Hot water

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Warm water is heated drinking or service water in the temperature range of usually 30 ° C to 60 ° C, in addition one speaks of hot water . A water temperature of 35 ° C is usually perceived as lukewarm , while the risk of scalding begins with infants at 38 ° C (see below ). Thermostatically controlled mixing valves in apartments are usually set to a normal temperature of 40 ° C. With the help of a mixer tap , the water temperature can be adjusted between the temperature of the incoming cold and hot water.

The cold water used for hot water preparation has the temperature of the ground at a depth of 1 to 3 meters, i.e. around 10 ° C. If it stands longer in the line, it adopts room temperature.

The provision of hot water is part of normal living comfort in industrialized countries today .


While most private households met their need for hot water by heating on the stove or in the watercraft and later with bath stoves until about the middle of the 20th century , central hot water storage tanks and electric or city ​​gas- operated instantaneous water heaters , which generated hot water, then became popular. For daily use (e.g. washing up , hand washing ) or for bathing, the highly heated water had to be mixed with cold water. In the case of fuel-operated water heaters, possible safety deficiencies in the air supply, gas lines or exhaust flues mean that these devices may only be used to a limited extent in living spaces .

Today, convenient, automated water heating is standard in developed industrial countries. In the last few decades, the energy for warming was mainly provided by burning oil or gas. In countries with very low electricity prices (France, Scandinavian countries), electric heating is also widespread. Thermal solar systems , wood boilers , heat pump systems and waste water heat recovery have been gaining in importance since the early 2000s .

The use of solar energy has a longer history than is often assumed. It is said that Archimedes already had around 250 BC. A heating system with focal mirrors was built in Syracuse , and in southern countries the rapid warming of dark containers has long been used. Since the turn of the last century, allotment gardeners have been tinkering their own primitive systems based on a dark-painted barrel or a hose lying in the sun . Even with a simple garden hose, you can enjoy a short "dripping bath" after a quarter of an hour in the sun. In countries like Israel or Greece there is almost no house roof that does not have a thermosiphon collector with a hot water storage tank that works without a pump according to the gravity circulation principle .

Today's hot water systems

If a modern hot water system for our latitudes is to work satisfactorily in winter too, it primarily requires the following components:

The heat supply for modern, health-conscious commercial operations also requires a gradation of the water temperature and, for reasons of environmental protection, often also that of the quality. Since a central heating system is more efficient if the lines are not too long , it should meet the following requirements:

The heat generation (hot water, steam ) is mostly done with boilers or systems with combined heat and power (CHP). With the above layout, the system usually includes several hot water boilers , partially separate pipes, fittings and a suitable chimney, etc. In addition to efficient individual components, optimal operation also requires an optimal combination of the individual system parts.

Drinking water heating systems

The German Drinking Water Ordinance in Section 13 of the version that has been in force since November 1, 2011 contains the concept of the system for "heating drinking water according to the definition of generally recognized rules of technology" . In order to monitor these systems with regard to any pathogens (see Legionella ), the operation of drinking water heating systems has been subject to notification to the health department since November 2011 . With the second amendment to the Drinking Water Ordinance in December 2012, the reporting requirement for existing systems was reversed. Now only systems with commercial and public use and large systems for heating drinking water have to be reported, the technical measure value of which exceeds 100 CFU (colony-forming units) per 100 ml.

Hygienic requirements

The articles drinking water hygiene , drinking water # hygienic aspects , drinking water # hygienic requirements , hot water # hygienic requirements , legionella and VDI / DVGW 6023 overlap thematically. Help me to better differentiate or merge the articles (→  instructions ) . To do this, take part in the relevant redundancy discussion . Please remove this module only after the redundancy has been completely processed and do not forget to include the relevant entry on the redundancy discussion page{{ Done | 1 = ~~~~}}to mark. Kai Kemmann ( discussion ) - Improving instead of deleting - 04:57, 11 Feb 2020 (CET)

EN 806-2 states that 30 seconds after opening a tap, the hot water temperature should not fall below 60 ° C, provided that other local or national regulations do not conflict with this, as harmful germs generally cannot multiply at this temperature. In addition, the hot water at each tap should be able to be heated to 70 ° C in order to thermally disinfect pipes and fittings in this way.

In Germany, according to the national supplementary standard DIN 1988-200, it is sufficient if the hot water temperature reaches 55 ° C 30 seconds after opening a tap. In this case, however, it is not a question of a “target” determination, but rather a determination.

It should be noted that the precipitation of lime from drinking water increases significantly at temperatures above 55 ° C. However, thicker deposits are usually only to be expected in galvanized steel pipes , which are no longer used for drinking water installations today.

Scald protection

DIN EN 806-2 requires a maximum temperature of the heated water at draw-off points for general use of 45 ° C for public buildings. In nursing homes and facilities for children, the temperature should generally not exceed 43 ° C and in showers 38 ° C. VDI 3818 generally recommends 40 ° C for public baths and toilets. The technical implementation is complicated by the fact that not only hot water storage tanks , but also hot water pipes should be operated at 55–60 ° C in order to prevent legionella from multiplying . Water at over 60 ° C can cause scalds within seconds, while water at 50 ° C only takes about two minutes.

A simple way of ensuring protection against scalding is to use thermostatically controlled mixer taps at each tap , which can limit the water temperature to around 40 ° C. Single-lever mixers according to DIN EN 817 are approved for residential buildings . This allows the hot water temperature in the supply line to be kept at the temperature level required to combat legionella by using a simple circulation line . The disadvantage is the cost of a larger number of thermostatically controlled mixing valves , which have a lower life expectancy than conventional outlet fittings .

