Viessmann VITOPLEX 100 Service Instructions Manual page 24

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 directive VDI 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
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, including the intake
side of the pump, the pressure must be
higher than the surrounding atmospheric
pressure. The pre-charge pressure of
the expansion vessel should be checked
at least during the annual service. For
pressure maintaining systems, see
page 15.
24
(cont.)
The use of permeable components, e.g.
permeable plastic pipes in underfloor
heating systems, should be avoided.
Provide system separation if such com-
ponents are nevertheless used. This
must separate the water flowing through
the plastic pipes from other heating cir-
cuits – e.g. from the boiler – by the pro-
vision 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 9
and 10.5.
Different conditions apply to systems
that contain aluminium components.
Where chemicals are used as part of the
corrosion protection, we recommend
that the manufacturer of the chemicals
issues a certificate of suitability of the
additives with regard to the boiler mate-
rials and the materials of the other heat-
ing system components. We recom-
mend you refer questions regarding
water treatment to an appropriate spe-
cialist.
For further details, see the directive
VDI 2035-2 and EN 14868.