Technical definition
Conductivity is a measurement of the ability of the heating system water to conduct an electrical current. It is primarily determined by the concentration of dissolved ionic substances in the water, including minerals, salts, treatment chemicals and corrosion products. Conductivity is measured in microsiemens per centimetre (µS/cm).
In a closed heating system, conductivity provides a useful indication of the overall concentration of dissolved ions within the circulating water. It does not identify which individual substances are present, but an unusually high or changing conductivity reading can indicate changes in water chemistry that may require further investigation.
Simplified definition
Conductivity measures how many electrically charged dissolved substances are present in the system water. These can come from the original fill water, treatment chemicals, corrosion products or other contaminants.
A change in conductivity can therefore provide an early indication that the condition of the system water has changed.
Conductivity is useful because the concentration of dissolved substances within heating system water can influence corrosion, scale formation and the overall stability of the water chemistry.
A high or unexpectedly increasing conductivity level may indicate:
High levels of dissolved salts or minerals in the system water.
Repeated topping-up with mains water, which can introduce additional dissolved minerals and oxygen.
Contamination from another water source or substance.
Excessive concentrations of certain treatment chemicals.
Dissolved corrosion products accumulating within the system.
Changes caused by system components, leaks or inappropriate filling water.
Elevated conductivity can increase the ability of the water to carry electrical currents associated with electrochemical corrosion, potentially increasing the risk of corrosion of metals within radiators, boilers, heat exchangers and pipework.
Conductivity should therefore be considered alongside other measurements such as pH, inhibitor concentration, iron, copper and other water-quality parameters to build a complete picture of the condition of the heating system.
A conductivity result should not normally be assessed in isolation. Different filling-water sources and water-treatment products can produce significantly different conductivity values. For example, appropriately treated system water may have a higher conductivity than untreated demineralised water without necessarily indicating a problem.
The most useful assessment is whether conductivity is appropriate for the system, treatment regime and applicable water-quality guidance, and whether it has changed significantly compared with previous results.
