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WATER IN CENTRAL HEATING SYSTEMS
STEEL BOILERS
with furnace power < 150 kW
Initial filling
water
Regular service
water (*)
Sulphides
mg/l
< 20
Nitrides
mg/l
< 20
Iron
mg/l
< 0,5
STEEL BOILERS
with furnace power > 150 kW
Initial filling
water
Regular service
water (*)
ph
6-8
7,5-9,5
Hardness
°fH
< 5°
< 5°
Electrical
conductivity
µs/cm
< 100
Chlorides
mg/l
< 10
Sulphides
mg/l
< 10
Nitrides
mg/l
< 10
Iron
mg/l
< 0,5
(*) values for water in system after 8 weeks of functioning
General note on water used to top up systems:
− if softened water is used to top up a system, 8 weeks of
functioning after topping up, verify that the water in the
system respects the above limits, in particular for electri-
cal conductivity.
− this check is not necessary if demineralised water is used
to top up the system.
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 instead and record
top-ups in the system service book.
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If there are more than one boiler, they must all be put into
service either contemporarily or with a very low rotation
time during the initial period of service, so as to evenly dis-
tribute the limited quantity of initial lime-scale.
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A flushing cycle must be programmed after the plant has
been installed to flush out any installation debris.
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Water used to fill a system for the first time and water used
to top it up must always be filtered (using synthetic or metal
mesh filters with a filtration rating of no less than 50 mi-
crons) to prevent sludge from forming and triggering deposit
corrosion.
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The heating system must be flushed out and cleaned with
good workmanship before filling up the existing systems.
The boiler may not be filled until after the heating system
has been flushed out.
2.1 New central heating systems
The system must be filled up slowly the first time; once it is filled
and the air expelled it should never need to be topped up again.
Systems should also be operated at maximum working tem-
perature the first time they are started up, in order to facilitate
de-aeration. (Gas is not released from the water at low temper-
atures).
2.2 Reconditioning old central heating systems
IIf a boiler has to be replaced, do not refill the entire central
heating circuit if the quality of water in it conforms to require-
ments. 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
Under-deposit corrosion is an electrochemical process, due to
the presence of sand, rust, etc., inside the mass of water. These
solid substances generally deposit on the bottom of the boiler
(sludge), on tube and pipe heads or in the gaps between pipes
and tubes.
Micro-corrosion phenomena may be triggered off owing to the
difference in electrochemical potential coming to be created be-
tween the material in contact with the impurity and the sur-
rounding one.
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 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 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 floor 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.
