Hot water heater
A Speichererhitzer , DHW ,, hot water tank , hot water tank , etc., often even for a short storage ( hanging storage , free storage , sub-table memory , etc.), is a heat storage for the production and delivery of heated process water or drinking water , typically in the temperature range between 50 and 100 ° C. Storage tanks are constantly full and should have thermal insulation , the efficiency of which is divided into energy efficiency classes (EEK) and shown in the energy label.
In contrast, boilers such as wall-mounted boiling water devices and bath stoves are usually only heated shortly before hot water is drawn off and therefore do not require any thermal insulation .
The amount of hot water that can be drawn off in a short time primarily depends on the size of the storage tank, while the connected load determines the amount of water that can be drawn permanently.
Terms and Variants
In contrast to the heat accumulators dealt with here , a buffer tank is not used for the direct extraction of heated (drinking) water, but for the intermediate storage of heat energy in closed circuits, for example in heating technology . A seasonal storage tank is a buffer storage tank that is so large that it can store thermal energy for months.
Buffer storage tanks, which contain an integrated heat exchanger for the direct extraction of heated water, and thus also serve as hot water storage tanks, are usually referred to as combination water storage tanks or hygiene storage tanks .
The terms hot water ( at ) ready and hot water ( at ) ready are not unique. You can designate different systems for water heating.
Terms such as storage , hot water tank , hot water tank , hanging storage , free storage , sub-table memory can not be found that these devices often can not only save water, but also warm. All that is required for electrical heating is a heating rod .
The comparatively apt designation storage heater , on the other hand, is rarely used by manufacturers or in retail.
If the heating by gas, this is in contrast always made conceptually identified, such as the designation gas directly heated storage water or water heaters , as this is a more complex technical equipment is required.
A hot water storage tank that does not automatically heat the drinking water can be heated by a hot water heater if it has built-in coiled tubing for heat transfer or an external heat exchanger is installed and a small pump "charges" the storage tank by passing the drinking water through the heat exchanger can circulate, while it is fed with heating water by the pump of the heating circuit . If there are built-in coiled pipes, the heating water is either branched off from the heating circuit by a three-way valve , or an additional pump allows the heating water to circulate through the pipe heat exchanger if required . A boiler, in which a device for charging the storage tank is already integrated, is also known as a circulating water heater .
The term boiler is incorrectly borrowed from English , where it denotes a steam boiler (i.e. a boiler for water temperatures above 100 ° C) ( to boil "to boil, simmer"), while a storage heater basically works below the water boiling temperature . In English, a storage heater is not referred to as a boiler , but as a water heater .
application
Small, predominantly electrically heated hot water tanks with a capacity of 5 to 15 liters are mainly used for the hot water supply for sinks and wash basins when a central hot water supply is not available or it is not worth connecting to it. This can be the case if the water extraction point is far away or is used rarely or irregularly and therefore appears to be energetically unfavorable or if it is hygienically problematic in the case of prolonged low temperatures (due to possible legionella formation). There are versions for under-table or wall mounting, the connected load is usually 2,000 watts. Small gas-powered equipment is also available for campers and caravans ; in today's standard devices, mostly combined with a warm air fan heater.
Medium-sized boilers with 70 to 200 liters are often used to supply hot water to a residential unit. At a heating temperature of 60–80 ° C, an 80 l boiler can be enough to fill a bathtub by adding cold water to achieve a pleasant temperature . They are often found in fully electrified apartments and houses. If the dimensions are adequate, heating them up once per day is sufficient. This can be done relatively cheaply with low-tariff electricity (night electricity). They are designed for wall mounting, as free-standing devices or storage units. For some time now there have also been shallow storage tanks with a tank made of a compressed pipe ring. The electrical connection power is usually 2,000 to 4,000 watts. There are also some models with gas operation in this size range.
Larger storage tanks with a few hundred to several thousand liters in the basements of single and multi-family houses are rarer. They are usually only operated with night power or bivalent heating.
With electrically heated hot water storage tanks, the investment costs are relatively low, but the operating costs are higher. In the case of systems for individual residential units, there is no costly cost allocation based on consumption. The heating temperature is usually around 60–82 ° C, more rarely up to 95 ° C. It is usually fixed depending on the amount of hot water required. If the temperature is increased, lukewarm water can be withdrawn for a longer time with the same storage volume by adding more cold water to the mixer tap. You can also set lower temperatures, which reduces the power dissipation emitted via the device insulation. However, legionella , which are hazardous to health, can also multiply at long-term temperatures below 50 ° C. These can get into the lungs mainly through finely sprayed water droplets when showering. According to DVGW worksheet W 551 of April 2004, a temperature of at least 60 ° C is required for hot water systems. There are also legionella circuits in which the storage tank temperature is briefly raised once a week to several times a day.
