Water heating
From Wikipedia, the free encyclopedia
Water heating is a thermodynamic process using a heating source to heat water above its initial temperature. Potable water is usually heated by a device known as a water heater or hot water system. (Hot water heater is an incorrect, redundant term). Water heaters for nonpotable use, both industrial and domestic, are also called hot water boilers. When a shell-and-tube heat exchanger is used for potable or nonpotable water heating, it is commonly called a hot water generator.
Most commonly, human-induced heating processes, such as combustion or electric-resistance, are relied upon to heat the water, but solar energy, or where possible, geothermal power may be used instead. Heat pumps and heat recovery may be used as well. Sometimes a combination is used, such as solar preheating, and then conventional combustion or electric heating.
In English-speaking countries, except in North America, water heaters are usually known as boilers, or "geysers" (though the latter term originally applied to a brand of tankless heaters). In cold climates, the water heating and hydronic space heating are commonly combined in one boiler; in much of North America, the water- and space-heating functions are through separate pieces of equipment.
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[edit] Water heating in the United States
Water enters residences in the US at about 10°C (50°F). An adult generally prefers a shower temperature of 40-49°C (105-120°F), requiring the water temperature to be raised about 30°C (55°F), or more, if the hot water is later mixed with cold water. The Uniform Plumbing Code reference shower flow rate is 2.5 gpm (gallons per minute); sink and dishwasher usages range from 1-3 gpm.
Natural gas in the U.S. is measured in CCF (100 cubic feet), which is converted to a standardized heat content unit called the therm, equal to 100,000 British thermal units. A BTU is the energy required to raise one pound of water one degree. A U.S. gallon of water weighs 8.3 pounds. So, to raise a 40-gallon tank of 55°F water up to 105°F would take 40 x 8.3 x 50 / 100,000 therms, or approximately 0.17 CCF, at 100% efficiency. A 40,000 BTU (per hour) heater would take 25 minutes to do this, at 100% efficiency. At $1 per therm, the cost of the gas would be about 17 cents.
To heat the same quantity of water with electricity in the same amount of time would require about 29 kW (120 amperes at 240 volts) for 25 minutes, at 100% efficiency. This is 12 kWh, which at 10 cents/kWh would cost $1.20.
A tankless water heater operating at those same power levels (at 100% efficiency) would be able to supply 1.6 gpm continuously, raising the temperature by 50°F. Or supply 1.3 gpm raising the temperature by 60°F. To be able to handle a full house load of multiple uses (at least 5 gpm) at the same time with a centralized water heater would require about three to four times this power level — somewhat difficult to achieve with natural gas, and very difficult to achieve with electricity.
Unfortunately, it takes a great deal of energy to heat water. Ordinary experience with heating water on a stove shows that you must apply quite a bit of energy for an extended time to heat up a gallon of water — or apply an immense amount of heat if you are in a hurry. Tankless on-demand water heaters need to have a very large energy source to be usable. The amount of energy one can get from an ordinary wall outlet is barely enough warm a slow trickle of water. (1500 watts can raise 0.1 gpm flow by 40°F, at 100% efficiency.) This may seem disappointing, since heating the water right at the sink tap sounds like a great idea.
Overall system energy efficiencies of tank water heaters in residential use are about 65%, and somewhat higher for tankless.
[edit] Tank-type water heaters
In household and commercial usage, most water heaters in North America are of the tank type. Also called storage water heaters, these consist of a cylindrical tank in which water is kept continuously hot and ready for use. Typical sizes for household use range from 75 to 400 litres (20 to 100 U.S. gallons). These may use electricity, natural gas, propane, heating oil, solar, or other energy sources. Natural gas heaters are most popular in the United States and most European countries, since the gas is often conveniently piped throughout cities and towns and currently is the cheapest to use. Compared to tankless heaters, storage water heaters have the advantage of using energy (gas or electricity) at a relatively slow rate, storing the heat for later use. Larger tanks tend to provide hot water with less temperature fluctuation at moderate flow rates.
Storage water heaters in the United States and New Zealand are typically vertical, cylindrical tanks, usually standing on the floor or on a platform raised a short distance above the floor. Storage water heater tanks in Spain are typically horizontal. In apartments they can be mounted in the ceiling space over laundry-utility rooms.
