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Adding inhibitor is second nature to heating engineers when they install or commission a heating system. But what happens after that? In reality, many systems never get checked again, and that’s where corrosion begins. Leaks, unnoticed system changes and routine top-ups can all dilute inhibitor protection without any visible warning. By the time symptoms show up, the damage is already underway.
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In fact, 93% of engineers agree that incorrect water treatment leads to future heating system issues and increased lifetime costs, yet inhibitor checks are still not routinely carried out at every annual service by some installers.
This blog explains when and how to check inhibitor levels during a boiler service, and why it’s a critical step in protecting both the system and your reputation.
According to BS 7593:2019+A1:2024, checking inhibitor levels during a service is now considered best practice under Approved Document L .
You should test and maintain system water quality throughout the life of the heating system, not just during installation. This means testing and topping up if necessary
BS 7593 recommends cleaning and flushing a system before installing the appliance; fitting an inline filter; and every five years carrying out a full laboratory test of the system water or re-dosing with the inhibitor.
One of the biggest challenges with inhibitor is that you can’t see when it’s reduced in concentration. It’s colourless, so you can’t see when central heating inhibitor levels are low. Dilution is usually caused because the system has been topped up when the pressure has dropped, or because there’s a micro-leak somewhere in the system, or work, such as removing and replacing a radiator, has been carried out.
As fresh water enters the system, the protection is gradually weakened. And once inhibitor levels drop below effective concentration, the system becomes vulnerable. By the time there are cold spots or noise, sludge is building up and corrosion has already been progressing internally.
This isn’t just about system health. It directly affects your liability and reputation.
Many manufacturers specify correct inhibitor levels as a condition of the appliance's warranty. If an appliance breaks down and inhibitor levels aren’t correct, claims under the warranty could be rejected.
Around 87% of boiler breakdowns are linked to poor water treatment, so, by preventing the build-up of sludge and scale, your customers won’t experience as many issues and that means fewer callbacks for you.
Checking the inhibitor takes less than five minutes, so adding it to your service checklist is a small step that could prevent major issues later. And documenting the check on Benchmark provides a record that shows due diligence and compliance with Part L on your part.
The easiest way to check inhibitor levels is using a simple test kit, such as the Sentinel InstaCheck kit with its mobile app, which generates a tailored water quality report with recommendations for ongoing system care.
The InstaCheck step-by-step testing process gives immediate results. It assesses water quality and inhibitor levels, identifying water quality issues that engineers can remedy while on site. The validation process is based on BS7593 best practice, designed to achieve high levels of system protection.
When topping up, use the same inhibitor brand already in the system. Mixing inhibitor brands is not recommended as compatibility cannot be guaranteed.
With the increasing number of heat pumps being installed, and a move towards lower temperature systems, including those with boilers, it is even more critical to ensure the whole heating system is well maintained and optimised for maximum efficiency.
Damage to components, such as steel radiators, copper pipework and aluminium heat exchangers, isn’t immediately obvious, but can begin once protection is reduced. Magnetite (black iron oxide), commonly known as sludge, accumulates in low-flow areas, reducing efficiency and circulation. Limescale build-up, especially in hard water areas, also reduces efficiency – just 1.6mm of limescale can reduce efficiency by 12%.
As the system water deteriorates, the customer may experience cold spots on radiators and a noisy system. Eventually, this can lead to premature component failure, including blocked heat exchangers. And then you’re dealing with repairs instead of prevention.
When you’re servicing the boiler, it’s a simple process to check, correct and record inhibitor levels. It’s one of the simplest ways to prevent long-term system issues before they start. You’ll be protecting the system, providing a professional service for your customer and helping them save money in the long run. The engineers who build a reputation for thorough, documented servicing are the ones customers call back. And, importantly, you will be supporting compliance with Part L of the Building Regulations.
