23
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 heating
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 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 overflowing,
■ negative pressure in the system,
■ gas-permeable components.
Sealed systems – e.g. systems with a
diaphragm expansion vessel – offer
good protection against the ingress of
airborne oxygen into the system if they
are correctly sized and operate 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 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 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.
No further anti-corrosion measures are
required for sealed hot water heating
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 sulphite (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.
Water quality requirements
Water quality requirements
(cont.)
5692 798
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