Instantaneous water heater
A flow heater , flow water heater , geyser or flow water heater (DWH) is a permanently installed device for hot water preparation . In contrast to the boiler , which provides a limited amount of hot water as a storage tank, a flow heater only heats the water when a mixer tap is opened. In order to bring the flowing water to the desired temperature immediately, a flow heater needs a high heat output.
Instantaneous water heaters are not suitable for preparing boiling water. Newer instantaneous water heaters can also be operated with preheated water, which was generated, for example, by a solar system and is then reheated to the desired temperature, if necessary.
Combination heaters are heating boilers that have a second heat exchanger that, like a flow heater, can continuously produce hot water.
A normal instantaneous water heater can supply at most one shower or two wash basins at the same time, so that instantaneous water heaters usually decentralized, i. H. used in the vicinity of the extraction point. In 1961, Vaillant was the first company to bring a wall-mounted heater onto the market: the “Circo-Geyser MAG-C 20”, which is why “Geyser” is used as a generic name for instantaneous water heaters in specialist circles .
Working principle
If the water is drawn from the tapping point, the device detects the flow and heats the water flowing through. If the water supply is interrupted by turning off the hot water tap, the energy supply must be switched off immediately. If this happens after a delay, the energy supplied cannot be dissipated by the water flowing through and the stagnant water overheats. In the event of a malfunction , the device is switched off by a safety temperature limiter and must be reset by a skilled craftsman after the cause of the error has been eliminated. A safety pressure switch, which is arranged between the non-return valve (cold water inlet) and the extraction valve, can serve as further safety measures. If the control fails, steam pressure builds up to which the safety pressure switch responds and interrupts the power supply until it is manually reset. In the picture below this is the central contact block with the red button.
The length of the water pipe between the water heater and the tap (s) should be kept short, as the water in the pipe cools down between the tapping processes and flows away unused when the water is drawn again until the desired outlet temperature is reached.
Instantaneous water heaters are differentiated according to the type of energy used, mode of operation (energy conversion, principle of the heat exchanger ) and the intended area of application.
Gas-powered and electric water heaters, which are equipped with either hydraulic or electronic detection of the water flow, are common. Electric instantaneous water heaters can be further divided into tubular heating elements and bare wire devices as well as according to controlled or regulated behavior.
The flow rate in the device is detected using a differential pressure switch in hydraulic devices and a turbine flow meter in electronic devices. The differential pressure switch works mechanically. It registers the difference in pressure when tapping and switches on the energy supply. Depending on the flow speed and pressure conditions, the membrane is deflected more strongly, which means that several power levels can be switched. In practice, however, only two stages are common. The turbine flow meter contains an electronic assembly for evaluating the flow rate, which allows continuous measurement regardless of the pressure conditions. Both systems require a minimum flow rate in order to perceive a withdrawal of water and to avoid the boiling of the water at the lowest intended power output.
The heat is transferred to the water through a heat exchanger , which is usually made of copper, as copper is a good conductor of heat and does not corrode.
Types by type of energy
Electric water heater
Operating conditions
High electrical power is required to warm up the water quickly . Market are usual water heater with 18, 21, 24, 27 and 33 kW connection value . So-called compact instantaneous water heaters have a connected load of 11 or 13.5 kW (also switchable). Plug-in single-phase devices have only 2.2 to 3.5 kW due to the usual load capacity of an earthed socket and are only suitable for operating a hand wash basin.
The higher the heating output, the more water can be heated to a desired temperature. The following rule of thumb is common for heating drinking water to a bath or shower temperature of 38 ° C based on a cold water temperature of 11 ° C :
With hydraulically controlled instantaneous water heaters, the temperature of the water usually fluctuates with the flow rate. Electronically controlled devices, on the other hand, offer a constant output temperature over the entire operating range.
Older devices contain an electric tubular heater that is located inside a container through which the water flows. Many modern devices have bare wire heating elements , the heating wires of which are washed directly by the water. Tap water has an isolating effect on the live wires, so that (up to the conductivity value of the locally available drinking water specified on the device label ) no impermissibly high fault currents occur. The heating elements are separated from the connected pipes by a pre-run and post-run section, so that there is a sufficiently long, insulating water path. Bare wire heating elements have the advantage over the previously used tubular heating elements of a much shorter reaction and thus heating time, and they do not tend to build up limescale . The disadvantage is the higher susceptibility to air inclusions and vapor bubbles, which can lead to partial overheating of the heating wires.
Hydraulically controlled instantaneous water heater
In models with hydraulic control, the heating takes place with up to three heat output levels. Smallest amounts of water cannot be heated with hydraulic instantaneous water heaters, because a minimum flow rate is required for switching on. Mixing cold water is therefore only useful to a limited extent. If there is not enough hot water running, the instantaneous water heater switches off completely.
