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, aluminium, and mixed circuits
It becomes essential as soon as there's steel or cast iron (radiators, boiler bodies), aluminium (exchangers), or a "mixed" copper-steel-aluminium 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.
In France, the energy saving certificate scheme goes further than good practice. On an individual heating installation whose nominal thermal output is 70 kW or less, certificate BAR-SE-108 describes a four-stage power flush and places the injection of an inhibitor, at the dose the manufacturer states, at stage d, the last one. For communal systems, BAR-SE-109 places it at stage c, the last of its three. Put another way, a power flush without an inhibitor is not an incomplete power flush, it is an operation that fails the certificate. One caveat for heat pumps: BAR-SE-108 excludes loops heated, wholly or partly, by an air-to-water, water-to-water, ground-to-water or hybrid heat pump, so the step stays technically necessary but earns no certificates.
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, aluminium), 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' behaviour. 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: aluminium, underfloor heating, heat pump, top-up water, and antifreeze
Choose a multi-metal inhibitor compatible with aluminium 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.
What is left is choosing which visit carries that check, and the interval is not the same depending on the generator.
| Generator | Mandatory interval | Reference text |
|---|---|---|
| Boiler of 4 to 400 kW | 12 months | French arrêté of 15 September 2009 |
| Thermodynamic system of 4 to 70 kW | 24 months at most | French arrêté of 24 July 2020, code de l'environnement |
On a boiler, the annual service gives the concentration reading a natural slot. On a heat pump, sticking to the regulatory deadline alone can leave 24 months without a check, while water top-ups do not space themselves out to match. That visit belongs in the service contract, not in the statute.
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 labelling, 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 labelling and safety data sheet. In 2026, this evidence also helps in case of a funding audit and for the maintenance certificate of the relevant equipment.
On a job funded by the French energy saving certificate scheme, that evidence has a set format. The document drawn up, dated and signed by the professional must carry, among other entries, the total water volume of the circuit along with the cleaning agent and the inhibitor used. Two lines to be recorded during the intervention, not that evening. Both certificates set a conventional lifetime of 12 years, and apply to operations committed before 1 August 2030 for BAR-SE-108 and before 1 April 2030 for BAR-SE-109.
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, aluminium, PEX underfloor heating. A mixed circuit needs an inhibitor compatible with all the metals present, without any risk of reaction with aluminium. 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 (aluminium, glycol, seals), and the target concentration in ml per litre. 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.
