Riello KV/200 20 ISC Solar Installation And Maintenance Manual page 22

2.2 Reconditioning old central heating systems
If a boiler has to be replaced, do not refill the entire central heating circuit if the quality of water in it conforms to requi-
rements. If the quality of water fails to conform to requirements, either recondition the old water or separate the water
circuits (water in the boiler circuit must conform to requirements).
3. Corrosion
3.1 Deposit corrosion
Deposit corrosion is an electro-chemical phenomenon caused by the presence of foreign bodies (sand, rust, etc.) in the
water mass. These solid substances generally form deposits (sludge) in the bottom of the boiler, in the tube heads and
in the joins of tube bundles.
These parts of the boiler can therefore be affected by micro-corrosion caused by the electrochemical potential difference
created between the metal parts in contact with impurities and other metal parts around them.
3.2 Stray current corrosion
Stray current corrosion is caused by the different electrical potentials of the water in the boiler and the metal body of the
boiler or piping. Stray current corrosion is easily identified by the regular tiny conical holes it leaves.
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 frequent
top-ups) always separate the boiler's water circuit from the central heating circuit.
Mistakes to avoid and precautions.
From what has been said above we can see that it is essential to avoid two main factors that could lead to corrosion.
These are contact between system water and air and the continuous addition of fresh water to the circuit.
To eliminate contact between system water and the air (thus avoiding oxygenation of the system water), we need to:
- ensure that the expansion vessel is a closed vessel, and of the correct size and pre-charge pressure (the pressure to
be checked periodically);
- ensure that the system is always kept at a pressure higher than atmospheric pressure at all points (including the pump
suction side) and under all operating conditions (precisely because the seals, gaskets and joins in a water circuit are
designed to resist pressure from within, but not to resist a vacuum within);
- ensure that no part of the system is made from materials that are permeable to gases (e.g. plastic pipes with no oxygen
barrier destined for under-floor heating systems).
b
Finally, never forget that boiler failures caused by encrustation and corrosion are not covered under the
terms of the warranty.
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INSTALLER