Water heating#Tankless heaters

A tankless water heater is a permanently installed device for heating water. In contrast to the boiler, which provides a limited amount of hot water in a tank, a tankless water heater heats the water only when a faucet is opened. Due to the high heat capacity, a water heater continuously generate hot water. Water heater are mostly for the decentralized hot water supply (eg in the bathroom, less frequently in the kitchen ) are used. They are not suitable for the preparation of boiling water. Does the equipment at the same time to operate the heating, so it is a combination boiler. Current models also allow connection to a solar system.

• 5.1 Operation with current
• 5.2 Operation with natural gas

Principle of operation

If the water is drawn off at the tap, the unit detects the flow. The flowing water is heated. If the water supply is interrupted, for example by closing the hot water tap, the power supply must be switched off immediately. Not this happen or seriously delayed, the energy supplied quantities can no longer be dissipated by the flowing water, the water is overheated. In such an incident, the unit is set by a safety cut out of service and must be taken by a qualified person again. As a further safety measures and safety pressure switches are used.

The length of the water line between the water heater and the tapping point (s) should be kept as short as possible, since the water standing in the pipe cools between the tapping cycles and has to flow only until the desired outlet temperature is reached.

Water heater after the type of energy used, how they work, energy conversion heat exchanger principle and differentiated according to the intended application.

So there is the gas-powered and electric water heater, which in turn are divided according to hydraulic and electronic detection of water flow. In electric water heaters is additionally according to the principles of heat transfer to the water in differentiated tubular heaters and bare wire devices as well as controlled or regulated behavior.

The detection of the flow in the device by means of a differential pressure switch with hydraulic equipment and means of a turbine flowmeter in the electronic. The differential pressure switch is purely mechanical. It registers the pressure difference at the pins, and turns on the power supply. Depending on flow rate and pressure conditions, the membrane will more or less deflected and multiple power levels can be realized. In practice, however, only two stages are typical. The turbine flow meter is required for the evaluation of the flow rate, an electronic module, but it has the advantage that the analysis can be carried out independently of the pressure conditions and steps. Both systems require a minimum flow rate to detect a water withdrawal and not to bring the small amount of water, even at the lowest possible power output to a boil.

Heat transfer to the water is performed through a heat exchanger, such as an electric tubular heaters, inside a container with two water connections. The water flows around the tube heater and is heated thereby. Heat exchangers are usually made ​​of copper because copper is a good conductor of heat. The electrical devices can be dispensed with a heat exchanger. The bare wire devices in contrast to the tubular heating elements heating coils are bathed directly from the water.

Types by fuel

Electric water heater

For the fast heating of the water, a large electric power is required. Usual in the market are water heater with 18, 21, 24, 27 and 33 kW power rating.

The higher the heating power, the more water can be heated to a desired temperature. For a chilled water temperature at 11 ° C, the rule of thumb for heating drinking water to bath or shower temperature of 38 ° C is common:

In hydraulically controlled instantaneous heaters the water temperature usually decreases with increasing flow rate. Electronically controlled systems, however, offer over the entire operating range of a constant output temperature.

Hydraulically controlled instantaneous water heaters

For models with hydraulic control heating takes place with up to three levels of heating power. The smallest amounts of water can not be heated with hydraulic instantaneous water heaters, because to turn on a minimum flow rate is required. The addition of cold water is therefore useful only within limits. Runs too little hot water, the water heater shuts off completely.

Electronically controlled instantaneous water heater

In the electronic instantaneous water heaters can select the water temperature at the device. The Dazu mixture of cold water can be dispensed with. But even if a mixing valve is used, even a thermostatically controlled, no longer subject to real electronically controlled instantaneous water heater to the limitations of the hydraulic water heater for which you needed to explicitly suitable for instantaneous heater thermostatic mixing valves.

Benefits are increased comfort and a slightly better efficiency, which enables energy savings of up to 20% compared to hydraulic instantaneous water heaters, because even small amounts of water are only as strong heated as necessary. The heating is largely variable.

The water is usually heated by a Blankdrahtheizelement, in which the heating wires are directly surrounded by water. Tap water is usually a good insulator, so that up to a certain apparent on the device conductance no significant fault currents can occur. The bare wire heating elements have in addition to the higher efficiency also has the advantage of a much shorter reaction and therefore heat-up time compared with the indirectly heated tube radiators used previously. However, a disadvantage is the higher susceptibility to entrapped air and vapor bubbles, which can lead to a partial overheating of the heating wires.

Fully electronic instantaneous water heater outlet temperature can be kept constant even over the power limit, by reducing by an electronically controlled motor valve, the flow rate to the extent that the set desired temperature is reached. The world's first fully electronic water heater was developed in 1987 by Stiebel Eltron and patented.

