Technical definition
Excess Chloride is a calculated measurement of the additional chloride present in the heating system water compared with the chloride concentration of the original or reference filling water, expressed in milligrams per litre (mg/L).
In practical terms, it distinguishes the chloride naturally present in the mains/fill water from chloride that appears to have been introduced or accumulated within the heating system after filling.
Fernox water-test results demonstrate this calculation directly. For example, a system with mains-water chloride of 39 mg/L and system-water chloride of 154 mg/L produced an Excess Chloride result of 115 mg/L. In another Fernox case study, 23.3 mg/L in the mains water and 26.8 mg/L in the system water produced an Excess Chloride result of 3.5 mg/L.
Conceptually:
Excess Chloride = System Water Chloride − Reference Fill Water Chloride
Where the system chloride is not greater than the reference water, the excess value would normally be reported as zero rather than a negative concentration, as demonstrated in Fernox test data.
Simplified definition
Excess Chloride measures how much additional chloride has appeared in the heating system compared with the water originally used to fill it.
This helps separate the chloride that was naturally present in the incoming water from chloride that may have entered or accumulated within the system. A high result can indicate contamination or a change in the system water and may increase the risk of corrosion.
Excess Chloride is particularly useful because a standard Chloride result alone does not show whether the chloride concentration is normal for the local filling water or has increased after the water entered the heating system.
Fernox includes both Chloride and Excess Chloride as separate parameters within relevant laboratory water tests.
An elevated Excess Chloride result may indicate:
Additional chloride contamination entering the heating system after filling.
Chemical residues remaining following cleaning, installation or maintenance.
Introduction of inappropriate chemicals or substances containing chloride.
Contamination from another water or fluid source.
Changes in the system water that cannot be explained by the original filling-water chemistry.
Accumulation of chloride-containing salts within the system.
An increased potential for chloride-related corrosion of susceptible metallic components.
This distinction can be diagnostically important. For example, a mains water supply may naturally contain 40 mg/L of chloride. Finding approximately 40 mg/L in the heating system would not necessarily indicate that chloride contamination has occurred. However, finding 140 mg/L in the system would represent approximately 100 mg/L of excess chloride and provide evidence that the water chemistry has changed since filling.
Elevated chloride levels are important because chloride ions can increase the risk of localised corrosion, including pitting corrosion, particularly in susceptible metallic materials. A historical Fernox System Health Check report identified elevated Excess Chloride alongside a finding of chloride contamination and a risk of pitting corrosion.
Excess chloride can therefore contribute to:
Pitting and localised corrosion.
Damage to susceptible stainless-steel components.
Increased corrosion risk for aluminium and other sensitive system materials under unsuitable conditions.
Corrosion of pipework, heat exchangers, fittings and other metallic components.
Increased release of corrosion products into the system water.
Leakage or premature component failure where corrosion becomes severe.
By comparing the system water against its starting point, Excess Chloride can therefore provide a more useful diagnostic indication of system contamination and chemical change than the absolute Chloride result alone.
Excess Chloride should not normally be assessed in isolation. It is a comparative parameter and depends on having an appropriate reference value for the chloride concentration of the original filling water.
Fernox reporting has used mains-water/reference-water results alongside system-water results when calculating this parameter. Historical Fernox reporting shows a recommended Excess Chloride level of less than 50 mg/L, and a more recent Fernox case study uses the same <50 mg/L criterion.
It is also important to distinguish between Chloride and Excess Chloride:
Chloride measures the total amount of chloride currently present in the system water.
Excess Chloride measures how much additional chloride is present compared with the reference filling water.
A system can therefore have a measurable Chloride result while having little or no Excess Chloride if that concentration is consistent with its original filling water.
The result should be considered alongside total Chloride, conductivity, pH, aluminium, iron, copper, inhibitor concentration and the materials present within the heating system when assessing the significance and potential corrosion risk.
