In a heating or air-conditioning installation, the fact that the circuit is closed does not mean it is completely protected against corrosion.
In fact, one of the problems that can appear over time is the build-up of corrosion products inside pipes, radiators, heat exchangers, pumps and other components. The result can be a progressive loss of hydraulic and thermal performance and, if the problem advances, breakdowns or more complex maintenance work.
Corrosion in these systems does not usually appear overnight. It is a process that can be related to oxygen ingress, water quality, pH, conductivity, the materials used or the operating conditions of the installation. That is why preventing it requires more than adding a product to the circuit: first you have to understand why it is happening.
Why does corrosion occur in a closed circuit?
Corrosion is an electrochemical process involving the metals in the system, the water and various chemical conditions.
One of the most important factors is dissolved oxygen. Even if an installation is designed as a closed circuit, oxygen can enter during filling, water top-ups, certain pressure problems or through materials and components that allow some diffusion. Air can also enter if there are sealing problems or if the system operates under certain pressure conditions.
That is why, when an installation has recurring corrosion problems, it is not enough to treat the consequences. It is also worth investigating why oxygen is entering or why the water quality is not under control.
Another important factor is the composition of the water itself. The pH, the conductivity and the presence of certain ions can influence the rate and type of corrosion. Temperature, the circulation regime and the combination of different metals can also change the behaviour of the circuit.
What happens when corrosion progresses?
Corrosion products do not disappear. They can remain suspended in the water or settle in certain areas of the circuit.
In systems with iron or steel components, magnetite can appear, a black iron oxide that forms part of the sludge that can build up in hydraulic installations. These deposits can concentrate especially in areas where the circulation favours their sedimentation.
And here one of the most common problems for maintenance professionals appears: an installation can seem to keep working normally while dirt and corrosion product are building up inside it.
Over time, these deposits can:
- reduce water flow;
- foul filters and separators;
- affect pumps and valves;
- hinder heat transfer;
- cause blockages;
- increase maintenance needs;
- contribute to a loss of system performance.
An experimental study published in 2026 on closed hydraulic circuits found that fouling caused by corrosion products could considerably increase pumping energy consumption and that mechanical filtration could recover part of those losses. This is a specific experimental result, so it should not be automatically extrapolated to any installation, but it shows that the corrosion problem is not only chemical: it can also end up having hydraulic and energy consequences.
The signs that should draw attention during maintenance
There is no single symptom that allows corrosion to be diagnosed in an installation. However, there are certain signs that justify a more thorough inspection.
For example, finding very dark water or water with particles, a large amount of dirt in filters, the presence of sludge or recurring circulation problems may indicate a build-up of corrosion products.
It is also worth paying attention when an installation starts to show performance problems with no obvious cause, especially if some components are repeatedly blocked or if the circuit requires frequent interventions.
Inspecting the water and knowing the materials that make up the circuit help determine what is happening. The parameters that can be used to assess the condition of the water include pH, conductivity and other indicators related to the chemistry of the circuit.
How to prevent corrosion
Prevention should begin before large amounts of sludge or deposits appear.
1. Control oxygen ingress
This is one of the fundamental points. A properly sealed installation with adequate pressure reduces the chances of air entering. It is also important to check for possible leaks and avoid unnecessary water top-ups.
If a circuit constantly loses pressure and needs to be refilled, the problem should not be solved simply by adding water every so often. The source of the loss has to be found.
2. Control water quality
The water in a closed circuit should not be considered simply a medium for carrying heat. Its composition has a direct influence on the materials and on the stability of the system.
That is why, depending on the installation, it is necessary to monitor parameters such as pH, conductivity, hardness and other relevant indicators to determine whether the water is suitable.
Specific recommendations depend on the design, materials and the equipment manufacturer. Standards and technical guidelines such as BS EN 14868 and VDI 2035 are used as references for addressing corrosion protection and water quality in different types of installations.
3. Clean the circuit when there is a build-up
Prevention does not replace cleaning when the installation already has a significant build-up of sludge, magnetite or other deposits.
In these cases it may be necessary to carry out a hydraulic cleaning by flushing or sludge removal, combined, where appropriate, with a suitable chemical treatment.
CH Química offers KAMCO solutions designed precisely for the cleaning and hydraulic recovery of closed heating and air-conditioning systems. Their equipment makes it possible to mobilise and extract deposits of magnetite, oxides, sludge and scale accumulated in radiators, heat exchangers, boilers and underfloor heating.
The choice of procedure will depend on the condition of the circuit. Carrying out preventive cleaning is not the same as recovering an installation with a significant build-up of deposits.
4. Protect the circuit after cleaning
Cleaning a circuit and refilling it should not necessarily be the end of the process.
Once the system has been recovered, you need to consider how to keep under control the conditions that favour corrosion. For this purpose there are specific treatments with corrosion and scale inhibitors.
In the Aqua Line range, CH Química offers solutions such as AQ PROTECT, formulated for heating systems and closed water circuits, and AQ PROTECT BIOKIL, which also incorporates a biocidal function. The range also includes AQ NEUTRAL for adjusting the pH after certain cleaning processes.
The right treatment must be determined according to the characteristics of each circuit and following the relevant technical guidelines. There is no single formulation that is valid for all installations.
Corrosion and maintenance: acting before the problem becomes visible
One of the main challenges of corrosion in closed circuits is precisely that part of the problem develops inside the installation.
By the time the technician finds a filter clogged with particles, a heat exchanger with deposits or a loss of flow, corrosion may have been taking place for some time.
That is why a good maintenance programme should not be limited to repairing a component when it fails. It should also take into account the condition of the water, the circuit pressure, the presence of sludge and sediment, the operating conditions and the history of interventions.
The most effective strategy combines several measures: avoiding unnecessary oxygen ingress, controlling water quality, cleaning when buildup is present and maintaining adequate protection of the circuit.
In this way, corrosion stops being merely a problem that appears when something fails and becomes part of the preventive management of the installation.
A protected installation starts with knowing the condition of the circuit
Not all problems in a closed circuit have the same origin or require the same treatment. There may be corrosion, scale, magnetite, biological contamination or a combination of several factors.
That is why, before choosing a treatment, it is essential to diagnose what is happening and understand the characteristics of the installation.
For the professional, this approach makes it possible to move from corrective action, cleaning once the system already has problems, to a maintenance strategy based on controlling the water and the condition of the circuit.
And that is where chemical treatment, hydraulic cleaning and periodic monitoring stop being independent actions and become parts of a single maintenance strategy.