Technical definition
Nitrate is a measurement of the concentration of dissolved nitrate ions (NO₃⁻) present in the heating system water, expressed in milligrams per litre (mg/L).
Nitrate occurs naturally in water and is formed through the oxidation of nitrogen-containing compounds such as ammonia. Its concentration in mains or filling water therefore varies according to the water source and local environmental conditions.
Within heating-system water analysis, nitrate is primarily useful as an informational and diagnostic parameter. Fernox laboratory testing includes nitrate as one of the common anions measured by ion chromatography, and current Fernox laboratory reporting categorises Nitrate as “For Information” rather than applying a universal pass/fail concentration.
Simplified definition
Nitrate is a dissolved substance that can naturally be present in the water used to fill a heating system.
Testing nitrate helps build a picture of the chemical composition and history of the system water. An unexpected difference from the original filling water can indicate that the water chemistry has changed or that another water or chemical source may have been introduced.
Nitrate is useful because it contributes to the overall chemical fingerprint of the heating-system water and can help identify differences between the circulating system water and the water originally used to fill the system.
An unusual or unexpected Nitrate result may indicate:
Naturally elevated nitrate in the original mains or filling water.
A different water source having subsequently been introduced.
Repeated topping-up or refilling with water of a different chemical composition.
Introduction of a nitrate-containing chemical, where applicable.
Residual chemicals following cleaning or treatment, depending on the products used.
Contamination from another water or chemical source.
Changes in nitrogen-containing compounds within the system water.
A difference between the current system-water chemistry and previous laboratory results.
Fernox's published laboratory example demonstrates the diagnostic nature of the parameter. In that analysis, Nitrate was recorded alongside Nitrite and Sulphates and was specifically classified as “For Information” rather than being given a defined acceptable limit.
Nitrate also exists as a dissolved ion and therefore contributes to the overall ionic content of the water, although its contribution should be considered as part of the wider conductivity and water-chemistry profile rather than interpreted on its own.
Testing nitrate can therefore help:
Establish the chemical characteristics of the original filling water.
Compare filling water with circulating system water.
Identify unexplained changes in water composition.
Support investigations into contamination or changes in filling-water source.
Provide additional context when interpreting Conductivity, Nitrite, Chloride, Sulphates and other dissolved-ion measurements.
Nitrate should therefore primarily be regarded as a supporting diagnostic parameter, rather than a direct standalone indication of scale or corrosion within a typical heating system.
Nitrate should not normally be assessed in isolation, and the presence of nitrate does not automatically indicate a problem with the heating system.
Fernox currently categorises Nitrate as “For Information”, meaning that there is not a single universal Fernox pass/fail concentration applicable to all systems.
It is also important to distinguish between Nitrate and Nitrite:
Nitrate (NO₃⁻) is a relatively stable oxidised form of nitrogen and may naturally be present in filling water.
Nitrite (NO₂⁻) is a different chemical species and can have different significance within water-treatment chemistry, including use in some corrosion-inhibitor formulations.
The two results should therefore not be treated as interchangeable.
A higher nitrate concentration in the circulating system water than in the original filling water may warrant further investigation, but it does not by itself demonstrate corrosion, contamination or treatment failure. The result is most useful when compared with the filling-water nitrate concentration, previous test results, Nitrite, Conductivity and the known treatment history of the system.
