Viessmann Vitocrossal 300 CA3 2.5 Installation And Service Instructions Manual page 120

Service
Checking the Water Conditions
120
Vitocrossal 300 CA3 Series 2.5 to 6.0 Installation/Service
(continued)
Avoiding damage caused by water corrosion
The corrosion-resistance on the heating-water side of
ferrous materials used in heating systems and heat
generators depends on the absence of oxygen in the
heating water. The oxygen that enters the heating
system upon initial and subsequent filling reacts with
the system materials without causing damage.
The characteristic blackening of the water after a certain
operating time is a sign that there is no longer any free
oxygen.
During operation, oxygen can enter due to:
Open vented expansion vessels due to flow through
Negative pressure in the system
Through permeable components
Correctly sized sealed unvented systems operating at the
correct pressure, e.g. systems with expansion vessel,
offer good protection against the ingress of airborne
oxygen. The pressure must be above the pressure of the
ambient atmosphere at all times during operation and at
every point in the heating system, including the suction
side of the pump. Check the precharge pressure of the
expansion vessel at least during the annual service.
Avoid the use of components permeable to gas, e.g.
permeable plastic pipes in underfloor heating systems.
If they are used, system separation should be provided.
This system separation 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 other protective measures against
corrosion are necessary in corrosion-resistant closed hot
water heating systems in which the previous points
have been taken into account. However, take additional
precautions where there is a risk of oxygen ingress,
for example by adding oxygen binder sodium sulphite
5 to 10 ppm (5 to 10 mg/L) into the excess. The pH
value of the heating water should be 8.2–9.5.