Prevention Of Damage Through Corrosion On The Water Side - Viessmann VITOPLEX 200 Service Instructions Manual

Water quality requirements

Prevention of damage through 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 blacken-
ing 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 there-
fore recommend that heating systems
are designed and operated so that a
constant ingress of oxygen into the
heating water is prevented.
During use, oxygen can normally only
enter due to:
&
open expansion vessels overflow-
ing,
&
negative pressure in the system,
& gas-permeable components.
(cont.)
Sealed systems, e.g. those equipped
with a diaphragm expansion vessel,
offer good protection against the
ingress of airborne oxygen into the
system, if correctly sized and operat-
ing 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 inlet pres-
sure of the diaphragm expansion ves-
sel 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.
Appendix
23