6. Which water should be used for the heating system?

The question related to the required properties of heating water is asked very frequently. Quality of water affects the run and service life of the entire heating system. In most cases, drinking water is used as the filling medium. But the fact that it is classified as drinking water does not guarantee its suitability for heating systems. Hardness, acidity, content of salts and solved gases are important parameters.

Hardness of water is related to the geological bedrock and soil conditions. At higher altitudes, with granite bedrock, water is softer; at lower altitudes it is harder thanks to limestone rocks. Hardness of water is determined by the content of calcium and magnesium salts in water. During operation of the system, just these salts form insoluble carbonates - scale on heat exchange surfaces of boilers. Usually, these are boiler heat exchangers. The layer of scale can grow on these surfaces and after a period of time, it is peeled and spread to the system, where it can cause clogging of individual valves. This results not only in reduction of boiler power output, but also in gradual putting of control valves out of operation. Moreover, during formation of scale, carbon dioxide is released to water that acts corrosively and at the same time enriches the system with undesired gases. This system must be continuously replenished with additional water. This cycle of replenishment and clogging of the system with scale results in congestion of the heat exchanger or valves.

Hardness of water is determined, for example in the German hardness scale (°d or dH°). On packages of washing powders, it is often divided to 4 degrees as follows:

10 - 7 °dsoft
27 - 14 °dmedium hard
314 - 21 °dhard
4over 21 °dvery hard

Hardness of water in Slovakia usually ranges from 5 °d to 35 °d.

Water with hardness that does not exceed 5,6 °d is suitable for heating systems. Other corresponding units can be stated for comparison:

German °d French °f English °e American gr / gal mmol / liter
5,6 10,0 7,0 5,8 1,0

The value of water hardness should be also given to you by your supplier of drinking water. But it is questionable, how reliable this data will be. Therefore it is always more certain to measure hardness of used water directly at the site. Hardness of water can be measured preliminary using the testing kits for aquarists. They can be based on indication papers or on special preparations (methyl orange) that are added to the sample of liquid in drops until it changes its color. Each drop represents for example 1°d. However, we recommend that water should be treated by the specialist. It is always cheaper than to pay later for remedy of the damages to the system.

The important factor affecting corrosion resistance of the heating system is acidity - pH. Thanks to instability of chemical bonds of water, its chemical composition can be changes under certain conditions, and in this way its acidity can be changed, too. Neutral water has value of pH = 7. The lower the pH value, the more acid water is and on the contrary, the higher the pH value, water is more alkaline. For heating water, it is necessary to adjust the pH value according to resistance of the materials used in the heating system. The pH value can be measured for example using the indication papers.

Hodnoty tvrdosti vykurovacej vody

In spite of the fact that for the system with different materials such as steel and copper we will choose pH that is as friendly from the viewpoint of corrosion processes as possible, electrochemical corrosion can take place, for example. This is initiated by high content of salts in water that is also called salinity. Salinity, or better said mineralization expresses the sum of all salts dissolved in water. These are represented mainly by cations Na+, K+, Fe+ and by anions Cl-, SO42-.

Their content in water can be specified by conductivity. Conductivity (content of ions) is given in µS (microsiemens) and can be measured using the conduct meter. Water with conductivity up to 0,5 µS / cm is suitable for heating systems.

Gases are dissolved in water. In heating water, they are represented mainly by nitrogen, oxygen and carbon dioxide. The content of gases dissolved in water depends on its temperature and pressure. Solubility is decreased with increasing temperature and decreasing pressure. This phenomenon is obvious when opening the bottle filled with saturated beverage. Sudden drop of pressure cause the origin of carbon dioxide bubbles that were dissolved in water. In nature, water usually contains dissolved gases under atmospheric pressure, namely oxygen up to 11 mg/l and nitrogen up to 18 mg/l. Nitrogen is inert gas, but its larger quantity in the system results in disorders of circulation. Problem free concentrations of nitrogen in water must be lower than 15 mg/l. Oxygen and carbon dioxide represent the greatest threat for the heating system. These gases cause corrosion of the system. A great portion of gases can be removed from the system by deaeration. Of course, it is not possible to do it to 100%. Residual oxygen and carbon dioxide are consumed in corrosion reactions that will be stopped after certain period of time. But if oxygen continues in penetration to the system, this is the most frequent cause of corrosion of the system.
One gram of O2 will corrode 2,6 g of Fe with the origin of 4,13 g of Fe(OH)3, i.e. the rust, with simultaneous origin of 1,4.10-3 m3 of hydrogen.

Leakages of individual elements of the system, unsuitable or incorrectly designed expansion vessel, permeability of some plastic components, but mainly replenished water are the sources of oxygen penetration.

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