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Water heating is a thermodynamic process using a heating source to heat water above its ambient temperature. Most commonly, human-induced heating processes such as combustion are relied upon, but in areas such as Iceland or near volcanic regions, geothermal power may be used instead. In industrial usage, as well when used to heat buildings through steam, large water heaters are called boilers; this term is also used for domestic water heaters in most English-speaking countries, but rarely in North America.
Tank heaters
In household and commercial usage, most water heaters in North America are of the tank type. They are also called storage water heaters. These consist of tanks in which a given amount of water is kept continuously hot and ready for use. Typical sizes for household use are 75 to 200 litres (20 to 50 US gallons). These may run on electricity, natural gas, propane, heating oil, or other energy sources. The most popular in the United States and most European countries is the natural gas type, since provided piped gas is available this is currently the cheapest to run.
Tank-type water heaters can be made more efficient by installation of a water heater blanket, also called a water heater jacket. The most common type is fiberglass insulation with a vinyl face on the outside. The insulation is wrapped around the tank and the ends are taped together. It is important that the blanket is the right size for the tank and does not block air flow or cover valves and controls. Other measures 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 boiler 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 without the need of a chimney. At high efficiencies a drain must be supplied to handle the water condensed out of the combustion products.
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 traditional plumbing in the United Kingdom the boiler is set up to feed a separate hot water tank. Such tanks are often fitted with a backup electrical heater for a quick boost, known as an immersion heater. It is mandatory that these two hot water storage vessels are 'indirect'. That means the water from the boiler circulates via a separate internal exchanger and does not come into contact with the stored hot water. This primary circulation is at a higher temperature than the final hot water supply, and it is common for first time American users to burn themselves on the primary circulation pipes if not warned. Most heaters in the United States are direct water heaters, which heat the water directly.
Tankless heaters
Tankless water heaters, also called instantaneous, 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.
Tankless water heaters can be broken down into two categories; "full on/full off" and "modulated". Full on/full off units do not change the power output at all. 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 will receive the same output temperature of water at differing velocities, usually within a close range of ±2 degrees Celsius.
Tankless heaters can be far more efficient than storage water heaters. In both kinds of installation, 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 US gallons/minute or GPM), 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 currently far more expensive than propane and 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 the 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 home 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 lot 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. Gas units are efficient but require a large volume of gas. 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.
Solar heaters
In some locales, solar powered water heaters are used. These are installed outside dwellings, typically on the roof or nearby, and consist of a tank and panel in which water circulates. 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.
Water heater safety
Water heaters potentially can explode and cause significant damage to a house if certain safety devices are not installed. When the water temperature exceeds 100 degrees Celsius, 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 will cause 3rd degree burns on contact. A safety device called a temperature pressure relief (TPR) valve, is normally fitted on the top of the water heater to dump water if the pressure becomes too high. Often a discharge pipe is connected to the valve to direct the flow of water to a drain. Most home inspectors recommend the discharge pipe extend to outside the home so that it does not cause flooding damage. Some building codes will allow for the discharge pipe to terminate in the garage. Where homes have basements, the water heater is usually in the basement and the discharge simply dumps to the floor, where the water runs to a floor drain or sump.
If a water heater is installed in a garage, it is recommended that it be elevated 18 inches off the ground so that the potential for catching fire due to low, ground flowing exhaust is eliminated.
For older homes where the water heater is centrally located, some plumbers will install a Watts 210 device in place of the TPR valve. When the device senses that the temperature reaches 210 °F (99° C), it will shut off the gas and eliminate the heat from water heater tank. In addition, an outside pressure relief valve is required to be installed on one of the cold water hose bibs on the house's exterior . If the water pressure is too great in the water heater, it will discharge out through the cold water system.
In Australia it is common practice to put a tempering valve on the outlet of the service. A tempering valve mixes enough cold water with the hot from the service to keep the outgoing water temperature fixed, often set to 50 degrees Celsius.
In California, state law mandates that all homes sold, whether new or old, have water heaters strapped and securely bolted to wall studs to prevent the tank from toppling over in a serious earthquake, causing burns or a gas leak.
See also
External links
- Tankless 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
"Water heating"