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VDI 2035, Part 2 - "Prevention of damage in water heating installations – water-side corrosion"

Area of applicability:

Hot-water heating systems according to DIN EN 12828,
Potable water systems according to DIN 4753,

General information

Just as the existing revision of Part 1 (already in existence for some time), Part 2 of this directive (which appeared in August) joins the "tradition" of the continuous further development of the VDI 2035.

With the objective of protecting the components of heating systems in buildings in an effective way, Part 2 has the same handwriting as Part 1 and is useful to the practitioner as a valuable orientation. Current and up-to-date knowledge of the system technology, the applied materials and new knowledge about corrosion procedures and their interrelation have been the basis and the background for the revision of this part.

With practical, well summarize information, Part 2 has the intention of protecting components of heating systems from destruction or corrosion attack or minimizing the probability of corrosion.

What is important or what has changed?

Anti-corrosive closed system

The essence of Part 2 is the finding that a main aspect for the prevention of corrosion is that there must be consistent prevention of continually introduced oxygen into the heating water. Great importance is attached here to the pressurization. This is not surprising, however, it is described here in clarity for the first time in a directive outside of the large system technology.

The directive requires the construction of anti-corrosion closed systems and "kicks" system components that bring heating water into direct contact with atmospheric oxygen "off the field". Because of this, pressurization systems with expansion vessels without diaphragms, for example, are now finally and clearly emerging in light of corrosion investigations in a way that is understandable for everyone.

Maintenance with focus on pressurization

Likewise, the long known aspect of the necessary maintenance is also brought into focus by expansion vessels. When they are not maintained or inoperative, they cause vacuum formation in the system and thus promote the entry of air – with the well-known consequences.

Professional planning

is also required with the information that has existed in DIN EN 12828 for many years. Proper planning is an elementary foundation for the prevention of damage.

No chemicals

Anti-corrosion procedures in conjunction with different materials are very clearly described for the practitioner. The VDI continuously and clearly indicates that the "chemical bomb" in the form of additives must or should only be used in the most seldom of cases. The VDI advocates precisely the opposite thus clearing up the jungle of water chemistry in heating systems.


Importantly high value is placed on the awareness and monitoring of the ph-value. Depending on the materials used, it should lie between 8.2 and 10. The expert will have to get used to the necessity of being aware about the necessary or existing ph-value of the heating water. In addition to the oxygen content, it is an important factor for the possibility of the presence of corrosion processes.

The ph-value is simple to check without a large amount of technical expenditure. Systems require a certain amount of time for the development of or stabilization of the ph-value of the heating water. Usually, no external assistance is necessary in order to adjust the required alkaline range. The so-called inherent alkaline treatment takes place on its own after the system is filled and should be checked after 8 to 12 weeks of operating time.

New - the electrical conductivity

The observation of the electrical conductivity of the heating water is new in the revision compared to earlier editions. References made to the interrelation already made in other rules about the "low conductivity minimizing the probability of corrosion by oxygen".

Particularly interesting and important in practice is, however, also the information about the fact that additives in the heating water, such as inhibitors or oxygen binders increase the conductivity. I.e. that heating water makes it more conducive for the feared oxygen corrosion. On the topic of inherent alkaline treatment specifically, however, the VDI also explains to the practitioner that in a anti-corrosion closed system uncritical concentrations of oxygen will adjust on their own.

Against a misinterpretation, as already mentioned numerous times in publications, that a secure system operation is only possible with low-salt (desalinated) heating water is to be warned about at this juncture.


The VDI 2035 suggests the use of a system book for the safety of the system creator and naturally for the operator. As already clearly required in Part 1, and important for the overview to the provided measures, the system book should document which measures are necessary or were performed. The system book is structured as model example and summarizes all important parameters of both parts of the directive once more.


The VDI 2035 Part 2 offers the professional good and clear information about the prevention of corrosion damage and heating systems. The information is displayed in a way that allows the interested professional to understand it and implement it. The most important principles are clearly presented in detail and the VDI is not afraid of making critical comments about chemistry. In all cases, it serves to create clarity in dealing with heating water treatment and preparation.

With this directive, the VDI has again made a work available for the professional which, in contrast to the predominantly European body of standards, is to be applied very specifically.