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. However, the heating element may be prone to burnout if it should be operated without water in the tank.
[edit] Insulation and other improvements
In general, the more tank insulation the better, since it reduces standby heat loss. Tanks are available with insulation ratings ranging from R-6 to R-24. It may be possible to add an extra insulating blanket or jacket on the outside of a poorly insulated tank to reduce heat loss.[1] 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.
Other improvements include check valve devices at their inlet and outlet, cycle timers, electronic ignition in the case of fuel-using models, sealed air intake systems in the case of fuel-using models, and pipe insulation. The sealed air-intake system types are sometimes called "band-joist" intake units. "High efficiency" condensing units can convert up to 98% of the energy in the fuel to heating the water. The exhaust gases of combustion are cooled and are mechanically ventilated either through the roof or through an exterior wall. At high combustion efficiencies a drain must be supplied to handle the water condensed out of the combustion products which are primarily carbon dioxide and water vapor.
In traditional plumbing in the United Kingdom the space-heating boiler is set up to heat a separate hot water cylinder or hot water tank for potable hot water. Such tanks are often fitted with an auxiliary electrical immersion heater for a quick temperature boost. Heat from the space-heating boiler is transferred to the potable water tank by means of a heat exchanger, and the boiler operates at a higher temperature than the potable hot water supply. Most potable water heaters in the United States are completely separate from the space heating units.
Residential combustion water heaters manufactured since 2003 in the United States have been redesigned to resist ignition of flammable vapors and incorporate a thermal cutoff switch, per ANS Z21.10.1. The first feature attempts to prevent vapors from flammable liquids and gasses in the vicinity of the heater from being ignited and thus causing a house fire or explosion. The second feature prevents tank overheating due to unusual combustion conditions. These safety requirements were made based on homeowners placing, and sometimes spilling, gasoline and other flammable gases near their water heaters and causing fires. Since most of the new designs incorporate some type of flame arrestor screen, they require monitoring to make sure they don't become clogged with lint or dust, reducing the availability of air for combustion. If the flame arrestor becomes clogged, the thermal cutoff may act to shut down the heater.
A wetback stove or wetback heater is the name (used in New Zealand at least) for a simple household secondary water-heater using incidental heat. It typically consists of a hot water pipe running behind a fireplace or stove (rather than hot water storage), and has no facility to limit the heating. Modern wetbacks may run the pipe in a more sophisticated design to assist heat-exchange.
[edit] Maintenance
It is important that a water heater, particularly a combustion type, be properly installed to avoid safety hazards.[2] The area around it must be kept clear. The T&P (Temperature and Pressure) relief valve should be tested at least yearly. Sediment that accumulates in the bottom of the tank should be drained out at least yearly, to maintain heating efficiency and to make the tank last longer.[3] [4] [5] [6]
In the past, tanks were made out of copper and were very corrosion resistant; they could last for decades. Now tanks are made out of steel lined with glass or porcelain.[7] A steel tank can rust and leak. To prevent this, sacrificial anode rods of magnesium or aluminum alloy are used to reduce corrosion of the steel tank. Tanks with longer guarantees have increased anode material — when the anode is use up, the tank will rust. It may be possible to check the anode condition, and replace it if it is wearing out.[8] Modern US water heater tanks last about ten years.[9] The tank should have a tray underneath to contain leaks and channel them into a drain. Simple, battery-powered alarms are available for $10-$30 that alert the homeowner to leaks.[10] All water heaters will eventually leak, so it is important to know where the water supply shutoff valve is located!
Electric water heaters made with plastic tanks (e.g., polybutylene) are available. Designed with corrosion-free and corrosion-resistant materials, they can conceivably last the life of a dwelling. The only maintenance requirement would be to periodically clean sediments from the bottom of the tank and clean mineral scale off the heating elements.[11]
[edit] Electric water heating power load shifting
Because tank-type water heaters store heat, electrical water heaters can be a good match for an intelligent electrical power distribution system, heating when the electrical grid load is low and turning off when the load is high. This could be implemented by allowing the power supplier to send load-shedding requests, or by the use of real-time energy pricing.
