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VDI 2035, Part 1 - "Prevention of damage in water heating installations – scale formation in water heating systems"

Area of applicability:

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

General information

With the 5th edition, the VDI 2035 Part 1 completes the update of the long-known directive. A revision, with focus on the determination of new recommendations for filling and make-up water in domestic hot water installations appeared to be necessary. This is also the substance of the following information. In addition, the directive also includes information about systems for the heating of potable water, however, this is not discussed here.

The background for the changed requirements for filling and make-up water in heating systems is the fact that the heat generators continue to become more compact while providing increased heating performance. Likewise, the trend to multi-boiler systems or the trend to the division of required heating performance onto multiple heat generators or even to modularly switched units is unmistakable.

Due to construction, the generator heat surface is correspondingly loaded more which means it is even more important that the heat dissipation be possible without hindrance. Any reduction of the heat flow is to be prevented, and thus the application of VDI 2035 Part 1 is obvious, particularly with regard to scale formation. It is supplemented by Part 2, water-side corrosion (09/1998), and Part 3 exhaust-side corrosion (9/2000).

What is important or what has changed?

Water treatment

The amount of the regional water hardness is decisive for the decision as to whether measures for the prevention of boiler scale need to be taken. Moreover, the size of the heating system determines the limit values that are to be complied with. If necessary, these limit values are to be realized by technical measures. In areas with medium to hard water hardness, looking at the specifications of the directive is recommended.

The directive attempts to create a practical foundation which results in a catalog of measures which are simple to apply. There are now limit values for the maximum water hardness to be utilized by heat generators for all power ranges. Requirements are also placed on the smallest units, provided these deal with circulating water heaters (performance specification boiler water content Vk<0.3 l/kW) or systems with electric heating elements (e.g. heat pumps). In principle, one can assume that all wall-mounted heat generators, be they condensing boilers, or heat value generators, but also heat pumps with electric emergency heating fall into this category. The following table shows the limit values of the water hardness based on the total power of the heat generator modeled after the texts of the directive:

However, column 3 of the table only applies if the specific water content of the complete system is < 20 l/kW based on the smallest heat generator output (for multi-boiler systems). With the knowledge that a system with compact radiators and a design temperature of 70/50 °C has approx. 11 l/kW specific water content, this limit is already exceeded with a two-boiler system and a power distribution of 50/50 %. Because based on one boiler, a specific system content of 22 l/kW would already result.

In this case, the requirements of the next higher power level of the table are already decisive (column 4). The use of hot-water buffer tanks has an additional massive influence on the characteristic value of the specific water content. In this context, a significant amount of water to be heated comes into play, which must be taken into account.

A softening to nearly 0°dH is required (column 5) when larger amounts of make-up water for the system are to be expected (> 3 times Va per lifecycle of the system) or if the specific water content is > 50 l/kW.

Constructive measures

To keep the quantity of a water change as low as possible, e.g. during partial repairs, sectional shut-offs are to be utilized. For systems > 50 kW, a water meter is to be provided in the make-up water feedline. For circulating water heaters and a water hardness of > 16.8°dH, softening is preferable.


For the protection and preservation of an effective heat generation, the new VDI 2035 Part 1 (as a result of the latest experiences) supports the requirements of many heat generator manufacturers to minimize calcifications, particularly in the boiler. This is surely done in the interests of all manufacturers, planners, technicians, and operators.

What is important is the understanding that there are specified parameters and recommendations down to even the smallest units, parameters which are to be complied with, so as not to immediately stumble here in the event of damage. Provided you have familiarized yourself with the regional conditions with regard to the existing water harnesses, you will quickly find the necessary measures that need to be initiated. For the area of applicability of the VDI 2035 Part 1, the water softening by means of ion exchange resin is the most simple version to the implemented.
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