It is more efficient to use a central thermostatic mixing valve in the outlet of the hot water tank or (unregulated) hot water heater , which limits the water temperature by adding cold water. In order to reach the temperatures of 55 to 60 ° C required to combat Legionella, however, special measures are required, such as temporarily heating the pipe system at night. If a circulation line is installed, the circulation pump must be switched off or reduced when the critical temperature is reached. A constant run of the circulation pump is possible if the return of the circulation is divided and at the same time connected to the hot water storage tank as well as to the cold water connection of the thermostatic valve. This enables the thermostatic mixing valve to minimize the supply of hot boiler water and largely limit the circulation to the pipeline system.

If the content of the hot water pipe from the hot water generator to the furthest point does not exceed 3 liters, it may be possible to dispense with heating the hot water pipes for disinfection.

save energy

By reducing the temperature of hot water storage tanks, energy consumption can be reduced significantly. This also means that there is no need to install a thermostatic mixing valve for adding cold water to protect against scalding .

40 ° C is sufficient as the service water temperature for showering and bathing. The higher the temperature in the hot water tank, the higher the radiation losses from the boiler (for example, a heat loss of 3% from 75 ° C is more than 3% from 45 ° C). However, it must be ensured that the boiler is heated to over 60 ° C at least once a week and that the pipes are also rinsed with this hot water in order to reduce the risk of legionella multiplying in the boiler and pipes.

Circulation pumps have come under fire because they often increase energy costs disproportionately with only moderate water savings. The hot water circulating in the pipe cools down permanently despite the thermal insulation of the hot water and circulation pipes. While the heat output in winter can still relieve the building heating system, it leads to unnecessary energy losses in summer. Since a circulation system basically has the same effect in the masonry as a heating pipe, it can be seen that outside of the heating season the masonry is unnecessarily heated and energy is wasted. This can be remedied by a switching line with which the circulation pump is switched on for a few minutes (using a manually operated button or connected to the lighting in a wet room) before you take a shower, before you use the toilet (and then wash your hands ) or before starting a manual wash in the kitchen. A useful alternative is a "circulation system without a circulation line", in which an existing cold water line is used as a return line for the circulation until the hot water target temperature is reached at the tap, which means that warm water is no longer directed into the drain.

Circulation systems are only recommended for long pipelines, which would otherwise cause a waiting time of over 20 seconds for the heated water to arrive. With the length of the line paths, however, the energy losses also increase. Therefore, installations and the line network should preferably be designed in such a way that line lengths and cross-sections are minimized. The smaller the pipe volume, the faster the heated water reaches the tap and the circulation may be unnecessary. A pipe with a nominal diameter of 15 millimeters is usually sufficient to supply hot water to an average apartment with a bathroom . If a pipe between the water heater and tap has a volume of more than 3 liters, increased requirements for legionella protection must be observed. This can be done through pipe heating or circulation. If a circulation pipe cannot be avoided, hot water and circulation pipes must be insulated as carefully and as possible. Menu-controlled and self-optimizing digital controls ("Zirkomat") use temperature sensors on the hot water line to determine the usual consumption pattern and only activate the circulation pump specifically when hot water is expected to be drawn off. During the night, for example, the circulation can remain switched off. A simpler option is to control the pump using a timer .

An alternative to circulation can be decentralized hot water preparation using instantaneous water heaters or smaller water heaters . With instantaneous water heaters, there are no production losses because the water is only heated when it is needed.

In the case of centralized hot water preparation via a boiler , the fuel losses due to frequent burner starts (so-called cycling ) must be taken into account (see also the annual degree of utilization and the degree of efficiency and degree of utilization of the condensing boiler ). The better the output of the boiler is adapted to the actual consumption, the lower the losses.

Approaches to reducing the heating energy consumption for hot and service water already existed at the time of the first energy crisis in the 1970s. However, the technical development was not that advanced and the market for integrated solutions was too small. Tried and tested solar energy technologies were not yet available. Anyone who already had a hot water storage tank back then - when the price of oil roughly tripled - could at least save energy through better thermal insulation , lower water temperatures and night-time lowering of the heating system.

Today systems for local waste water heat recovery are offered, which make it possible to save a significant part of the primary energy consumption for hot water.

See also

Web links

Individual evidence

  1. LGL Bayern: FAQ "Notifiable systems according to the changed drinking water ordinance"  ( page no longer available , search in web archivesInfo: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice.@1@ 2Template: Toter Link /  
  2. Overview of the planning of drinking water installations and the set of rules according to DIN EN 806-2 and DIN 1988-200 in the publication "Sanitärtechnisches Symposium 2010" of the Central Association of Sanitary, Heating, Air Conditioning
  3. a b Overview of the planning of drinking water installations and the set of rules according to DIN EN 806 and DIN 1988-200 in the publication Sanitärtechnisches Symposium 2010 of the Central Association of Sanitary, Heating, Air Conditioning, p. 13
  4. FAQ Thermostatic mixing valves - Why is it important to have a thermostatic mixing valve? ( Memento of the original from September 20, 2018 in the Internet Archive ) 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. , ESBE AB, Sweden @1@ 2Template: Webachiv / IABot /
  5. Discussion "Scalding Protection" , In:, May 2016
  6. Overview of the planning of drinking water installations and the set of rules according to DIN EN 806 and DIN 1988-200 in the publication Sanitärtechnisches Symposium 2010 of the Central Association of Sanitary, Heating, Air Conditioning, pp. 14 and 15