Technical definition
Total Dissolved Solids (TDS) Calculated is an estimated measurement of the total concentration of dissolved ionic substances present in the heating system water, expressed in milligrams per litre (mg/L).
These dissolved substances can include calcium, magnesium, sodium, potassium, chloride, sulphate, bicarbonate, carbonate and dissolved treatment chemicals, together with other soluble salts and minerals.
The word “Calculated” is important. Rather than physically evaporating the water and weighing the remaining dissolved material, calculated TDS is normally derived from the water's electrical conductivity using an appropriate conversion factor:
Calculated TDS (mg/L) ≈ Conductivity (µS/cm) × conversion factor
The exact conversion factor depends on the ionic composition of the water. Published analytical guidance typically places conductivity-to-TDS conversion factors in approximately the 0.55–0.8 range, demonstrating that TDS calculated from conductivity is an estimate rather than a direct measurement.
Fernox Laboratories specifically lists “Total Dissolved Solids (calculated)” as one of its general water-quality parameters.
Simplified definition
Total Dissolved Solids tells you roughly how much dissolved material is present in the heating system water.
This includes dissolved salts, minerals and treatment chemicals that cannot necessarily be seen in the water. A high or changing TDS result can indicate that the system water contains more dissolved material than expected and can help identify contamination, inadequate flushing or changes in water chemistry.
TDS is important because it provides a useful overall indication of the amount of dissolved material within the system water.
Rather than identifying one particular chemical, TDS gives a broad indication of how concentrated the dissolved salts and treatment chemicals are.
An abnormal or unexpectedly high TDS result may indicate:
High concentrations of dissolved salts or minerals in the filling water.
Residual cleaning chemicals remaining after a system clean.
Insufficient flushing following chemical cleaning.
High concentrations of treatment chemicals.
Contamination from another water or chemical source.
Repeated topping-up with mains water.
Concentration of dissolved substances within the system over time.
Changes in system-water chemistry since the system was originally filled.
Introduction of hard or highly mineralised filling water.
An unexpected increase in dissolved corrosion products or other ionic substances.
One particularly important use of TDS in heating systems is assessing whether a system has been adequately flushed following chemical cleaning.
Fernox specifically states that the primary use of its TDS Meter is to determine whether a system has been correctly flushed after chemical cleaning by comparing the TDS readings of the mains water and the system water.
For example, if the incoming filling water has a relatively low TDS but the system water remains significantly higher after cleaning, this can indicate that cleaner, dissolved contaminants or other chemical residues remain within the system.
Elevated dissolved solids can also affect the overall characteristics of the system water because dissolved ions influence electrical conductivity and electrochemical behaviour. Conductivity increases as the concentration of dissolved ionic substances increases.
Testing calculated TDS can therefore help:
Assess whether a system has been properly flushed.
Compare system water with the original filling water.
Detect unexpected changes in dissolved mineral or salt content.
Identify possible chemical contamination.
Monitor changes following cleaning, treatment or maintenance.
Support interpretation of an unusually high Conductivity result.
Build a broader picture of overall system-water quality.
Identify systems where further chemical analysis may be required.
TDS should therefore be considered alongside parameters such as Conductivity, Chloride, Sodium, Potassium, Calcium, Magnesium, Sulphates and treatment-related measurements when assessing the overall chemistry of the system water.
Calculated TDS should not normally be assessed in isolation, and a high value does not automatically mean that the heating system water is unsuitable.
This is particularly important because beneficial treatment chemicals also contribute to TDS. A correctly treated system containing corrosion inhibitor, glycol or other dissolved treatment chemicals may legitimately have a higher TDS than the untreated filling water.
The composition of the dissolved material is therefore often more important than the total quantity.
It is also important to distinguish between Calculated TDS, Conductivity and Suspended Solids:
Calculated TDS estimates the concentration of dissolved substances in the water.
Conductivity measures how effectively the dissolved ions allow the water to conduct electricity.
Suspended Solids measures undissolved particles such as sludge, rust and corrosion debris.
A system can therefore have clear-looking water with very low Suspended Solids but still have a high TDS, because dissolved salts and chemicals are invisible.
Conversely, heavily contaminated or sludged water can contain high Suspended Solids without the same proportional increase in calculated TDS.
Because Calculated TDS is derived from conductivity, it should also be understood as an estimate rather than a direct gravimetric measurement. The relationship between conductivity and TDS varies according to which ions are present. Analytical guidance notes that conversion factors can vary significantly depending on water composition.
For the Fernox App, the conversion factor used by the Fernox laboratory should therefore be the authoritative basis for any calculated value or associated pass/fail threshold rather than applying a generic conductivity-to-TDS conversion.
The most useful interpretation is obtained by comparing TDS with the original filling-water result, Conductivity, known treatment products and previous system-water results.
