Water heating

A Storage heaters, hot water tanks, hot water tanks, etc., often only for a short storage ( hanging storage, small storage, under- table storage, etc.) designated in the budget a fixed installation for the production of warm / hot water (adjustable up to almost 100 ° C). Stores are constantly filled, are continuously heated and insulated. Boiler means any devices that are filled and heated for the moment the hot water extraction and are therefore not insulated. Examples are stoves and hot water heaters.

Also in circulation but are unclear ( at ) heaters and hot water (on ) the terms hot water heaters.

All devices can provide large amounts of water in a short time at high power input.


Small, mostly electrically heated hot water tank with 5 to 15 liters are used primarily for hot water supply at kitchen sinks and washbasins, if a central hot water supply is not available or the connection to it is not worthwhile. This can be the case if the water supply is far away or in seldom-used rooms and it appears energetically unfavorable or prolonged low temperatures hygienically problematic (legionella ). Models are available for under- table or wall mounting, the connected load is usually 2,000 watts. Small gas-fired boiler is also available for RVs and campers; with conventional devices today usually combined with a hot air blower heating.

Medium Boiler with 70 to 200 liters are often used for hot water supply of a residential unit. At a heating temperature of 60 ° to 80 ° with a boiler 80 l may be enough by mixing in cold water for a comfortable temperature even for the filling of a bathtub. They are often found in vollelektrifizierten flats and houses. With sufficient dimensioning a unique heating per day is sufficient. This can be with low tariff electricity ( electric night ) done relatively cheap. You can run them are designed for wall mounting, as stand equipment or deck storage. For some time, there are also flat memory with reduced depth, the boiler consists of an upset tube ring. The electrically connected load is usually 2,000 to 4,000 watts. In this magnitude, there are some models with gas operation.

Less common are even more memory with a few hundred to several thousand liters in basements of single and multi- family homes. They are usually operated with off-peak electricity or bivalent heating.

For electrically heated hot water tanks, the investment costs, operating costs are relatively low, but higher. In systems for individual dwelling units a complex distribution of costs according to consumption is eliminated. The heating temperature is usually at 60 ° to 82 °, rarely up to 95 °. It is usually set to fix depending on the required amount of hot water. If the temperature is increased, it can be removed through addition of more cold water in the faucet longer lukewarm water me the same boiler contents. You can also set lower temperatures, the power dissipation of the device isolation given reduced. With prolonged temperatures below 50 ° C, however, can also increase harmful legionella. This can get especially through finely atomized water droplets in the shower in the lungs. So after the DVGW worksheet W 551 is provided in April 2004, a temperature of at least 60 ° for hot water generation equipment. There are also Legionella circuits where once a week to several times a day the store temperature is raised briefly.

Unlike boilers use pure thermal storage ( hot water tank) energy from external heat sources. There is also combined with multi-fuel heating systems work, so an external source and an electric immersion heater. For example, there are at solar panels despite insufficient sunlight warm water or if more hot water is consumed as the heat pump makes it eliminates the standby heat losses of the boiler during the heating season and a wood boiler must not be extra- fueled.

Contrast, water heater not store hot water. They are sometimes referred to as a boiler anyway. However, electrical small instantaneous water heater for sink can be connected without further due to the higher power consumption for short-term heating of the water quantities in the old building no. Small gas appliances usually operate according to this principle, as well as combination boilers, which have only a small, non-insulated water tank.


Small hot water tank are usually electrically heated, formerly with gas; larger memory mostly with electricity or gas, formerly with solid fuel ( hot water tank ). Newer models have a heater for additional heating by solar energy or a heat pump.


In electrically heated hot water tank is an electric heating element. It consists of a heating element ( resistance wire ) in a protective, corrosion-resistant metal tube. In addition to the heating element is the sensing tube of a temperature- controlled switch for temperature control. Meanwhile setpoint can be adjusted with a rotary knob. The switch is usually an electro-mechanical system in which the difference in thermal expansion coefficients of different materials can be used (e.g., glass rod into a metallic tube ) together with a snap-action switch. Often a liquid is used which operates on a tube onto a membrane. In addition, located in the vicinity of the heating element, a temperature switch that prevents failure in the controller of fire and destruction. This thermal fuse can be reset or it must be replaced after the response.

Temperature sensor and heater are located in a vessel made of copper, plastic, or - at pressure boilers - made ​​of stainless steel sheet, galvanized sheet steel ( for corrosion protection with a sacrificial anode made ​​of an aluminum -magnesium alloy fitted ) or enameled steel.