In contrast to boilers, pure heat storage (hot water storage ) use energy from external heat sources. There are also combined systems that work with bivalent heating, i.e. an external source and an electrical heating element. With solar collectors, for example, there is warm water in spite of insufficient solar radiation or if more warm water is consumed than the heat pump can provide, there are no standby losses of the boiler outside of the heating season and a wood boiler does not have to be heated up.
On the other hand, instantaneous water heaters do not store hot water . Nevertheless, they are sometimes referred to as boilers. However, small electrical instantaneous water heaters for wash basins cannot easily be connected because of the higher power consumption for brief heating of the water in old buildings. Small gas appliances mostly work according to this principle, just like combi-boilers that only have a small, non-insulated water tank.
Heating
Small hot water storage tanks are usually heated electrically, in the past also with gas; Larger storage tanks mostly also with electricity or gas, as well as indirectly through heating water (heating insert or double jacket), earlier also with solid fuels (bath heater). Newer models have additional heating inserts for additional heating using solar energy or other heat sources, e.g. B. a heat pump .
construction
There is an electrical heating element in the electrically heated hot water tank . It consists of a heating conductor (resistance wire) in a protective, corrosion-resistant metal tube. Next to the heating element is the sensor tube of a temperature-controlled switch for temperature control. Its setpoint can be adjusted with a rotary knob. The switch is usually an electromechanical system in which the different coefficients of thermal expansion of different materials (e.g. glass rod in a metal tube) are used together with a snap switch . Often a liquid is also used that works on a membrane via a pipe. In addition, there is a temperature switch near the heating element, which prevents fire and destruction if the controller fails. This thermal fuse can be reset or it has to be replaced after it has responded.
The temperature sensor and heating element are located in a vessel made of copper, plastic or - in the case of pressure boilers - made of stainless steel sheet, galvanized sheet steel (equipped with a sacrificial anode made of an aluminum-magnesium alloy for corrosion protection ) or enamelled sheet steel.
The cold tap water is fed into the bottom of the vessel; the hot water is taken from the top of the vessel. This is necessary because the warm water always collects at the top due to its lower specific weight and can be taken from there. The pipes are color-coded on the outside (blue for the inlet and red for the outlet) or with arrows.
Modern hot water storage tanks almost always have thermal insulation .
There are two fundamentally different types of hot water storage tank:
Closed or pressure-resistant hot water storage tanks
A hot water storage tank that is installed as part of the pressurized water pipe must be designed as a pressure vessel and can usually withstand a pressure of around 10 bar. If a lower pressure is specified on the nameplate, a pressure reducer may have to be installed. To prevent hot water from flowing back into the supply line, a check valve must be installed in the supply line . To protect against overpressure caused by heating and as an outlet for the water that expands during heating, a safety valve must therefore be installed behind the non-return valve in the direction of flow , which continuously drips when the storage tank is heated up. In order not to be sucked back into the drinking water pipe in the event of a pipe burst , the leaking water should overcome a fall distance of at least 20 mm before it is caught in an odor trap and drained into the sewer pipe . The drop can be prevented if, in addition, a suitable for potable water expansion tank is installed.
The capacity is usually at least 30 to 120 liters for a larger apartment. A pressure-tight hot water storage tank is usually more expensive than a pressureless storage tank of a similar size. The preparation is often done in the bathroom, near a wall-mounted combination boiler as a group can provide several taps or in the boiler room when the boiler central heating.
Open or pressureless hot water storage tanks (low pressure system)
This about i. d. Storage tanks with a capacity of 5 to 15 liters usually supply a single tap (individual supply), because the shut-off valve is located in the direction of flow in front of the storage tank. In order to prevent the storage tank from being loaded with line pressure, the outlet behind the storage tank must not have any further shut-offs. If there were several outlets, water would always flow from them at the same time.
In order to be able to place the shut-off valve for cold and hot water directly next to the spout, special low-pressure mixer taps with three connections are usually used today. If the hot water tap of the mixer tap is opened, the cold water flowing in first flows to the hot water tank. It is introduced into the bottom area and displaces the hot water in the container upwards (= pressureless displacement) from where it is directed to the open outlet of the mixer tap. Non-pressurized accumulators must not be used with ordinary two-port fittings, as this would pressurize the accumulator and burst.
