--- title: "What is Odour?" description: "Odour is an assessment of any unusual smell from the system water. Strong or unexpected odours can indicate chemical contamination or microbiological activity; for example, a rotten egg smell may be associated with hydrogen sulphide produced by sulphate-reducing bacteria. Odour should always be considered alongside other chemical and microbiological test results." url: https://help.fernox.app/en/articles/181-what-is-odour collection: "Chemical Parameters" updated: 2026-10-02 language: en alternates: [fr, nl, fr-be, es, de, nl-be, ro, cs, tr, pl, sv, sk] --- # What is Odour? Odour is an assessment of any unusual smell from the system water. Strong or unexpected odours can indicate chemical contamination or microbiological activity; for example, a rotten egg smell may be associated with hydrogen sulphide produced by sulphate-reducing bacteria. Odour should always be considered alongside other chemical and microbiological test results. ### Odour **Technical definition** Odour is a **qualitative assessment of any noticeable smell associated with the heating system water**. Unlike parameters such as pH, chloride or iron, it is not normally expressed as a numerical concentration. In a correctly treated closed heating system, the circulating water would not normally be expected to have a strong or unusual odour. An abnormal smell can provide an early indication of **microbiological activity, chemical contamination, degradation of treatment products or other changes in the system water**. A particularly important example is a **hydrogen sulphide or “rotten egg” type odour**, which Fernox associates with the presence of corrosive **sulphate-reducing bacteria** within heating systems. **Simplified definition** Odour describes **whether the heating system water has an unusual or noticeable smell**. System water should not normally have a strong unpleasant odour. A bad or unexpected smell can indicate **bacterial activity, contamination or a change in the chemicals present within the system**. ### Why is Odour important to test? Odour is useful because certain changes in heating-system water can produce distinctive smells that provide an immediate indication that **further investigation may be required**. An abnormal Odour result may indicate: - **Microbiological activity** within the heating system. - Sulphate-reducing bacteria producing hydrogen sulphide. - Chemical contamination. - Residual cleaning or treatment chemicals. - Breakdown or degradation of chemicals already present in the system. - Introduction of an inappropriate chemical or external fluid. - Changes in water chemistry following cleaning, maintenance or refilling. - Stagnant conditions that have encouraged biological growth. - Contamination of the system from another water source. One of the most diagnostically significant odours is a **rotten egg or sulphurous smell**. Fernox states that hydrogen sulphide can occur in heating systems and associates its characteristic bad-egg smell with **corrosive sulphate-reducing bacteria**. This is important because sulphate-reducing bacteria can be associated with **microbiologically influenced corrosion (MIC)**. Their activity can create local conditions that encourage deterioration of metallic components. Where microbiological contamination is present, potential consequences can include: - **Accelerated localised corrosion**. - Formation of hydrogen sulphide. - Black deposits or sludge. - Biofilm formation. - Fouling of system components. - Deterioration of pipework, heat exchangers or fittings. - Reduced effectiveness of the water-treatment programme. - Persistent unpleasant odours. - Premature component failure in severe cases. Fernox also provides products specifically intended to control bacteria, algae and biofilm within water systems, highlighting the importance of microbiological contamination as a water-quality consideration. Other odours can potentially originate from treatment chemicals themselves. For example, Fernox System Neutraliser is described as having a slight **ammoniacal odour**, demonstrating why the treatment history needs to be known before an unusual smell is automatically interpreted as contamination. Odour should therefore be considered alongside parameters such as **Appearance, Sulphates, Ammonia, Nitrite, Nitrate, pH, Suspended Solids and microbiological test results** when assessing the overall condition of the system water. ### Important interpretation note Odour should **not normally be assessed in isolation**. It is a subjective observation and generally cannot identify a specific contaminant or chemical without supporting laboratory analysis. For example: - A **rotten egg smell** may indicate hydrogen sulphide associated with sulphate-reducing bacteria. - An **ammonia-like smell** may be associated with ammonia or an ammoniacal treatment chemical, but odour alone cannot confirm the cause. Fernox System Neutraliser, for example, has a slight ammoniacal odour as part of its normal product specification. - A chemical smell may result from a legitimate cleaner, inhibitor, glycol or other treatment product. - The absence of any noticeable odour does **not** confirm that the system water is chemically healthy. Odour should therefore be used as an **observational warning indicator rather than a definitive diagnostic test**. Any unusual odour should be assessed alongside the system's **treatment history, appearance, laboratory chemistry and microbiological results** before determining its cause. Samples containing unknown chemicals should also not be deliberately inhaled to identify an odour.