The cold tap water is introduced into the vessel down; the hot water is removed in the upper region of the vessel. This is necessary because the warm water due to its lower specific gravity gathers always on top and can be found there. The tubes are labeled accordingly outside color (blue for the inflow and red for the schedule) or with arrows.

Modern hot water tank almost always have a thermal insulation.

There are two fundamentally different hot water storage types:

Closed or flameproof hot water tank

A hot water tank with a closed system, has strong walls and keeps the line pressure up to 10 bar stood easily. In the worst case, he is constantly under the pressure. To prevent hot water from flowing back into the supply line, this memory must be installed with a check valve. To protect against overpressure due to the heating and as an outlet for the expanding when heated water is therefore a safety valve must be fitted, which must be constantly drips when heated and discharged directly into the sewer. The classical capacity of this type is from 30 to 120 liters. A pressure-tight displacement hot water tank is more expensive than a non-pressurized 5 -liter displacement hot water tank. He finds application, for example in the bathroom for cheap old building renovations as a group can provide several taps or for the central housing hot water source.

Open or non-pressurized hot water tank (low pressure system )

This approximately 5 to 15 liter storage usually supply an object ( Single Supply ): If you open the hot water tap the mixer, it is introduced into the bottom part of the hot water tank cold water, so that in the container hot water up from the reservoir ( = non-pressure displacement ) is forced to permanently open outlet of the mixer. Such mixers are so-called low-pressure mixing batteries and always have three connecting hoses. They are absolutely necessary for the operation of such a non-pressurized water heater.

There are non-pressurized hot water tank for mounting above or below the withdrawal point ( mixer ). They are referred to as superstructure or substructure water heaters. They are also available in larger sizes to supply a bathroom. Non-pressurized water heaters are relatively simple and inexpensive, since they are not (up to 10 bar) exposed to the water pressure in the water pipe.

Because water expands when heated, the connection between the outlet and water heaters must not be shut off. Heats the water heater, the water target temperature, so it expands from. This step small amounts of water on the - from the outlet of the mixer - as mentioned above permanently open. The drops of water taps to open store is inherent in the system - it does not constitute a defect and can not be prevented by stronger Crackin '.

Special designs which accommodate the expansion of water in a conservator of the water heater, avoid detrimental drops and often associated with rust and lime stains in the sink, caused by a precipitation reaction of atmospheric oxygen with the iron and manganese dissolved in hot water.

Calcium deposits occur mainly in carbonated water.

In non-pressurized water heaters, the carbon dioxide collects the top of the container and explains the escape of gas for hot water withdrawal after prolonged non-use.

Water boiler (boiler)

A special form of non-pressurized system is the water boiler dar. It is not built to store hot water, but the water is heated just before use it. Therefore it is not insulated against heat loss and is not regarded as a memory. Unlike storage devices, it is built for discontinuous operation, since it must be refilled and heated constantly, while a memory remains throughout all filled with water and heated at all times.


Electrically operated water heater use of almost 100% of the electric power at the heating. Losses caused by heat transfer through the thermal insulation. It must also be taken into account when assessing the efficiency of current- powered hot water boilers that power generation has a different efficiency depending on the type of power plant, in Germany this is about 30 %. Even with a higher efficiency is critical to question, to use the lavishly produced and distributed electricity in the Provision of warm water for long periods. For hygiene reasons, a reduced below 60 ° C operating temperature for energy saving with low water consumption differs from most because otherwise microorganisms and bacteria (eg legionella) can multiply in the hot water tank very quickly.

The hot water by electric energy is many times more expensive than other methods (district heating, oil, gas, biomass ). Therefore, electric water heaters are only due to lower installation costs and the localized heat losses towards distant hot water taps.

If rarely use, but constant readiness, the heat losses are considerable; they are, depending on quality, temperature and size of about 10-50 watts and correspond to the required temperature to maintain the average electrical power. The current cost for continuous operation are thus between typical standby services of other consumer and those of a refrigerator. It is quite rewarding, rarely used hot water tank when not in use ( business, holidays, etc.) off, ( bacteriologically unproblematic if any) to switch to reduce the temperature setpoint or even in rare but time- definite, repetitive use by a timer.

The generated during operation deposition of lime reduces the efficiency of the boiler. The heat transfer from the heating medium for the hot water is hindered by the lime. Regular descaling measurable increases in efficiency can be achieved.