Technical definition
Magnesium is a measurement of the concentration of dissolved magnesium ions (Mg²⁺) present in the heating system water, expressed in milligrams per litre (mg/L).
Magnesium occurs naturally in mains water and, together with calcium, is one of the principal contributors to water hardness. Its concentration depends largely on the characteristics of the original filling water and the local geology of the water supply.
Within a heating system, magnesium is relevant because it contributes to the overall hardness of the water and can participate in the formation of mineral deposits and scale, particularly when combined with suitable alkalinity, pH and temperature conditions.
Simplified definition
Magnesium is a naturally occurring mineral found in water and is one of the main minerals that makes water hard.
Like calcium, it can contribute to the formation of scale and mineral deposits inside the heating system, particularly when the water is repeatedly heated.
Magnesium is important because it contributes to Total Hardness and can therefore influence the overall scale-forming potential of the heating system water.
An abnormal magnesium result may indicate:
Hard filling water containing naturally elevated levels of magnesium.
Increased overall water hardness.
A greater potential for mineral deposits to form within the system.
Repeated topping-up of the system with fresh mains water.
Changes to the system water following draining, refilling or maintenance.
Ineffective use of demineralised or otherwise treated filling water where low mineral content is expected.
Differences between the current system water and the original filling-water characteristics.
Although calcium is generally the dominant mineral associated with limescale formation in heating systems, magnesium can also contribute to deposits and should be included when assessing the overall hardness and mineral content of the water.
Mineral deposition can contribute to:
Reduced heat-transfer efficiency.
Restricted flow through narrow waterways.
Deposits on heat exchangers and other hot surfaces.
Increased boiler or system noise.
Reduced appliance performance.
Increased energy consumption.
Premature component problems where deposits become significant.
Magnesium can also provide useful diagnostic information when compared with Calcium and Total Hardness. Together, these parameters help build a clearer picture of the mineral composition of the system water and whether changes have occurred since the system was originally filled.
Magnesium should therefore be considered alongside parameters such as Calcium, Total Hardness, M-Alkalinity, pH and conductivity when assessing the likelihood of scale formation and the overall condition of the system water.
Magnesium should not normally be assessed in isolation. A high magnesium concentration does not automatically mean that scale is forming, as mineral deposition depends on several interacting factors including temperature, pH, alkalinity and the presence of other hardness-forming minerals.
It is also important to distinguish between Magnesium and Total Hardness. Magnesium measures the concentration of magnesium ions specifically, whereas Total Hardness represents the combined contribution of magnesium, calcium and other multivalent ions, expressed as calcium carbonate equivalent.
The expected magnesium level will vary considerably depending on the source of the filling water. Systems filled with naturally hard mains water may contain appreciable magnesium, while systems filled with demineralised water would normally be expected to contain much lower levels.