Electronic water heater
TW = outlet to the tapping points
TWW = supply line with shut-off valve
L1 / L2 / L3 / PE = electrical connections
1) heating cartridge
2) pressure switch
3) central fastening screw
The electrical terminal connections must be tightened firmly, otherwise the contact points can become very hot: At connection L2 The core insulation is visibly melted onto the upper clamp connection of this device.
With electronic devices, the heating power is continuously controlled by means of power electronics. The high operating currents do not allow control via phase control , because such a considerable distortion reactive power would result. Instead, a vibration packet control is used. The power semiconductors are cooled with the cold water supply. Adding cold water to these devices can u. U. omitted. But even if a mixer fitting is used, even a thermostatically controlled one, electronic instantaneous water heaters are no longer subject to the restrictions of hydraulic instantaneous water heaters, for which thermostatic mixer fittings explicitly suitable for instantaneous water heaters would be required.
With the electronically controlled instantaneous water heaters, the heating output is continuously electronically controlled based on a flow measurement (turbine wheel) and the measured inlet temperature.
Electronically controlled instantaneous water heaters also work with a temperature sensor on the outlet and regulate the outlet temperature. There may be an external setpoint potentiometer.
The advantages of electronic variants are increased comfort and energy savings of up to around 30 percent compared to hydraulic instantaneous water heaters, since the flow rate has a lower minimum and the setpoint is reached more quickly and more accurately.
The power control can only keep the temperature constant up to a maximum flow rate, then it drops. Therefore, some devices have a motorized valve, which in this case limits the flow rate in order to be able to maintain the temperature.
Electrical connection
Due to the high power consumption , instantaneous water heaters are usually intended for connection to the three-phase network . Because of the high currents, the connection lines have to be renewed for installation in many old buildings . Lines that are too small represent a fire hazard .
Instantaneous water heaters may only be connected by a qualified electrician. In some areas of Germany, approval must be obtained from the relevant energy supply company before an electrical instantaneous water heater is installed.
Instantaneous water heaters are three-phase current consumers with symmetrical loads, so that a connection to the neutral conductor of the power grid is usually not provided.
Small instantaneous water heater
Small instantaneous water heaters are devices with an output of 3.5–6.5 kilowatts that are used to supply individual tapping points . The low electrical output only leads to a low hot water output: A 3.5 kW device achieves a temperature increase of 25 Kelvin at a flow rate of 2 liters per minute . These devices are suitable for places where small amounts of warm water with a lower temperature are required, such as in the guest toilet. They are a sensible alternative to little-used, classic 5-liter hot water boilers , as there are no standby losses. However, only a 16 A fuse is required, as is otherwise usual. It is connected to a socket, which means that no qualified electrician is required for installation. The low water flow rate is by beamformer at the outlet of the armature laminated.
Gas water heater
In the case of a gas flow heater , a gas burner is ignited and operated when required , either by a constantly burning pilot flame or an automatically activated piezoelectric igniter . The water to be heated is usually passed through copper pipe coils, which are connected to sheet metal fins for improved heat transfer , through which the heated fuel gases flow (see also heat exchangers ).
The latest generation of gas water heaters does not require the pilot flame that was used in the past, nor a power connection for electrical ignition. They contain a battery or generator ignition, which uses the flow energy of the flowing water.
It is usually necessary to connect the water heater to a chimney . If the device does not perform well, in exceptional cases it may be sufficient to supply fresh air and discharge the exhaust gases through an external wall.
Gas instant water heaters are operated with natural gas if a gas connection is available. Liquefied gas from cylinders or stationary systems is used less often. The installer changes the gas type by changing the nozzle.
Gas instantaneous water heaters require significantly more space for installation than their electric counterparts.
Fresh water station
In a so-called fresh water station , the energy required to heat the drinking water is supplied by a heating circuit, the medium of which is heated elsewhere. Solar collectors or district heating serve as energy sources . A fresh water station requires very little space if it only contains a plate heat exchanger. Often they also contain drainage and shut-off fittings, as well as temperature sensors and, if necessary, heat meters.
Efficiency
The electric instantaneous water heater converts the electrical energy into water heating with an efficiency of up to 99 percent. There are only minor radiation losses from the device and conduction losses in the form of heat. However, the generation of electrical power and distribution up to the house connection is subject to considerable losses. With primary energy sources such as coal, natural gas, oil or nuclear energy, this results in an overall efficiency of only 30–40 percent.
Gas water heaters have an overall efficiency of around 75-85 percent. These losses are made up of the waste heat escaping through the chimney and the radiation losses from the device.
With the same hot water output, the gas appliance therefore only needs a third of the primary energy amount of an electric instantaneous water heater that is operated with electricity from fossil fuels .
Instantaneous water heaters installed near the point of consumption can be used to save energy and water compared to a central hot water supply, since with the latter, the drainage losses (until hot water comes in), the heat losses through pipes and hot water storage tanks and the electrical energy requirement for circulation pumps must be taken into account. The scope of the hot water installation is also reduced. The effect is particularly noticeable in remote consumption points with infrequent use or low purchase quantities (e.g. guest toilet), so that the use of electricity-operated devices can often be justified here.
