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3.2 Stray current corrosion
Corrosion from stray currents can occur due to the differ-
ing electrical potentials between water in the boiler and the
metallic mass of the boiler or piping. This process leaves
unmistakeable traces i.e. small regular conical holes.
b
All metallic parts should be grounded by an efficient
earth cable for this reason.
4. Eliminating air and gas from central heating systems
If oxygen enters a circuit continuously or even intermittently
(e.g. in under-floor heating systems whose pipes are not
protected by impermeable synthetic sheaths, in circuits
with open expansion vessels, or in circuits that require fre-
quent top-ups) always separate the boiler’s water circuit
from the central heating circuit.
Mistakes to avoid and precautions.
From what we have seen it is therefore important to avoid
two factors possibly leading to the above mentioned pro-
cesses i.e. contact between air and water in the installation
and regular topping up with fresh water.
To eliminate contact between air and water (and to prevent
the latter from becoming oxidized), it is necessary:
- For the expansion system to be a closed vessel type,
correctly scaled and with the correct pre-loading
pressure (to be regularly checked)
- For the installation to always be at a higher pressure
than that of the atmosphere at any point (comprising
the pump suction side) and under all running con-
ditions (all the water sealing and couplings in the
installation are designed to resist pressure towards
outside, but not for depression)
- That the installation will not be made with materials
permeable to gas (e.g. plastic pipes for floor systems
without anti-oxygen barrier).
b
Lastly we would remind you that the warranty does
not cover breakdowns incurred by the boiler due to
deposits and corrosion.
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