Is tap water hard or soft

Water hardness: German cities in comparison & risks

The water hardness results from the amount of the minerals calcium and magnesium contained in the water.

The higher the proportion, the more the water hardness increases. The water hardness in Germany is measured and designated with the scale in degrees of German hardness (° dH). The values ​​are divided into 3 groups:

  • up to 8.4 ° dH the water is considered soft
  • at 8.4 to 14 ° dH it is called medium-soft
  • from 14 ° dH it is hard water

Due to a high mineral content, deposits form in vessels and pipes through which the water moves. If the water hardness is too high, the water should or must be softened before it can be used in heating systems.

Water hardness in German cities

Degree of hardness


19.5 ° dHBerlin
7 ° dHHamburg
14.9 ° dHMunich
15.2 ° dHCologne
8.5 ° dHFrankfurt
10.5 ° dHStuttgart
15.6 ° dHDusseldorf
7.9 ° dHDortmund
7.1 ° dHeat
13.5 ° dHLeipzig
10.1 ° dHBremen
10.3 ° dHDresden
11.8 ° dHHanover
11.8 ° dHNuremberg
15.3 ° dHDuisburg
6.7 ° dHBochum
8.6 ° dHWuppertal
11.4 ° dHBielefeld
5.8 ° dHBonn
15.5 ° dHMuenster

* soft water, medium soft water, hard water

What causes hard water

Too much water hardness affects the heating circuit and efficiency in several ways. The stones that form in pipes and boilers reduce the cross-section and clog flow processes. Deposits act like natural insulation barriers on the direct heat transfer. The heat generated by the heating water can only be transported into the environment to a limited extent. Corrosion of the inner walls is promoted.

There is a risk of functional restrictions and defects for seals, nozzles, valves, switches and all other mechanical components of a heating installation. If the water from the public supply is too hard, it must be softened.

If water that is too hard is used, warranty and insurance claims are void. This also applies to refills to level the system pressure.

Possible measurement methods

Common methods of finding out the hardness of your tap water is to obtain information from the local supplier or to measure it yourself. It is easy to determine the hardness of the water.

A water hardness test strip is briefly dipped into the water or held under the water jet. After about a minute, the discoloration of the mostly 5 colored fields shows the water hardness in ° dH. A color scale is used to precisely interpret the discoloration.

Since the test strips only cost a few euros, you should also carry out a counter test after obtaining the general information from the utility or the local water authority. The ideal degree of hardness that you should achieve for the circulating water is specified in the technical documentation of your heating system.

Water hardness in Germany

In the six federal states of Bremen, Lower Saxony, North Rhine-Westphalia, Saarland, Saxony and Saxony-Anhalt, the average water hardness is in the single-digit ° dH range. Rhineland-Palatinate and Schleswig-Holstein are still in the medium hardness range with 10.5 and 10.6 ° dH respectively. In all other federal states the water is on average hard and in Thuringia the hardest with 20.5 ° dH.

You should take a measurement anytime, anywhere. Remote supply, overhead lines and utility processing can lead to large deviations locally. The technical rule VDI 2035 defines the requirements and treatment methods for heating water.

In addition to the hardness measurement, the pH value of the heating water should also be recorded. Any value deviating from the neutral PH 7 is responsible for corrosion and seal damage.

Ideal water hardness varies

The optimal degree of hardness for the heating also depends on the size and performance of the boiler and pipe systems. As a rule of thumb, the larger the boiler heating surfaces, the softer the water must be. For systems with a total output of less than 50 kilowatts and heating surfaces of less than 20 liters per kilowatt, VDI 2035 defines a ° dH of less than 16.8.

For boiler heating surfaces between 20 and 50 liters per kilowatt with the same total output, the value drops to 11.2 ° dH. For large systems with the smallest boiler heating surfaces, the water must be softened to 0.11 ° dH. The professional optimization of the hardness of your heating water also increases the heating efficiency and thus helps to reduce energy costs.