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Buderus SB625WS Manual

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14 | Water treatment
SB625WS/SB745WS6 720 805 218 (2016/02)
of only 1 mm can cause severe overheating in metal parts and
consequent damage through thermal stress. It is continuous topping up
(automatic fill from incoming water) that causes thick deposits to form,
leading to boiler breakdown.
3.2.2 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.
The lower parts of the boiler can therefore be affected by a chemical
reaction of micro-corrosion caused by the electrochemical potential
difference created between the metal (steel) and the impurities around
it.
3.2.3 Stray current corrosion
Stray current corrosion is not common, but can be caused by the
different electrical potentials of the boiler water and the metal body of
the boiler or piping creating a cathode/anode effect.
All metal parts of the boiler should therefore be connected to an efficient
ground (earth) point, even though this form of corrosion is actually
caused by the passage of DC current, no longer used for domestic
power. Stray current corrosion is easily identified by the regular tiny
conical holes it leaves.
3.2.4 Diffused and localized acid corrosion
Other forms of corrosion exist that are harder to see but nonetheless
dangerous because they affect the entire heating system and not just the
boiler.
These forms of corrosion are generally due to the water becoming acidic
(pH < 7), and are caused by:
• Incorrect water softening and the presence of carbon dioxide (which
lowers the water’s pH). Carbon dioxide is released more easily in
softened water and also forms during the limescale formation
process. Acid corrosion is diffused and attacks the entire system
more or less uniformly
• Incorrect acid washing (e.g. washing done without a passivating
agent). Acid introduced into the system can cause localized
perforation if it is not properly removed from all parts of the system.
The formation of corrosion can easily be detected by analyzing the
chemical composition of the water. Even a minimal iron content is a
clear sign that corrosion is occurring.
3.3 New central heating systems
Mistakes to avoid and precautions.
To eliminate contact between system water and the air, the following is
required:
• ensure that the expansion vessel is a closed vessel, and of the correct
size and pre-charge pressure (the pressure should 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 at all operating conditions (precisely because the seals, gaskets
and joints 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 used in
floor heating systems).
The heating system should not need any further topping up once it is
filled and bled of all air.
Any top-ups need to be monitored (by a meter), treated and recorded in
the heating system’s technical log. The presence of a water softener in
conjunction with an automatic filling system is not sufficient to ensure
proper performance.
If more than one boiler is installed in a large system, all boilers must be
switched on at the same time to ensure that any possible limescale
formation is uniformly distributed.
3.4 Reconditioning old heating systems
Frequent mistakes and necessary precautions.
If a boiler must be replaced, do not refill the entire central heating circuit
if the quality of water in it conforms 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).
Conclusions
Never forget that proper water conditioning and proper heating system
design not only guarantee safety and security but also ensures
significant savings in maintenance costs and overall thermal efficiency.
3.5 Elimination air and gas from central heating system
When designing new heating systems, it is necessary to eliminate the air
and other gases that form in the system.
Recently added fill or top-up water loses much of its volume in the first
few days because it releases gases. With new systems you should
therefore initially check the heating water pressure on a daily basis, and
then at gradually longer intervals. Air and gas in the water system not
only causes the corrosion problems listed above, but also reduces
thermal efficiency, causing pump failure and noise and vibration
throughout the heating system. Air bubbles and gas inevitably form in
heating circuits during normal functioning, especially if the precautions
listed above are not fully respected.
In particular:
• as temperature increases, oxygen becomes less watersoluble and
bubbles begin to form.
The technical details provided in this section refer
specifically to domestic and industrial hot water heating
systems with working temperatures up to 210 °F
(98,8 °C).
NOTICE:
The original system filling water and any topping up
water must always be filtered (using synthetic or metal
mesh filters with a filtration rating of no less than 50
microns) to prevent sludge from forming and triggering
deposit induced corrosion.
NOTICE:
Loss of water from the system, and the consequential
need to add water, can be caused not only by leaks from
the circuit, but also from the incorrect sizing of the
expansion vessel or precharge pressure. (If normal
thermal expansion causes pressure in the system to
increase beyond the setting of the safety valve, that
safety valve will open continuously.)
The expansion vessel size should be corrected to
prevent unnecessary safety valve blow-off.
… 14 …


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