Water Hardness And Corrosion - eta SH Installation Manual

Water hardness and corrosion

Softened water and shut-off valves
If a buffer storage tank is installed in the heating
system, the system should be filled with softened
water. The lime is deposited on a very small area in
the boiler and forms layers of insulating limescale.
The boiler wall is no longer sufficiently cooled and
stress cracks can result. To keep the lime content to
a minimum during minor repairs, shut-off valves
are essential at all buffer connections and all heat
distribution branches.
When is decalcifying necessary?
For the initial fill-up of the heating system with
the boiler, the lime content of all water in the
heating system may not exceed the value of
20,000 lt°dH (system volume in litres multiplied by
the hardness in degrees of German hardness).
20,000 lt°dH
°dH
litres of water
Example:
To maintain the limit of 20,000 lt°dH, the system
must be softened to 10°dH.
Softening with a salt-regeneration ion exchanger
We recommend softening water with salt-
regeneration ion exchangers, just as drinking water
is softened. This method does not remove salt
from the water. It exchanges the calcium in lime
against sodium from the salt and has considerable
advantages. It is cheap and chemically stable
against contamination. In addition, it has a natural
alkalinity that generally results in a sufficiently
non-corroding pH value of around 8.
pH value between 8 and 9
may require dosing with trisodium phosphate
If the heating water's pH value is not greater
than 8 after a week of operation, increase it by
adding 10 g/m³ of trisodium phosphate (Na3PO4)
or 25 g/m³ of trisodium phosphate dodecahydrate
(Na3PO4.12H2O). Wait another 2-4 weeks before
making further corrections. The pH value may not
exceed 9.
14
= permitted hardness in
20,000 lt°dH
= 10 °dH
2000 litres
No hybrid installations
A disadvantage of salt-regeneration ion exchange
is the salt content with its high electrical
conductivity, which can lead to electrolytic
corrosion, especially of aluminium or galvanized
steel.
If only steel, brass, gunmetal and copper are used
in the heating system and the use of stainless
steel is limited to small areas, then no corrosion
problems should be expected even with salty
water.
Inside a heating system, galvanized and aluminium
parts are always in danger of corrosion, especially
in combination with copper tubing. In practice
this means no hot-galvanized fittings and no
mix of galvanized tubing with copper tubing.
There is an illogical exception: galvanized steel
tubing combined with boilers or buffer storage
tanks made of steel. Presumably the uniform zinc
layer dissolves uniformly and is dispersed evenly
throughout the system without localised corrosion.
Complete desalination not required
If there is no aluminium (heat exchanger in the
gas boiler or aluminium radiator) in the system,
then no expensive desalination with ion exchange
cartridges or osmosis is required.
Lime stabilisation can be dangerous
The addition of lime stabilising agents prevents
limescale. However, we advise against doing so.
These agents increase the salt content and result in
an undefined pH value. If large amounts of water
are added, exactly the same agent must be used
again. Mixing with other water additives or with
antifreeze can result in corrosion.
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