In contrast, gas-powered instantaneous water heaters and central water heaters with boilers are more energy-efficient and cost-effective to operate with regular and higher hot water requirements . Gas-powered combined hot water / heating instantaneous water heaters are often used as floor heating in apartment buildings.
Maintenance costs
The heating components of electric instant water heaters do not require any maintenance.
It is often recommended to have gas-powered devices serviced annually , with the combustion chambers being cleaned. Gas appliances are usually checked by the chimney sweep every two years .
Depending on the exact composition, it may be necessary to the depositing in calcareous water scale removing professionally. If the decalcification is not carried out , the degree of efficiency also deteriorates with the heat transfer .
Electric instantaneous water heaters with bare wire heating elements are considered maintenance-free and do not need to be descaled. Because of the particularly low-mass heating elements (wire), there is hardly any residual heat left after switching off, which is the main reason for the reduction in scale deposits. As the heating wire expands when heated, a scale deposit is regularly blown off.
Consumption and maintenance costs
These examples are based on the usual costs in Germany .
Operation with electricity
Electric water heaters are usually operated with household electricity. The power consumption of a flow heater can be roughly determined based on the maximum output of the device and the service life. A water heater with a power consumption of 18 kilowatts (kW) will consume 18 kilowatt hours (kWh) in one hour at full power . If four people shower ten minutes at full power each day, this results in a consumption of around 4400 kWh per year. An average price of 28 cents per kWh would result in annual costs of € 1,226.
Operation with natural gas
The energy costs of natural gas are around 6 cents per kWh. The costs for hot water from a water heater operated with natural gas are made up as follows:
- Chimney sweep and maintenance (approx. € 60 per year for individual devices; reduced proportionally if additional gas devices are available)
- Basic charge for the gas meter (pro rata if there are several gas devices)
- Electricity costs for operating the gas water heater (according to the example above, 240 operating hours per year with around 100 watts of electrical power, i.e. 24 kWh per year at € 0.28 per kWh results in € 7.80 per year)
- Costs for natural gas (according to the above example 4400 kWh with an assumed efficiency of 65% result in 6740 kWh gas; at 6 ct per kWh there are costs of 404 € per year)
With a sufficiently high proportion of electricity from renewable sources ( electricity mix ), the electric instantaneous water heater has a more favorable CO 2 balance . The pure energy costs for a natural gas-powered instantaneous water heater are significantly lower and, even if the gas instantaneous water heater has a poor efficiency of less than 70 percent, only around a third of the electricity costs. If, overall, only little hot water is required, the higher purchase, installation and maintenance costs of gas appliances can compensate for this advantage.
literature
- Hermann Recknagel, Eberhard Sprenger, Ernst-Rudolf Schramek (eds.): Pocket book for heating and air conditioning 03/04 . including hot water and refrigeration technology. Oldenbourg Industrieverlag, Munich 2003, ISBN 3-486-26534-2 , 4.2.1 Individual and group supply, p. 1699-1710 .
- Karl Volger, Erhard Laasch (edit.): House technology. Basics, planning, execution . 10th edition. Teubner, 1999, ISBN 3-519-15265-7 .
Web links
Individual evidence
- ↑ Lajos Joos: Energy saving in buildings: state of the art; Development tendencies , Vulkan-Verlag GmbH, 2004.
- ↑ a b Ernst-Rudolf Schramek, Eberhard Sprenger, Hermann Recknagel: Pocket book for heating and air conditioning. 03/04. Verlag Oldenbourg, 2002, ISBN 3-486-26534-2 , pp. 1699-1710.
- ↑ TomTank: How does an electric water heater work. In: SBZ fitter. Alfons W. Gentner Verlag, March 8, 2013, ISSN 0342-8206 (sbz-monteur.de , accessed October 21, 2018)
- ↑ Which heating systems are there for instantaneous water heaters? Corporate typeface from Stiebel-Eltron . (stiebel-eltron.at accessed on Oct. 21, 2018)
- ↑ Dimplex service manual (PDF)
- ↑ Savings potential for instantaneous water heaters. Stiebel Eltron company typeface. (stiebel-eltron.de accessed on Oct. 21, 2018)
- ↑ From fully electronic to electronically controlled instantaneous water heaters. Stiebel Eltron company publication on fully electronic DLE. (stiebel-eltron.at , accessed on Oct. 21, 2018)
- ↑ Gas instant water heater. In: www.durchlauferhitzer-wissen.de. Retrieved March 25, 2018 .
- ↑ Power consumption of instantaneous water heaters. (PDF; 113 kB) In: energieexperten.org. December 29, 2009. Retrieved October 14, 2012 .
- ↑ Electric instantaneous water heaters cost a third of the electricity costs. In: http://www.octobernews.de/ . January 15, 2015, accessed February 11, 2016 .
- ↑ Water heater power consumption. Power consumption and cost calculator. In: www.durchlauferhitzer-wissen.de. November 15, 2016, accessed March 16, 2018 .