There are pressureless hot water tanks for installation above or below the tapping point (mixer tap). They are referred to as superstructure and substructure water heaters. Pressureless water heaters are relatively simple and inexpensive as they do not have to withstand the water pressure of the water pipe (up to approx. 10 bar ).
Since there must be no shut-off valve between the water heater and the outlet , the water that expands when it is heated can drip off freely via the outlet. A dripping tap during the heating phase of the water storage tank does not mean that there is a leak.
Special types of pressureless storage tanks have a small expansion vessel that absorbs the increase in volume of the water contained in order to prevent dripping. At the same time, this avoids rust and lime streaks in the sink, which occur after a long period of time due to a precipitation reaction between the oxygen in the air and the iron and manganese dissolved in the hot water .
Limescale deposits occur mainly in carbonated water.
- Ca (HCO 3 ) 2 (lime; dissolved in water) → H 2 O + CO 2 (escapes) + CaCO 3 ( scale )
In pressureless water heaters, the carbon dioxide collects in the top of the container and leads to the escape of gas when hot water is drawn off for the first time after a long period of non-use.
Boiling water device (boiler)
The boiling water device is a special form of the pressureless system . It is not intended to keep water warm for long periods of time, but for discontinuous operation, in which it is only filled with water shortly before use and heated. Therefore it is not insulated against heat loss and is not considered a storage. The advantage is that only the required amount of water is heated and thus energy is saved. In addition, the water can be heated until it boils and can be used immediately to make tea or coffee. The fact that heated water can only be withdrawn after it has been filled manually and waited for heating up complicates operation. The cumbersome operation usually means that less warm and more cold water is tapped, which further reduces energy consumption.
Boiling water devices are intended for installation above the table. They are often used in old buildings where the water connection is in the wall above the sink.
Efficiency
Electrically operated hot water storage tanks use almost 100% of the electrical power when heating. Losses arise through the heat transfer through the thermal insulation and, in the case of under-counter units with ascending lines, also through the circulation that forms within the pipe of the hot water connection.
When assessing the efficiency of electricity-operated hot water boilers, it must be taken into account that electricity generation has very different degrees of efficiency depending on the type of power plant . That of lignite power plants is less than 44%, while hydropower plants have around 90%. Even with a higher degree of efficiency, it must be critically questioned whether the elaborately produced and distributed electrical current is used to hold hot water over long periods of time. For reasons of hygiene , an operating temperature below 60 ° C to save energy with low water consumption is usually ruled out, as microorganisms and bacteria (e.g. Legionella ) can multiply quickly in the lower heat range.
The production of hot water using electrical energy is many times more expensive than with other methods (district heating, oil, gas, biomass). Therefore, electric water heaters are only suitable because of the lower installation costs and the spatially limited heat losses compared to distant hot water taps.
When used infrequently, but constantly on standby, the heat losses are considerable; Depending on the quality, temperature and size, they are approx. 10–110 watts and correspond to the average electrical power required to maintain the temperature. The electricity costs for continuous operation are thus between the typical standby performances of other consumers and those of a cooling device. It is definitely worthwhile to switch off seldom used hot water storage tanks when they are not in use (after work, vacation, etc.), to reduce the temperature setpoint (if bacteriologically unproblematic) or to switch them on with a timer when they are used rarely but repeatedly . However, attention must be paid to a possible risk of frost.
The performance of the boiler is reduced by the limescale deposits that occur during operation. The heat transfer from the heating medium to the domestic water is hindered by the lime. Regular descaling can lead to measurable increases in efficiency.
Dimensioning
The DIN 4708 and DIN EN 12831 standards must be observed when dimensioning heat generators and storage tanks (e.g. for single and multi-family houses).
Web links
Individual evidence
- ↑ a b c Lajos Joos: Energy saving in buildings: state of the art; Development tendencies, pages 260ff, Vulkan-Verlag GmbH, 2004
- ↑ Andreas Weingand: ABC around the mobile home: mobile home technology explained in an understandable way , 5th edition, BoD - Books on Demand, 2012, ISBN 978-3-8370-3736-4 ( limited preview in the Google book search)
- ↑ Suspended storage - comfort flat storage , Austria Email
- ↑ http://www.siemens-home.bsh-group.com/de/produktliste/warmwasser/standspeicher
- ↑ Hot water storage tank: calculate size. Retrieved January 1, 2017 .
- ↑ Prof. Dr.-Ing. Boris Kruppa: Sanitary technology - water supply. (PDF; 4.7 MB) (No longer available online.) Technische Hochschule Mittelhessen, April 5, 2013, archived from the original on January 1, 2017 ; accessed on January 1, 2017 . Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
