Heatpumpinstaller: 2014

Rules and recommendations for the installation and operation of heating systems, such as the VDI 2035, SWKI, ÖNORM H5195 or AGFW, used by our colleagues in other parts of Europe, pay special attention on the composition of heating water, and not without reason. For it is the quality of the heating water, which contributes mostly to corrosion.

By the comprehensive treatment of heating water not only water side corrosion, damage and efficiency loss due to limescale can effectively be prevented, but older systems can be preserved in their value and function as well. The corrosiveness of heating water is influenced by the interaction of several parameters. Basically these are dissolved gases, pH, electrical conductivity, hardness and the redox potential, which is often overlooked.

Dissolved gases

There are several gases present in heating water, such as carbon dioxide, nitrogen, sulphur-containing gases and oxygen. Any of these have different impacts on corrosion or corrosion related action. From the view of corrosion protection, dissolved oxygen is the most harmful substance, as oxygen is the food for the corrosion processes and thus responsible for sludge in heating systems. Guidelines, such as the VDI 2035, clearly state that the likelihood of corrosion damage is reduced in waters which are low in dissolved oxygen. If a heating system is designed, operated and maintained according to latest recommendations, the constant ingress of oxygen is mostly avoided. If still corrosion occurs it can be assumed, that oxygen is been drawn into the system, due to either a vacuum due to poor pressure keeping, with top-up water or via diffusion through plastic parts, such as seals or plastics UFH pipes. Whenever constant oxygen ingress into the heating system causes corrosion problems, preventive action must be taken by means of reliable oxygen elimination and a water quality which allows for a higher amount of oxygen by lowering the general risk of corrosion.

pH of the heating water

The acidity and the alkalinity of the heating water is measured with the pH-value. The pH simply expresses if there is an surplus of acidic hydrogen- or alkaline hydroxide-ions in the heating water. In acidic waters (pH <7) most metals cannot keep their natural layers stable and at pH <5 corrosion is possible even under the absence of oxygen. If not disturbed by other factors in the heating system, common metals can keep their natural layers stable in alkaline waters. Thus the pH has an important role in the field of water treatment for heating systems. According to VDI 2035 the pH should be in a range between 8.2 an 10, whereas it should not exceed 8.5 in the presence of aluminium.

The electrical conductivity of the heating water

Corrosion as an electrochemical process that only occurs if the metal has direct contact to a conductive medium (electrolyte). As a general rule of thumb, it is widly accepted, that with a low conductivity of the heating water the probability of corrosion decreases. The conductivity of the heating water is determined by the dissolved salts, which have an effect on the pH and the hardness of the heating water as well. Pure water contains no salts and is theoretically not conductive, however the pH is low and thus acidic corrosion can occur. Methods of water treatment where chemicals are used, such as oxygen scavengers or other inhibitors, may lead to an increase of conducivity – which increases the risk of corrosion if the inhibitor is not dosed correctly. Considerations for the electrical conductivity of the heating water can never be contemplated one-sided, as any water treatment has an impact on this value. For natural corrosion protection without chemicals a low salt operation of the heating system (low conductivity) is considered to be an effective an manageable way.

The redox-potential

The process of corrosion is an electrochemical oxidation-reduction reaction. One element of the reaction loses electrons (oxidation), the other partner picks up electrons (reduction). If iron corrodes the metal loses electrons and reduces oxygen from the water. Precondition for an corrosion process is the contact of the metal to an electrolytic solution containing dissolved oxygen, such as water. The redox potential is measured in mV od Eh (millivolts) and is influenced by the ions existing in the electrolyte and describes its tendency to receive electrons and to be reduced. In connection with the pH and the temperature it can be estimated, if an metal in contact with the electrolyte is able to build up stable layers or if it is chemical stable or if it corrodes. The measurement of the redox-potential is a rather uncharacteristic measurement in heating systems and is mainly used in mining.

The hadness of the heating water

The sum of alkaline earth ions dissolved in water determines its total hardness. At high degrees of hardness scale deposits may arrise causing a loss of energy efficiency. A water softening with ion exchange replaces calcium and magnesium ions with sodium ions. Water softening lowers the risk of scaling but the conductivity of the water still remains high, if the raw water has a high total hardness. The probability of corrosion usually decreases with a decreasing conductivity of the heating water (VDI 2035 sheet 2 paragraph 6). The reduction of the water hardness doesn’t protect a system against corrosion as a single action and therefore can only be taken into consideration alongside other measures. In case of a high total hardness demineralisation of the heating system make-up water can be the solution to lower the total hardness and the conductivity as well.

The colour of the heating water

Brown or black heating water is a sign of corrosion in the heating system, because the corrosion products of iron oxide colour the water. The undissolved iron components and other particles can cause sludge, siltation, can damage moving parts or can lead to malfunction of magnetic devices. Metal particles can settle to other metals an can cause galvanic corrosion. For these reasons the ideal heating water should be clean and clear.