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WATER IN CENTRAL HEATING SYSTEMS
3.2 Stray current corrosion
Corrosion from stray currents can occur due to the differing elec-
trical 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.
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All metallic parts should be grounded by an efcient 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-oor heating systems whose pipes are not pro-
tected by impermeable synthetic sheaths, in circuits with open
expansion vessels, or in circuits that require frequent top-ups)
always separate the boiler’s water circuit from the central heat-
ing 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 processes 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, cor-
rectly 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 conditions (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 per-
meable to gas (e.g. plastic pipes for oor systems without
anti-oxygen barrier).
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Lastly we would remind you that the warranty does not cov-
er breakdowns incurred by the boiler due to deposits and
corrosion.
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