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.
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.
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.
