Viessmann Vitocrossal 200 Service Instructions For Contractors page 77

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 when it is filled for the
first time or topped up reacts with system
materials without causing any 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
directive VDI 2035-2 therefore recom-
mend that heating systems are designed
and operated so that a constant ingress
of oxygen into the heating water is pre-
vented.
During operation, oxygen can normally
only enter due to:
■ Open expansion vessels receiving a
flow
■ Negative pressure in the system
■ Gas-permeable components
(cont.)
Sealed unvented systems – e.g. those
equipped with an expansion vessel –
offer good protection against the ingress
of airborne oxygen into the system, if
correctly sized and operating at the cor-
rect pressure. At all points in the heating
system, including on the intake side of
the pump, and in all operating states, the
pressure must be higher than the pres-
sure of the surrounding atmosphere. The
pre-charge pressure of the expansion
vessel should be checked at least during
the annual service. The use of permea-
ble components, e.g. plastic pipes that
are not diffusion-proof in underfloor heat-
ing systems, should be avoided. Provide
system separation if such components
are nevertheless used. This must sepa-
rate 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
danger of oxygen ingress, e.g. by adding
oxygen binder sodium sulphate
(5-10 mg/l into the excess). The pH value
of the heating water should be between
8.2-9.5.
Different conditions apply to systems
that contain aluminium components.
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