System Inhibitor: What It Is and Why You Need It

Central heating inhibitor is a chemical treatment added to the central heating water to prevent corrosion and scale buildup. If your system doesn't have it, it's corroding from the inside right now.

How it works

Inhibitor chemicals (the most popular UK product is Fernox F1) work in two ways:

• Corrosion inhibition: Creates a thin protective film on metal surfaces, preventing iron oxide formation
• Scale inhibition: Prevents limescale deposits on heat exchanger surfaces (important in hard water areas like Northamptonshire)

How much should be in the system?

Typically 1 litre of inhibitor per 100 litres of system water. The engineer checks the inhibitor level using a test strip during the annual service. If levels have dropped (due to water top-ups), they can dose the system through a radiator or dosing point.

How do you add inhibitor?

It can be added through a drain valve, a magnetic filter service port, or through a radiator bleed point. It's something your engineer does at service time — you don't need to add it yourself.

Does inhibitor expire?

Chemical inhibitors gradually deplete over time, especially if the system has had water added. Annual testing at service time ensures levels remain effective. A full dose of inhibitor typically lasts 2–3 years before needing top-up.

Can I use any brand?

We recommend Fernox F1 or Sentinel X100. Don't mix different brands without checking compatibility. We include inhibitor dosing as part of all new boiler installations and annual services.