Viessmann VITOCROSSAL 300 Service Instructions Manual page 37

Water quality requirements
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
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.
(cont.)
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.
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.
Further details can be found in VDI
Directive 2035-2 and EN 14868.
37