There are also as "electronic" advertised Heater without expensive power electronics, with only an electronically controlled motor valve. By controlling the flow rate can be kept constant with the temperature limitations, therefore, such devices have similar limitations as the hydraulically controlled instantaneous water heater on.

Electrical connection

Due to the high power water heaters are provided almost exclusively for connection to three-phase AC power. Due to the high required currents, they are useful in many older buildings only after replacement of housing or house connection to the mains. If the pipe is too small, a house fire may result.

Water heater may only be installed by trained personnel. In some areas, a permit from the respective utility company must be obtained before an electric water heater is installed.

Small Heater

As a small water heater devices are referred to with a capacity of 3.5 to 6.5 kilowatts. These are used to supply individual taps.

Because of the low electric power is also receives only a small hot water performance: A 3.5 kW device obtained at a flow rate of 2 liters per minute, a temperature rise of 25 Kelvin. These units are suitable for places where small amounts of hot water are needed with a not so high temperature, such as in the guest bathroom. You are a sensible alternative to little-used, classic 5 -liter small stores, since no standby losses. The low water flow is masked by beamformer with narrow nozzle at the outlet of the valve. For small water heaters is the necessary electrical fuse max. 16 amps (at 230 V ) must be ensured. The cross section of the line should be at least 2.5 mm ².

Gas water heater

With a gas water heater is either ignited and operated by a constantly burning pilot light or a self- employed zuschaltenden piezo ignition, a gas burner when needed. The water to be heated is passed into tubes with fins by means of the flames and thereby heated (see heat exchanger ). The latest generation of gas water heaters come in igniting without additional power connection: here, a battery or generator ignition is used (using the flow of energy from the flowing water); but this is only for models for exclusive hot water of interest as combination boilers anyway require power because of the circulation pump for the heating circuit. Depending on the structural situation and depending on the performance of the device, it is necessary to connect the water heater to a chimney. Otherwise, a fresh air supply and an exhaust gas discharge through an exterior wall to the outside enough.

Gas water heaters are operated for the most part on natural gas, if an adequate supply is available; addition also LPG from bottles or fixed installations can be burned, such as when no gas connection is available. To change the type of gas, the nozzle must be changed only by the installer.

Gas water heaters are usually much larger than their electric counterparts.

Efficiency and application areas

The electric water heater is the electrical energy with an efficiency of up to 99% for water heating. The remaining energy are conduction losses in the form of heat and radiation losses of the device. However, the nature of the electric power generation and its distribution are subject to the house connection with significant losses. This results in a much lower total efficiency obtained based on the primary energy.

Gas water heaters have an overall efficiency of about 75 to 85%, which means that from a paid kilowatt hour loses only about 15 to 25 %. These losses consist of waste heat through the chimney and radiation losses of the device.

Comparing the method to be used for both primary energy, so with the same hot water output is with an electric water heater about three times as much primary energy as the gas appliance (electricity from a steam power plant accepted).

Water heater can save energy and water, as they usually heat the water near the point of consumption. Compared with a central hot water supply with hot water tank, the flow losses ( "to warm water is" ) and the heat losses through pipes and the electrical energy required for circulation pumps are reduced. Also usually reduces the amount of hot water piping to be installed. This is particularly at off -lying consuming units with a small decrease (eg, guest toilet ) noticeable, so that here the use of power operated equipment is justified.

Gas-fired water heater and central hot water with boilers can be significantly more energy efficient and therefore cheaper to run with regular and high hot water consumption. To find, for example, gas-fired combined Warmwasser-/Heizungs-Durchlauferhitzer as floor heating in apartment buildings use.

Maintenance costs

The heating components of electric water heater not require that. 'Gas appliances to annual maintenance and cleaning of the combustion chambers In addition, the gas appliances are regularly checked by the chimney sweep.

The water side must be subjected in either case, a maintenance as are deposited there with the time scale, which should be removed professionally. If such decalcification, the heat transfer deteriorates, so the efficiency drops.

Electric water heater with bare heating elements are considered to be maintenance-free and do not need to be descaled. Due to the extremely low mass heating element (wire) remains hardly decay heat after shut down, which is the main reason for the reduction in the scale precipitates. Furthermore, due to the expansion ( heating ) and contraction ( cooling) of the heating wire broken off too large a scale covering.

Consumption and maintenance costs

These examples made ​​on the basis of current costs in Germany.

Operation with current

Usually, electric water heater be operated with household current. The costs amounted to 2012 about 26.4 cents / kWh.

Operation with natural gas

The energy cost of natural gas amount to about 6 cents / kWh ( 2012). The cost of hot water from a natural gas fired water heater set up as follows:

• Chimney Maintenance
• Fee / rent for the gas meter ( pro rata)
• Electricity costs for the operation of the gas flow heater
• Cost of natural gas

If the overall efficiency is included cuts in the consumption cost of natural gas-powered water heater cheaper than from an electric water heater.