Viessmann VITOROND 200 Service Instructions Manual page 26

Water quality
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 heat generators
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 dam-
age.
The characteristic blackening of the
water after a certain time in operation
indicates that there is no more free oxy-
gen present. The technical rules and in
particular VDI Directive 2035-2 therefore
recommend that heating systems are
designed and operated so that a con-
stant ingress of oxygen into the heating
water is prevented.
During operation, oxygen can normally
only enter due to:
■ Open expansion vessels receiving a
flow
■ Negative pressure in the system
■ Gas-permeable components
26
(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. In all operating states and
at all points in the heating system, includ-
ing the intake side of the pump, the pres-
sure must be higher than the surround-
ing atmospheric pressure. The pre-
charge pressure of the expansion vessel
should be checked at least during the
annual service. The use of permeable
components, e.g. permeable plastic
pipes 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 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 -
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.