Viessmann VITOCROSSAL 300, Service Instructions Manual

Page: 32 / 36

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 heat-
ing water. The oxygen introduced into
the heating system with both the first
fill and with 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 recom-
mend that heating systems are
designed and operated so that a con-
stant ingress of oxygen into the heat-
ing water is prevented.
Oxygen can normally only enter dur-
ing operation via:
& open expansion vessels overflow-
ing,
&
negative pressure in the system,
&
gas-permeable components.
Sealed systems, e.g. those equipped
with a diaphragm expansion vessel,
offer
–
if correctly sized and operating
at the correct pressure
–
good protec-
tion against the ingress of airborne
oxygen into the system.
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
inlet pressure of the diaphragm
expansion vessel should be checked
at least during the annual service. For
pressure maintaining systems, see
page 12.
The use of permeable components, e.
g. plastic pipes that are permeable to
gas in underfloor heating 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 cir-
cuits, e.g. from the boiler, by the provi-
sion of a corrosion-resistant heat
exchanger.
No further anti-corrosion measures
are required for sealed hot water heat-
ing 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 sul-
phite (5-10 mg/litre 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.
Where chemicals are used as part of
the corrosion protection, we recom-
mend that the manufacturer of the
chemicals issues a certificate of suit-
ability of the additives with regard to
the boiler materials and the materials
of the other heating equipment com-
ponents. We recommend you refer
questions of water quality/treatment
to an appropriate specialist.
Further details can be found in the
VDI Directive 2035-2 and in EN
14868.
Water quality requirements
(cont.)
32
Specification
56
92
405
GB