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Water quality
Water quality requirements
If failure to observe guideline VDI 2035 results in
harmful limescale deposits, the service life of the
installed boilers will in most cases have already been
reduced. Removing limescale deposits may be an
option to restore the operating capability. This must be
carried out by Viessmann Industrieservice or a special-
ist contractor. Inspect the heating system for possible
damage prior to returning it into use. The faulty operat-
ing parameters must be corrected in order to prevent
excessive scale build-up from reoccurring.
Prevention of damage through corrosion on the water side
The corrosion resistance of ferrous materials on the
heating water side of heating systems and heat gener-
ators depends on the absence of oxygen in the heating
water.
The oxygen introduced into the heating system with
the first fill and subsequent top-ups reacts with the sys-
tem materials without causing damage.
The characteristic blackening of the water after a cer-
tain time in operation indicates that there is no more
free oxygen present.
In accordance with the technical rules and in particular
VDI guideline 2035-2, we recommend that the heating
system is designed and operated so that a constant
ingress of oxygen into the heating water is prevented.
During operation, oxygen can enter due to:
Open expansion vessels receiving a flow
■
■
Negative pressure in the system
■
Gas-permeable components
Correctly sized sealed unvented systems operating at
the correct pressure, e.g. systems with a diaphragm
expansion vessel, offer good protection against the
ingress of airborne oxygen.
Under all operating conditions and at all points in the
heating system, including the intake side of the pump,
the pressure must be higher than atmospheric pres-
sure. Check the pre-charge pressure of the diaphragm
expansion vessel at least during the annual service.
See the relevant section in the chapter on commission-
ing, inspection and maintenance.
Using antifreeze in boilers
Viessmann boilers are designed and built for water as
a heat transfer medium. To protect boiler systems from
frost, it may be necessary to treat the boiler water or
circulating water with antifreeze.
When doing so, observe the following:
■
In general, follow the specifications given by the anti-
freeze manufacturer.
■
The properties of antifreeze and water are very dif-
ferent.
20
(cont.)
Avoid the use of gas-permeable components, e.g. per-
meable plastic pipes in underfloor heating systems.
Provide system separation if such components are
nevertheless used. This system separation separates
the water flowing through the plastic pipes from other
heating circuits, e.g. from the heat generator, by the
provision of a corrosion-resistant heat exchanger.
No further anti-corrosion measures are required for
sealed unvented hot water heating systems subject to
the above points being observed.
However, take additional precautions where there is a
risk of oxygen ingress, for example by adding oxygen
binder sodium sulphite (surplus of 5-10 mg/l). The pH
value of the heating water should be between 9.0 and
10.5.
Different conditions apply to systems that include alu-
minium components.
Where chemicals are used as part of the corrosion
protection, we recommend that the manufacturer of the
chemicals issues a certificate of suitability of the addi-
tives with regard to the boiler materials and the materi-
als of the other heating system components.
We recommend you refer questions regarding water
treatment to Viessmann Industrieservice or an appro-
priate specialist.
Further details can be found in guideline VDI 2035 part
2 and EN 14868.
■
The temperature stability of the antifreeze must be
sufficient for the particular application.
Check the compatibility with sealing materials. If
■
other sealing materials are used, take this into
account when designing the system.
■
Antifreeze developed especially for heating systems
contains inhibitors and buffer substances for corro-
sion protection as well as glycol. When using anti-
freeze, always observe the manufacturer's instruc-
tions regarding minimum and maximum concentra-
tions.
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