulation through the pipes.

Tankless water heaters can be divided into two categories: "full on/full off" and "modulated". Full on/full off units do not have a variable power output level; the unit is either fully on or completely off. Modulated tankless water heaters base the heat output on the flow of water running through the unit. This is usually done through the use of a flow sensor, modulating gas valve, inlet water temperature sensor and an outlet water temperature sensor-choke valve and means that the occupants should receive the same output temperature of water at differing velocities, usually within a close range of ±2 °C.

The high-efficiency condensing combination boiler[7] provides both space heating and water heating, an increasingly popular choice in UK houses. In fact, combination boilers now account for over half of all the new domestic boilers installed in Britain.

Under current North American conditions, the most cost effective configuration from an operating viewpoint is usually to use a central tankless water heater for most of the house, and install a point of use tankless water heater at any distant faucets or bathrooms. However, this may vary according to how much electricity, gas and water costs in the area, the layout of the house, and how much hot water is used. Only electric tankless water heaters were available at first and they are still used for almost all point of use heaters, but natural gas and propane heaters are now common. When consumers are considering a whole house gas tankless unit, they are advised to look at how the unit functions when raising the water temperature by about 42 °C (75–77 °F). Thus, if they live in a cold weather climate, they are advised to look at the unit's capacity with 3-10 °C (38–50 °F) inlet water temperatures, and find a size that produces approximately 15 litres/minute (4 gpm) even in winter if they have a typical-sized house and desire what is called a 2-appliance heater. This same unit may produce 25-30 litres/minute (6.3–6.9 gpm) in summer with higher inlet temperatures, but there is greater interest in year round production and usability.


There are certain advantages to tankless water heaters :

  • 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.[8]


Tankless heaters also have several disadvantages:

  • 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 (showers, baths, washing machine) this is not an issue. However, for intermittent-use applications (for example when a hot water faucet is turned on and off repeatedly at a sink) this can result in periods of hot water, followed by some small amount of cold water as the heater activates, followed quickly by hot water again. The period between hot/cold/hot is the amount of water which has flowed though the heater before becoming active. This cold section of water takes some amount of time to reach the faucet and is dependent on the length of piping.
  • Installation cost: 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.
  • Heat source flexibility Tankless heaters are sometimes limited to a choice between CO2 problematic energy sources: gas and electricity. This sometimes makes it difficult to include other heat sources, sometimes including certain renewable energy options. One exception is solar water heating, which can be used in conjuncion with tankless water heaters. However, tank-type systems have a much wider choice of heat sources available, 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.[9] 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.

Hybrid water heaters

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.[10]

The hybrid water heater maintains 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.[11]

Solar water heaters

Direct-gain solar heater panels with integrated storage tank

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-plate solar thermal collector, viewed from roof-level

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

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

Water heater safety

Water heaters potentially can explode and cause significant damage, injury, or death if certain safety devices are not installed. When the water temperature exceeds 100 °C (212 °F), the water will remain a liquid inside the tank, but when the pressure is released as the water comes out the tap the water will boil, potentially inflicting steam burns. Water above about 88 °C (190 °F) will cause burns on contact. A safety device called a temperature and pressure relief (T&P or TPR) valve, is normally fitted on the top of the water heater to dump water if the temperature or pressure becomes too high. Most plumbing codes require that a discharge pipe be connected to the valve to direct the flow of discharged hot water to a drain, typically a nearby floor drain, or outside the living space. Some building codes will allow for the discharge pipe to terminate in the garage.[12]

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.[13]

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

Scalding is a serious concern with any water heater. Human skin burns quickly at high temperature 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 from the heater to keep the outgoing water temperature fixed, often set to 50 °C. Without a tempering valve, reduction of the water heater's setpoint temperature is the most direct way to reduce scalding. However, for sanitation, hot water is needed. Most residential dishwashing machines, for example, include an electric heating element for increasing the water temperature above that provided by water heaters. The result of mixing hot and cold water via a tempering valve is also referred to as tempered water.[14]

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.[15] 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,[16] 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 effective 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


  1. ^ a b Early History of Water Heater Technology
  2. ^ http://www.search.eb.com/eb/article-3085
  3. ^ http://www.google.com/search?q=%282.5+gallon%2Fminute%29+*+%281+calorie%2F%28cm%5E3+*+1C%29%29++*+20+C
  4. ^ "Clean-air rules could threaten wetback fires". 24 August 2005. http://www.nzherald.co.nz/energy-efficiency-and-conservation-authority/news/article.cfm?o_id=285&objectid=10342132. 
  5. ^ "How A Tankless Gas Water Heater Works". www.rinnai.us. http://www.rinnai.us/tankless-water-heater/learn-about-tankless/how-tankless-works/. Retrieved 2010-03-29. 
  6. ^ Phillip Inman (April 2, 2005). "The new boiler that's causing a heated row". The Guardian. http://www.guardian.co.uk/money/2005/apr/02/consumerissues.jobsandmoney. Retrieved 2011-04-08. 
  7. ^ "UK.DIY Combination Boilers". Diyfaq.org.uk. http://www.diyfaq.org.uk/plumbing/combi.html. Retrieved 2009-04-23. 
  8. ^ http://eemax.com/WhyTankless
  9. ^ Electric tankless water heaters
  10. ^ http://home.howstuffworks.com/home-improvement/energy-efficiency/hybrid-water-heaters1.htm
  11. ^ http://www.reevesjournal.com/CDA/Articles/Feature_Article/BNP_GUID_9-5-2006_A_10000000000000605957
  12. ^ Water Can Be Dynamite 1951 article with illustrations on basics of water heater safety pressure relief valve
  13. ^ "California Plumbing Code". International Association of Plumbing and Mechanical Officials. pp. 58–59. http://www.iapmo.org/California%20Plumbing%20Code/Chapter%2005.pdf. Retrieved 23 Feb 2010. 
  14. ^ http://www.armstronginternational.com/files/products/lynnwood/EMC/articles/ASPE5-2003.pdf
  15. ^ "European Guidelines for Control and Prevention of Travel Associated Legionnaires’ Disease". European Working Group for Legionella Infections. 2005-01-01. Archived from the original on 2007-09-22. http://web.archive.org/web/20070922164043/http://www.ewgli.org/data/european_guidelines/european_guidelines_jan05.pdf. Retrieved 2008-02-12. 
  16. ^ "A Consumer's Guide to Energy Efficiency and Renewable Energy: Lower Water Heating Temperature for Energy Savings". U.S. Department of Energy. 2005-09-12. http://www.eere.energy.gov/consumer/your_home/water_heating/index.cfm/mytopic=13090. Retrieved 2007-10-14. 

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