Viessmann Vitocrossal 300 Service Instructions For Contractors page 32

Water quality requirements
Water quality requirements
Any limescale deposits that have formed because of a
failure to observe the requirements of guideline
VDI 2035 will in most cases already have caused a
reduction in the service life of the installed heating
appliances. Removing the limescale deposits is one
option for restoring operational viability.
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. Oxygen introduced into the heating system with
the first fill and subsequent top-ups reacts with the sys-
tem 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. The technical rules and in partic-
ular Directive VDI 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 operation, oxygen can enter due to:
■ Open expansion vessels receiving a flow
■ Negative pressure in the system
■ Gas-permeable components
Sealed unvented systems
vessel offer good protection against the ingress of
–
airborne oxygen into the system, if correctly sized and
operating at the correct pressure. 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 pressure. Check the
pre-charge pressure of the expansion vessel at least
during the annual service. The use of gas-permeable
components, e.g. permeable plastic pipes in underfloor
heating systems, should be avoided. Provide system
separation if such components are nevertheless used.
This must separate the water flowing through the plas-
tic pipes from other heating circuits, e.g. from the
boiler, by the provision of a corrosion-resistant heat
exchanger.
32
(cont.)
e.g. with an expansion
–
This measure must be carried out by a qualified con-
tractor. Inspect the heating system for possible dam-
age prior to returning it into use. It is essential that the
incorrect operating parameters are corrected to pre-
vent renewed formation of excessive scale deposits.
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, add sodium sulphite as an oxy-
gen binder (5 - 10 mg/litre 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 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 rec-
ommend you refer questions regarding water treat-
ment to a relevant qualified contractor.
For further details, see VDI 2035-2 and EN 14868
guidelines.