Viessmann VITOCROSSAL 300 Type CT3 Service Instructions Manual page 29

Water quality requirements
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 boilers depends on
the absence of oxygen in the heating
water. The oxygen introduced into the
heating system with the first filling and
subsequent top-ups reacts with the sys-
tem materials without causing damage.
The characteristic blackening of the
water after some time in use indicates
that free oxygen is no longer present.
The technical rules and in particular VDI
Directive 2035-2 therefore recommend
that heating systems are designed and
operated so that a constant ingress of
oxygen into the heating water is preven-
ted.
During operation, oxygen can normally
only enter due to:
■ Open expansion vessels receiving a
flow
■ Negative pressure in the system
■ Gas-permeable components.
Sealed unvented systems– e.g. those
equipped with a diaphragm expansion
vessel– offer good protection against the
ingress of airborne oxygen into the sys-
tem, if correctly sized and operating at
the correct pressure.
At every part of the heating system, even
at the suction side of the pump and under
all operating conditions, the system
pressure should be above ambient
atmospheric pressure. The pre-charge
pressure of the diaphragm expansion
vessel should be checked at least during
the annual service. For pressure main-
taining systems, see page 12.
(cont.)
The use of gas-permeable components,
e.g. plastic pipes that are not diffusion-
proof in underfloor heating systems,
should be avoided. Provide system sep-
aration if such components are never-
theless used. This 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
additional precautions where there is a
risk of oxygen ingress, for example by
adding oxygen binder sodium sulphite
(5 - 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 contain aluminium 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
additives with regard to the boiler mate-
rials and the materials of the other heat-
ing system components. We recom-
mend you refer questions regarding
water treatment to an appropriate spe-
cialist.
Further details can be found in VDI
Directive 2035-2 and EN 14868.
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