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Water quality requirements
If limescale deposits have formed because of a failure
to observe the requirements of Guideline VDI 2035 the
service life of the installed heating appliances will, in
most cases, already have been reduced.
Removing the limescale deposits is one option for
restoring operational viability. This measure must be
carried out by a qualified contractor. Inspect the heat-
ing system for possible damage prior to returning it into
use.
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 gener-
ators 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 damage.
The characteristic blackening of the water after a cer-
tain time in operation indicates that there is no more
free oxygen present. In accordance with the technical
rules and in particular VDI guideline 2035-2, we rec-
ommend that the heating system is designed and
operated such that a constant ingress of oxygen into
the heating water is prevented.
During operation, oxygen can enter due to:
■
Open expansion vessels receiving a flow
Negative pressure in the system
■
Gas-permeable components
■
Correctly sized sealed unvented systems operating at
the correct pressure, e.g. systems with a diaphragm
expansion vessel, offer good protection against the
ingress of airborne oxygen.
Under all operating conditions and at all points in the
heating system, including the intake side of the pump,
the pressure must be higher than atmospheric pres-
sure. Check the pre-charge pressure of the diaphragm
expansion vessel at least during the annual service.
(cont.)
It is essential that the incorrect operating parameters
are corrected to prevent renewed formation of exces-
sive scale deposits.
Avoid the use of gas-permeable components, e.g. per-
meable plastic pipes in underfloor heating systems.
Provide system separation if such components are
nevertheless used. This system separation separates
the water flowing through the plastic pipes from other
heating circuits, e.g. from the heat generator, by the
provision of a corrosion-resistant heat exchanger.
No further anti-corrosion measures are required for
sealed unvented 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 (surplus of 5 - 10 mg/l). The pH value of the
heating water should be between 8.2 and 9.5.
Different conditions apply to systems that contain alu-
minium 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 addi-
tives with regard to the boiler materials and the materi-
als of the other heating system components.
We recommend you refer to a qualified contractor for
questions regarding water treatment.
For further details, see Guideline VDI-2035-2 and
EN 14868.
Water quality
33
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