Viessmann VITOCROSSAL 200 Service Instructions Manual page 69

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 fill and the
top-up water reacts with the system
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 use, oxygen can normally only
enter due to:
■ open expansion vessels overflowing
■ negative pressure in the system
■ gas-permeable components
(cont.)
Sealed unvented systems – e.g. sys-
tems with a diaphragm expansion vessel
– offer good protection against the
ingress of airborne oxygen into the sys-
tem if they are correctly sized and oper-
ate at the correct pressure. At every part
of the heating system, even at the suc-
tion side of the pump and under all oper-
ating conditions, the system pressure
should be above ambient 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. plas-
tic pipes that are permeable to gas in
underfloor heating systems, should be
avoided. Provide system separation if
such components are nevertheless
used. This must separate the water flow-
ing 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 heating
water should have a pH value between
8.2 and 9.5.
Different conditions apply to systems
that contain aluminium components.
Water quality requirements
69