Bedienungsanleitung Buderus SB625WS (Seite 14 von 52) (Englisch)

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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.

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