29
INSTALLER
2 CENTRAL HEATING SYSTEMS
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Do not use automatic filling devices to add water to
central heating systems. Use a manual device inste-
ad and record top-ups in the system service book.
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If a system comprises a number of boilers, when
that system is first put into service, make sure that
all the boilers function simultaneously or on a short
interval rotation basis in order to evenly distribute
initial limescale deposits.
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When you finish installing a system, always flush it
out to remove installation residues.
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Water used to fill a system for the first time and wa-
ter used to top it up must always be filtered (using
synthetic or metal mesh filters with a filtration ra-
ting of no less than 50 microns) to prevent sludge
from forming and triggering deposit corrosion.
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Before re-filling an existing system, clean and flush
it out thoroughly. Only fill the boiler after the central
heating circuit has been thoroughly flushed out.
2.1 New central heating systems
Initial filling of the system must be performed slowly. In
theory, once filled and bled of all air, a heating circuit
should not need topping up.
Systems should also be operated at maximum working
temperature the first time they are started up, in order
to facilitate de-aeration. (Gas is not released from the
water at low temperatures).
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 con-
forms to requirements. 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 poten-
tial difference created between the metal parts in con-
tact with impurities and other metal parts around them.
3.2 Stray current corrosion
Stray current corrosion is caused by the different electri-
cal 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 le-
aves.
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All metallic parts should be grounded by an effi-
cient earth cable for this reason.
4 ELIMINATING AIR AND GAS FROM CENTRAL HEATING
SYSTEMS.
If oxygen enters a circuit continuously or even intermit-
tently (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 ves-
sel, and of the correct size and pre-charge pres-
sure (the pressure to be checked periodically);
− -ensure that the system is always kept at a pres-
sure 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. pla-
stic pipes with no oxygen barrier destined for
under-floor heating systems).
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Finally, never forget that boiler failures caused by
encrustation and corrosion are not covered under
the terms of the warranty .
