Corrosion inhibitor: protect the heating system

Understanding the inhibitor and its role in a heating circuit

Why corrosion attacks closed circuits (sludge, gas, efficiency losses)

Even "closed," a heating circuit breathes a little. Water top-ups, micro-leaks, certain oxygen-permeable materials, and dissimilar-metal pairs all fuel corrosion. The result: magnetite particles form black sludge, gas builds up (noise, loss of prime), and exchangers foul. Heat transfer suffers, consumption rises, and the control components work against the current.

What an inhibitor does: passivation, pH stabilization, multi-metal protection

An inhibitor is a blend of additives that limits electrochemical reactions. It creates a durable passivation film on the surfaces, buffers the water to keep a stable pH, and protects several families of metals at once. Properly dosed, it also reduces the suspension of fine particles, which makes filtration easier and prevents sludge from returning too quickly.

When the inhibitor is essential: steel, cast iron, copper, aluminum, and mixed circuits

It becomes essential as soon as there's steel or cast iron (radiators, boiler bodies), aluminum (exchangers), or a "mixed" copper-steel-aluminum network. In 2026, with high-efficiency generators (condensing boilers, heat pumps), a clean exchanger is a real gain. After a power flush, a radiator replacement, a partial drain-down, or frequent top-ups, adding an inhibitor avoids starting from scratch.

Diagnosing a circuit before treatment: the checks that prevent bad surprises

Spotting the signs of corrosion: sludge, black water, cold radiators, pump noise

Before any power flush or inhibitor addition, start with a full walkthrough of the system. Visible sludge at the filter or vents, black water when draining, radiators that stay lukewarm at the bottom, or cold zones often indicate disrupted circulation. In the boiler room, cavitation noise, a pump that's struggling, or repeated air bleeding are signals to take seriously.

Useful on-site measurements: pH, conductivity, presence of magnetite, and oxygen entry points

A quick check on a sample saves time. Check a pH consistent with the materials present (steel, copper, aluminum), a conductivity that isn't spiking, and the presence of magnetite (magnet, sludge trap). Also look for oxygen entry points, such as frequent water top-ups, micro-leaks, faulty automatic air vents, or pipework without an oxygen barrier.

Checking the condition of components: exchanger, circulator, expansion vessel, vents, and fittings

Inspect the exchanger (fouling, flow loss), the circulator (play, noise, speed), and the expansion vessel (pressure, membrane). Check vents, valves, fittings, and seals. If a component is already weakened, overly aggressive cleaning can finish it off. It's better to secure it first, then treat and stabilize the circuit.

Applying the inhibitor correctly: on-site method and points of attention

Prior cleaning or simple addition: how to decide based on the circuit's condition

Before adding an inhibitor, look at the network water and the emitters' behavior. Black water, sludge, radiators lukewarm at the bottom, noise, recent work (boiler, heat pump, or radiator replacement) all point to a cleaning and rinse, with a power flush if needed. If the circuit is healthy, a simple addition during maintenance is enough. The right reflex is a visual diagnosis, not an automatic habit.

Dosage, injection, and circulation: avoiding underdosing and dilution

Base the dosage on the circuit's actual water volume and the product instructions. Inject the inhibitor at a high point (radiator, injection pot, filter) after bleeding the air. Re-pressurize, open all the valves, then circulate to homogenize. Watch out for repeated water top-ups, as they dilute the treatment. Keep simple traceability on the boiler room label.

Compatibilities to respect: aluminum, underfloor heating, heat pump, top-up water, and antifreeze

Choose a multi-metal inhibitor compatible with aluminum and check compatibility with seals, PEX pipe, and underfloor heating. With a heat pump, the treatment remains useful, especially in mixed circuits. If using glycol antifreeze, avoid risky mixes and confirm compatibility, since antifreeze changes the chemistry and can reduce effectiveness. When in doubt, stick to a single product range.

Keeping it effective over time: monitoring, maintenance, and 2026 obligations

Periodic checks in 2026: rechecking pH and concentration after water top-ups

Good reflex. With every water top-up, recheck the circuit's pH and inhibitor concentration (test strip, pH meter, refractometer depending on the product). Note the volume added. If top-ups keep recurring, look for the leak or the expansion-vessel fault before "recharging" the chemistry.

Limiting oxygen ingress: airtightness, vents, automatic top-up, and water quality

Key point. Oxygen feeds corrosion. Aim for genuine airtightness, bleed the system at the right time, then limit air ingress (suitable vents, an automatic top-up that's adjusted and not left running continuously). Use water that meets the manufacturer's recommendations, especially with underfloor heating and steel radiators.

Traceability and documentation: service report, product labeling, maintenance recommendations

To keep. Hand over a service report with the measurements (pH, conductivity if relevant), the product, the dosage, the date, and the actions taken. Keep the product labeling and safety data sheet. In 2026, this evidence also helps in case of a subsidy audit (CEE, MaPrimeRénov') and for the maintenance certificate of the relevant equipment.

Choosing a suitable inhibitor: simple criteria to avoid mistakes

Circuit type and materials: solutions for mixed circuits and older installations

Start by identifying your network. Steel radiators, copper, aluminum, PEX underfloor heating. A mixed circuit needs an inhibitor compatible with all the metals present, without any risk of reaction with aluminum. On an older installation, don't add the inhibitor on top of sludge. Do a rinse or a power flush first, then add the inhibitor into clean water.

Format and application: canister, cartridge, doser, injection via a decanting pot

The canister is dosed at fill-up or after a top-up. The cartridge is placed on a bypass or a designated point, handy when access to the network is limited. A doser prevents oversights on sites with regular top-ups. Injection via a decanting pot is often done through the service point, respecting pressure and air bleeding.

Reading the labels: standards, compatibilities, target concentration, and safety in use

Check the stated standard, the list of compatibilities (aluminum, glycol, seals), and the target concentration in ml per liter. Also check the CLP pictograms, the safety data sheet, and the disposal instructions. A well-chosen inhibitor protects the system without turning the technical room into a laboratory.