23
Removing the limescale deposits is one option for
restoring operational viability. This measure must be
carried out by a qualified contractor. Inspect the heat-
ing system for possible damage prior to returning it into
use. It is essential that incorrect operating parameters
are corrected to prevent renewed excessive scaling.
Prevention of damage due to 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
system 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 guideline VDI 2035-2, we rec-
ommend that heating systems are designed and oper-
ated so that a constant ingress of oxygen into the heat-
ing water is prevented.
During operation, oxygen can only 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 expansion ves-
sel, offer good protection against the ingress of air-
borne 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 expansion
vessel at least during the annual service. For pressure
maintaining systems, see chapter "Checking the
expansion vessel and system pressure".
Avoid the use of permeable components, e.g. permea-
ble plastic pipes in underfloor heating systems. Pro-
vide system separation if such components are never-
theless used. This system separation must separate
the water flowing through the plastic pipes from other
heating circuits, e.g. from the boiler, 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 addi-
tional precautions where there is a risk of oxygen
ingress, for example by adding oxygen binder sodium
sulphite (5 to 10 mg/l into the excess). The pH value of
the heating water should be between 8.2 and 9.5.
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 other components. We recommend you refer
questions regarding water treatment to a qualified con-
tractor.
For further details, see guideline VDI 2035-2 and
EN 14868.
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.
■
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.
■
The concentration must never fall below the prescri-
bed minimum level, subject to the required frost pro-
tection temperature. Check and adjust the pH value
and frost protection (measure the density) regularly,
at least once a year, according to the manufacturer's
instructions.
Water quality
Water quality requirements
(cont.)
5692818
Appendix
