Technical definition
pH is a measurement of the acidity or alkalinity of the heating system water. It is based on the activity of hydrogen ions in the water and is expressed on a logarithmic scale, meaning that a change of one pH unit represents approximately a tenfold change in hydrogen-ion activity.
A pH of around 7 is neutral, lower values are increasingly acidic and higher values are increasingly alkaline. In heating systems, pH is particularly important because it strongly influences corrosion rates, metal stability, scale formation and the effectiveness of corrosion inhibitors.
Different metals perform best within particular pH ranges. This is especially important in modern mixed-metal systems containing steel, copper, brass and aluminium. Fernox states that excessive acidity or alkalinity can accelerate metal corrosion, particularly in mixed-metal heating systems.
Fernox Protector F1 uses pH buffering to help keep domestic system water between pH 6.5 and 8.5, which Fernox identifies as an optimum range for preventing corrosion and limescale formation when using that treatment.
Simplified definition
pH tells you whether the heating system water is too acidic, too alkaline or within a suitable balanced range.
Keeping the pH at the correct level is important because water that is too acidic or too alkaline can attack metals inside the system. The correct pH helps protect boilers, heat exchangers, radiators and pipework from corrosion and scale-related problems.
pH is one of the most important heating-system water-quality measurements because it directly affects the chemical environment experienced by every metallic component within the system.
An abnormal pH result may indicate:
Acidic system water, which can increase corrosion of metallic components.
Excessively alkaline system water, which can also increase corrosion risk for certain metals.
Inadequate or depleted water-treatment chemicals.
Incorrect chemical dosing.
Residual cleaning or installation chemicals.
Contamination from another water or chemical source.
Changes in water chemistry following draining, flushing or refilling.
Loss of the system's normal chemical buffering capacity.
A treatment product that is no longer maintaining the expected water conditions.
If the pH becomes too low, the water becomes increasingly acidic and can accelerate the dissolution and corrosion of metals. This may contribute to increased concentrations of Iron, Copper or Aluminium within the circulating water.
If the pH becomes too high, some materials can also become vulnerable. This is particularly important for aluminium-containing components, which require appropriate water chemistry and inhibitor protection. Fernox specifically formulates Protector F1 for mixed-metal systems including aluminium and uses pH buffering as part of its corrosion-protection strategy.
Incorrect pH can therefore contribute to:
Corrosion of radiators and steel components.
Corrosion of aluminium heat exchangers and other aluminium components.
Corrosion or deterioration of copper-containing materials.
Increased concentrations of dissolved metals.
Formation of corrosion products and sludge.
Reduced effectiveness of corrosion inhibitors.
Scale or deposit formation under suitable water-chemistry conditions.
Fouling of heat exchangers, pumps and valves.
Leakage and premature component failure.
Reduced heating-system efficiency and service life.
pH also affects scale-forming chemistry. The behaviour of calcium, carbonate and other dissolved minerals changes according to pH, meaning an unsuitable pH can increase the tendency for mineral deposits to form under appropriate conditions. Fernox identifies maintaining an appropriate pH as important for controlling both corrosion and limescale formation.
Testing pH can consequently provide an early indication that the system's overall water chemistry is becoming unsuitable, sometimes before significant physical corrosion or deposits have developed.
pH should therefore be considered alongside parameters such as M-Alkalinity, P-Alkalinity, Conductivity, Calcium, Total Hardness, Iron, Copper, Aluminium and inhibitor concentration when assessing the overall condition of the heating system.
pH should not normally be assessed completely in isolation. Although it is a highly important parameter, the significance of the result depends on the materials present, treatment product, filling-water chemistry and appliance manufacturer's requirements.
There is also no single pH range that should automatically be applied to every heating or cooling system. For example, current Fernox residential Protector F1 literature describes pH 6.5–8.5 as its optimum controlled range, while Fernox's light-commercial Protector F1 literature specifies an optimum range of pH 7–8.5 for that application.
This demonstrates why the application's pass/fail criteria should reflect the specific system type, treatment regime and applicable guidance, rather than relying on one universal threshold.
It is also important to distinguish between pH and alkalinity:
pH indicates how acidic or alkaline the water is at the time of measurement.
Alkalinity indicates the water's capacity to resist changes in pH.
A system can therefore have an acceptable pH but relatively little buffering capacity, making it more vulnerable to future changes.
Temperature can also influence measured pH, so laboratory and field results should ideally be assessed using an appropriate and consistent measurement method.
The most useful interpretation therefore considers pH alongside the system materials, inhibitor concentration, alkalinity, corrosion-metal results and previous water-test history.
