![Natural gas storage water heater](https://upload.wikimedia.org/wikipedia/commons/2/29/Natural_gas_storage_water_heater.gif)
A storage water heater, or a hot water system (HWS), is a domestic
water heating
Water heating is a heat transfer process that uses an energy source to heat water above its initial temperature. Typical domestic uses of hot water include cooking, cleaning, bathing, and space heating. In industry, hot water and water heated t ...
appliance that uses a
hot water storage tank
A hot water storage tank (also called a hot water tank, thermal storage tank, hot water thermal storage unit, heat storage tank and hot water cylinder) is a water tank used for storing hot water for space heating or domestic use.
Water is a con ...
to maximize water heating capacity and provide instantaneous delivery of hot water.
Conventional storage water heaters use a variety of
fuels
A fuel is any material that can be made to react with other substances so that it releases energy as thermal energy or to be used for work. The concept was originally applied solely to those materials capable of releasing chemical energy but ...
, including
natural gas
Natural gas (also called fossil gas or simply gas) is a naturally occurring mixture of gaseous hydrocarbons consisting primarily of methane in addition to various smaller amounts of other higher alkanes. Low levels of trace gases like carbo ...
,
propane
Propane () is a three-carbon alkane with the molecular formula . It is a gas at standard temperature and pressure, but compressible to a transportable liquid. A by-product of natural gas processing and petroleum refining, it is commonly used a ...
,
fuel oil
Fuel oil is any of various fractions obtained from the distillation of petroleum (crude oil). Such oils include distillates (the lighter fractions) and residues (the heavier fractions). Fuel oils include heavy fuel oil, marine fuel oil (MFO), bun ...
, and
electricity
Electricity is the set of physical phenomena associated with the presence and motion of matter that has a property of electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as described ...
. Less conventional water heating technologies, such as
heat pump
A heat pump is a device that can heat a building (or part of a building) by transferring thermal energy from the outside using a refrigeration cycle. Many heat pumps can also operate in the opposite direction, cooling the building by removing h ...
water heaters and
solar water heaters, can also be categorized as storage water heaters.
Difference between a storage heater and an instant heater
The primary difference between a storage heater and an instant heater is that a storage system can deliver hot water instantly, and an instant heater takes an amount of time to heat the water first.
Instant water heaters, as the name suggests, provide hot water almost instantaneously. There is hardly 1 or 2 minutes of heating time after which hot water can be accessed. But given the low storage capacity (max at a given point of time) of these types of heaters, you cannot expect a bucket or a barrel full at the same speed. They are priced significantly higher than storage heaters but have a longer life period. This stands at an average of 15-20 years. Instant water heater provides water as you need. There are no heat losses and cheaper than the storage water heater.
Storage heaters, are not as fast as the instant versions, as they are required heat a large tank of water that is then stored for later usage. Storage tank water heaters are ideal for large volume water usage (large tanks can store about ) and are popular due to their low upfront costs and long average life-span of 10-15 years. But these systems are the least energy efficient, making them expensive to run long-term. They also require more space for installation.
Solar
![Solar thermal roof](https://upload.wikimedia.org/wikipedia/commons/f/f1/Solar_thermal_roof.png)
Solar heat is clean and renewable. This is the most modern system. Increasingly,
solar powered
Solar energy is radiant light and heat from the Sun that is harnessed using a range of technologies such as solar power to generate electricity, solar thermal energy (including solar water heating), and solar architecture. It is an essenti ...
water heaters are being used. Their
solar thermal 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
solar panels
A solar cell panel, solar electric panel, photo-voltaic (PV) module, PV panel or solar panel is an assembly of photovoltaic solar cells mounted in a (usually rectangular) frame, and a neatly organised collection of PV panels is called a phot ...
, 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
A heat exchanger is a system used to transfer heat between a source and a working fluid. Heat exchangers are used in both cooling and heating processes. The fluids may be separated by a solid wall to prevent mixing or they may be in direct contac ...
, 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
circulator 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
A skylight (sometimes called a rooflight) is a light-permitting structure or window, usually made of transparent or translucent glass, that forms all or part of the roof space of a building for daylighting and ventilation purposes.
History
Open ...
, are the most durable type of collector, and they also have the best performance for systems designed for temperatures within 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 ). 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
Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkis ...
or
condensation
Condensation is the change of the state of matter from the gas phase into the liquid phase, and is the reverse of vaporization. The word most often refers to the water cycle. It can also be defined as the change in the state of water vapor to ...
-driven). The ''evacuated'' description refers to the
vacuum
A vacuum is a space devoid of matter. The word is derived from the Latin adjective ''vacuus'' for "vacant" or "void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure. Physicists often dis ...
created in the glass tubes during the manufacturing process, which results in very low heat loss and lets evacuated tube systems achieve extreme temperatures, far in excess of water's boiling point.
Fossil fuel fired water heaters
![Burner assembly of a water heater](https://upload.wikimedia.org/wikipedia/commons/d/d7/Burner_assembly_of_a_water_heater.jpg)
Natural gas and propane storage water heaters operate identically with a gas or propane burner located at the bottom of the storage tank heating the water. Fuel oil fired storage water heaters are configured similarly by igniting a vaporizing
mist
Mist is a phenomenon caused by small droplets of water suspended in the cold air, usually by condensation. Physically, it is an example of a dispersion. It is most commonly seen where water vapor in warm, moist air meets sudden cooling, such a ...
of
oil
An oil is any nonpolar chemical substance that is composed primarily of hydrocarbons and is hydrophobic (does not mix with water) & lipophilic (mixes with other oils). Oils are usually flammable and surface active. Most oils are unsaturated ...
and air with an
electric spark
An electric spark is an abrupt electrical discharge that occurs when a sufficiently high electric field creates an ionized, electrically conductive channel through a normally-insulating medium, often air or other gases or gas mixtures. Michael F ...