[edit] Tankless heaters
Tankless water heaters, also called combination or "combi" boilers, instantaneous, continuous flow, inline, flash, on-demand or instant-on water heaters, are also available and gaining in popularity. These water heaters heat the water as the water flows through the device, and do not retain any water internally except for what is in the heat exchanger coil. Point-of-use tankless (POU) heaters may be installed at the faucet or in the bathroom, far from the central water heater, or larger models can be used to provide all the hot water requirements for an entire house.
The high efficiency condensing combination boiler[12] 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.[13]
Tankless water heaters can be divided into two categories: "full on/full off" and "modulated". Full on/full off units do not have a variable power output level; the unit is either on or off. Modulated tankless water heaters base the heat output on the flow of water running through the unit. This is usually done through the use of a 'flow sensor', modulating gas valve, inlet water temperature sensor and an outlet water temperature sensor-choke valve and means that the occupants should receive the same output temperature of water at differing velocities, usually within a close range of ±2°C.
Tankless heaters can ideally be somewhat more efficient than storage water heaters. In both kinds of installation (centralized and POU) the absence of a tank saves energy as conventional water heaters have to reheat the water in the tank as it cools off, called standby loss. With a central water heater of any type, water is wasted waiting for water to heat up because of the cold water in the pipes between the faucet and the water heater.
Point of use tankless water heaters are located right where the water is being used, so the water is almost instantly hot, which saves water. They also save even more energy than centrally installed tankless water heaters because no hot water is left in the pipes after the water is shut off. However, point of use tankless water heaters are usually used in combination with a central water heater since they are usually limited to under 6 litres/minute (1.5 U.S. gallons/minute), as the expense of buying a heater for every kitchen, laundry room, bathroom or sink, often outweighs the money saved in water and energy bills. In addition, point of use water heaters until recently were almost always electrical, and electricity is often substantially more expensive than natural gas.
Under current North American conditions, the most cost effective configuration from an operating viewpoint is usually to use a central tankless water heater for most of the house, and install a point of use tankless water heater at any distant faucets or bathrooms. However, this may vary according to how much electricity, gas and water costs in the area, the layout of the house, and how much hot water is used. Only electric tankless water heaters were available at first and they are still used for almost all point of use heaters, but natural gas and propane heaters are now common. When consumers are considering a whole house gas tankless unit, they are advised to look at how the unit functions when raising the water temperature by about 42°C (75–77°F). Thus, if they live in a cold weather climate, they are advised to look at the unit's capacity with 3-10°C (38–50°F) inlet water temperatures, and find a size that produces approximately 15 litres/minute (4 gpm) even in winter if they have a typical-sized house and desire what is called a 2-appliance heater. This same unit may produce 25-30 litres/minute (6.3–6.9 gpm) in summer with higher inlet temperatures, but there is greater interest in year round production and usability.
Since the water must be heated instantly, tankless water heaters use a very large amount of electricity or gas while they are on. If a storage water heater is being replaced with a tankless one, the size of the electrical wiring or gas pipeline may have to be increased to handle the load, adding significant expense to the installation.[14] Gas units are efficient but require a large volume of gas when operating at high firing rates. Many tankless units have fully modulating gas valves that can range from as low as 10,000 to over 1,000,000 BTUs. For electrical installations, AWG 10 or 8 wire, corresponding to 10 or 6 mm2, is required for most POU heaters at North American voltages. In gas appliances both pressure and volume requirements must be met for optimum operation.
Since a tankless water heater is not active when hot water is not being used, they are typically incompatible with hot water recirculation systems or applications where the water is turned off and on repeatedly. With that being said, tankless water heaters can be used in recirculation applications when designed correctly. There is often a short delay before the water flow detector is triggered and the heating element activated. This can result in a brief flow of cold water when one reopens the hot water tap after having closed it. There are also typically minimum flow requirements that can result in a gap between the coolest warm water that can be achieved with a hot and cold water mix and straight cold water. On the other hand, more sophisticated tankless heaters often come with remote thermostats that can allow the water temperature to be changed interactively to suit the immediate purpose. This is impossible with tank heaters (but also not needed, since they have no trouble supplying hot water at a very low flow rate).