.
Emissions from
fossil fuel
A fossil fuel is a hydrocarbon-containing material formed naturally in the Earth's crust from the remains of dead plants and animals that is extracted and burned as a fuel. The main fossil fuels are coal, oil, and natural gas. Fossil fuels m ...
fired water heaters are expelled using a variety of venting technologies. Atmospheric vented systems use room air as
combustion
Combustion, or burning, is a high-temperature exothermic redox chemical reaction between a fuel (the reductant) and an oxidant, usually atmospheric oxygen, that produces oxidized, often gaseous products, in a mixture termed as smoke. Combusti ...
air and exhaust air. The exhaust air is expelled through the exhaust flue by
buoyancy
Buoyancy (), or upthrust, is an upward force exerted by a fluid that opposes the weight of a partially or fully immersed object. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus the p ...
forces resulting from the combustion. Power vent models operate similarly to atmospheric vent systems, but an exhaust fan is added to aid in the expulsion of combustion gases. Direct vent systems do not use room air for combustion; instead, buoyancy forces air from the outside through the water heater combustion system and finally exhausts the combustion gases to the outside. Powered direct-vent systems include an exhaust fan to aid in the expulsion of combustion gasses.
Electric water heaters
Most electric water heaters use
electric resistance
The electrical resistance of an object is a measure of its opposition to the flow of electric current. Its reciprocal quantity is , measuring the ease with which an electric current passes. Electrical resistance shares some conceptual parallels ...
elements to heat the water in the storage tank. A two-element electric water heater has one element at the top of the storage tank, and one element at the bottom. Each element is controlled by an independent
thermostat
A thermostat is a regulating device component which senses the temperature of a physical system and performs actions so that the system's temperature is maintained near a desired setpoint.
Thermostats are used in any device or system tha ...
. The lower element provides recovery from standby losses, and the upper element provides extra heating when a lot of hot water is being used. Some heaters contain only a lower element.
Electrical water heaters that store hot water can be a good match for a
smart grid
A smart grid is an electrical grid which includes a variety of operation and energy measures including:
*Advanced metering infrastructure (of which smart meters are a generic name for any utility side device even if it is more capable e.g. a f ...
, so that it heats when the
electrical grid
An electrical grid is an interconnected network for electricity delivery from producers to consumers. Electrical grids vary in size and can cover whole countries or continents. It consists of:Kaplan, S. M. (2009). Smart Grid. Electrical Power ...
load is low and turns off when the load is high. This could be implemented by allowing the power supplier to send
loadshedding
A rolling blackout, also referred to as rota or rotational load shedding, rota disconnection, feeder rotation, or a rotating outage, is an intentionally engineered electrical power shutdown in which electricity delivery is stopped for non-overla ...
requests, or by the use of real-time energy pricing. See
Economy 7
Economy 7 is the name of a differential tariff provided by United Kingdom electricity suppliers that uses base load generation to provide cheap off-peak electricity during the night.
Houses using the Economy 7 tariff require a special electricity ...
.
Heat pump water heaters use an
air source heat pump
An air source heat pump (ASHP) is a type of heat pump that can absorb heat from outside a structure and release it inside using the same vapor-compression refrigeration process and much the same equipment as air conditioners but used in the oppo ...
to transfer
thermal energy
The term "thermal energy" is used loosely in various contexts in physics and engineering. It can refer to several different well-defined physical concepts. These include the internal energy or enthalpy of a body of matter and radiation; heat, d ...
from the air around the unit into the storage tank. Electric resistance element(s) are typically included to provide backup heating if the heat pump cannot provide sufficient heating capacity.
Wood
As with fossil fuels, burning wood causes
greenhouse effect
The greenhouse effect is a process that occurs when energy from a planet's host star goes through the planet's atmosphere and heats the planet's surface, but greenhouse gases in the atmosphere prevent some of the heat from returning directly ...
gases. However, wood is a renewable source of energy. A sustainable heat system would be to use solar heat in the summer, and the minimum of wood in the winter.
Corrosion and its prevention
The storage tanks of water heaters are usually made out of
steel
Steel is an alloy made up of iron with added carbon to improve its strength and fracture resistance compared to other forms of iron. Many other elements may be present or added. Stainless steels that are corrosion- and oxidation-resistant ty ...
with a lining of glass inside them. Water will
corrode
Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials (usually a metal) by chemical or electrochemical reaction with their environment. Corrosion engine ...
exposed steel, so the glass lining prevents or delays corrosion.
The tanks also have magnesium anode rods. The anode rod will slow down corrosion of the steel tank by corroding in its place.
When the anode rod is completely corroded, the steel tank will corrode much faster.
Given the constant contact of water, corrosion eventually happens anyway. If corrosion of the tank creates holes in it, there are some temporary fixes to try to patch it, but the long-term solution is to replace the tank altogether.
Water with a lower
pH value will corrode the anode rods and steel tank faster. For proper maintenance of the tank, know the pH level of the water stored, watch for corrosion of the anode rod, and replace the anode when it becomes too corroded.
See also
*
Tankless water heating
Tankless water heaters — also called instantaneous, continuous flow, inline, flash, on-demand, or instant-on water heaters — are water heaters that instantly heat water as it flows through the device, and do not retain any water internally e ...
References
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Energy storage
Residential heating appliances
Heating