Although tankless water heating sounds ideal, the peak energy demands and increased complexity compared to tank-based systems can result in significantly larger purchase, installation, and maintenance expenses, particularly in retro-fit applications.[15] [16] They tend to be particularly expensive in areas such as the US where they are not dominant, compared to the established tank design. In general, because the water heater is only part of a larger infrastructure, there can be significant drawbacks to deviating from established practice in the local area.
[edit] Solar water heaters
In some locales, solar powered water heaters are used. Their solar collectors are installed outside dwellings, typically on the roof or nearby. Some direct-gain designs consist of flat panels in which water circulates, while others may use dish or trough mirrors to concentrate sunlight on a collector tube filled with water, brine or other heat transfer fluid. A storage tank is placed indoors or out. Circulation is caused by natural convection or by a small electric pump. At night, or when insufficient sunlight is present, circulation through the panel can be stopped by closing a valve and/or stopping the circulating pump, to keep hot water in the storage tank from cooling. Freeze protection, as well as prevention of overheating, must be addressed in their design, installation, and operation.
[edit] Geothermal heating
In countries like Iceland and New Zealand, and other volcanic regions, heating of water may be done using geothermal power, rather than combustion.
[edit] 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 third-degree burns on contact. A safety device called a temperature and pressure relief (T&P or TPR) valve, is normally fitted on the top of the water heater to dump water if the temperature or pressure becomes too high. Most plumbing codes require that a discharge pipe be connected to the valve to direct the flow of discharged hot water to a drain, typically a nearby floor drain, or outside the living space. Some building codes will allow for the discharge pipe to terminate in the garage.
If a water heater is installed in a garage, it is recommended, and many codes require, that it be elevated at least 18 inches (0.46 m) above the floor to reduce the potential for fire or explosion due to spillage or leakage of combustible liquids in the garage. Furthermore, some local codes mandate that tank-type heaters in new and retrofit installations be braced to an adjacent wall with a strap to prevent them from tipping over and breaking the water and gas pipes in the event of an earthquake.
For older houses where the water heater is part of the space heating boiler, and plumbing codes allow, some plumbers will install a "Watts 210" device in place of a TPR valve. When the device senses that the temperature reaches 99°C (210 °F), it will shut off the gas supply and prevent further heating. In addition, an expansion tank or exterior pressure relief valve must be installed to prevent pressure buildup in the plumbing from rupturing pipes, valves, or the water heater.
Scalding is a serious concern with any water heater. Human skin burns quickly at high temperature, e.g., only 60°C (140°F), but also at lower temperatures, e.g., 50°C (120°F), if the exposure times are sufficient. Older people and children often receive the most serious scalds due to disabilities or slow reaction times. In Australia and elsewhere it is common practice to put a tempering valve on the outlet of the water heater. A tempering valve mixes enough cold water with the hot from the heater to keep the outgoing water temperature fixed, often set to 50°C. Without a tempering valve, reduction of the water heater's setpoint temperature is the most direct way to reduce scalding. However, for sanitation, hot water is needed. Most residential dishwashing machines, for example, include an electric heating element for increasing the water temperature above that provided by water heaters.
There are two seemingly conflicting safety issues around water heater temperature — the risk of scalding from excessively hot water, and the risk of incubating bacteria colonies, particularly Legionella, in water that is not hot enough to kill them. Both risks are potentially life threatening, and both can be addressed easily: Set the water heater's thermostat to at least 60°C (140°F), and install a tempering valve set to no more than 55°C to avoid scalding. At 60°C, the water is at a commonly accepted minimum temperature for sanitizing water and cooked foods.
[edit] See also
[edit] External links
- How It Works - Water Heater
- Tankless Water Heater Interior Components
- PATH Tech Inventory: Tankless Water Heaters
- PATH Tech Inventory: Solar Water Heaters
- Tankless Water Heaters are Brutal on the Grid When Popularized
- Water Heating for Dairies in NZ
- Tankless Water Heater Guide
- U.S. Department of Energy Water Heating Guidance
- Antique water heater museum
- Tankless Water Heater Video
