Central Heating Inhibitors for Corrosion & Rust Build Up

Central heating inhibitors are one of the most overlooked parts of a UK wet heating system — yet they’re also one of the cheapest ways to protect a boiler, keep radiators heating evenly, and avoid breakdowns that run into the hundreds.

Put simply, an inhibitor is a chemical additive mixed into your system water. Its job is to slow corrosion, reduce scale, and stop sludge forming inside radiators, pipework and heat exchangers. When it’s doing its job properly, everything lasts longer and runs more efficiently — often by a noticeable margin.

Most modern manufacturers assume an inhibitor is present. Without one, even a new boiler can start suffering internal damage far sooner than expected.

Why Central Heating Systems Corrode in the First Place

Water inside a heating system isn’t static. It heats up, cools down, expands, contracts and slowly reacts with the metals it passes through.

Over time:

• Oxygen dissolved in the water reacts with steel radiators and iron components

• Microscopic rust particles form and circulate

• Those particles combine into a black residue often called sludge

In harder water areas — including much of Yorkshire and the North — mineral content adds another layer of stress. Scale can form alongside corrosion, while bacteria and installation debris accelerate wear, especially in systems that haven’t been serviced regularly.

None of this happens overnight. It’s gradual, which is why many homeowners don’t realise anything’s wrong until radiators stop heating properly or a pump starts complaining.

How Central Heating Inhibitors Actually Work

Rather than “cleaning” the system, inhibitors change the chemistry of the water so damage happens much more slowly.

A good inhibitor will:

• Create a thin protective layer on metal surfaces

• Reduce how easily oxygen reacts with iron and steel

• Keep tiny particles suspended so they don’t clump together

• Stabilise pH so the system stays chemically balanced

Once added, the inhibitor circulates with the heating water and protects everything it touches. In a typical UK home, one litre treats around 100–130 litres of system water, which covers most 8–12 radiator setups.

Under current UK best practice (BS 7593), protection should be checked regularly and topped up when needed.

The Most Trusted Central Heating Inhibitors in the UK

In real-world installs, three products dominate — not because of marketing, but because they consistently work across mixed-metal systems and modern boilers.

All three are compatible with aluminium heat exchangers and sealed systems. In practice, the difference between them is small — most engineers match the inhibitor to the filter or cleaner already in use rather than chasing marginal performance gains.

What Inhibitors Do for Homeowners (In Plain Terms)

When an inhibitor is present and maintained, systems tend to behave better across the board.

• Lower bills
  Clean heat exchangers transfer heat more efficiently. Many households see 10–20% less energy loss, which can mean £70–£150 a year saved without changing habits.

• Fewer breakdowns
  Pumps, valves and diverters last longer when they’re not grinding away at sludge. That alone can avoid £200–£500 callouts.

• More even heating
  Radiators warm up faster, cold spots reduce, and circulation noise is far less common.

• Longer system life
  Boilers and radiators protected from corrosion routinely last several times longer than neglected systems — and warranties remain valid.

Signs Your Heating System Is Missing Inhibitor

You don’t need lab tests to spot when protection has dropped off. Common signs include:

• Radiators heating slowly or unevenly

• Brown or dirty water when bleeding radiators

• Boiler or pump noise that wasn’t there before

• Energy bills creeping up without usage changes

During an annual service, engineers often check inhibitor levels. If protection is low and nothing’s done, damage tends to accelerate quietly in the background.

How to Add Central Heating Inhibitor (DIY-Friendly Method)

For reasonably clean systems, adding inhibitor is straightforward and doesn’t require a full drain.

Typical radiator dosing method:

1. Turn off the boiler and let the system cool

2. Isolate one radiator using the valves

3. Drain a small amount of water (1–2 litres)

4. Remove the bleed plug and add the inhibitor using a dosing tool or funnel

5. Refit the plug, reopen valves and repressurise to 1–1.5 bar

6. Run heating for 30–60 minutes and bleed air

For dirty systems, it’s best added after a chemical clean or power flush, once debris has been removed.

How Often Should Inhibitor Be Replaced?

Inhibitors don’t last forever. They slowly deplete as they do their job.

• Check levels once a year, usually during a service

• Re-dose after draining, leaks, radiator changes or system refills

• Expect 1–2 years maximum protection per dose

Test kits are available, but many homeowners simply top up annually as preventative maintenance.

Inhibitor vs System Cleaner: Not the Same Job

These two products are often confused, but they do opposite things.

If a system is already dirty, clean first, then inhibit. Swapping the order rarely ends well.

When Inhibitors Matter Most

Inhibitors aren’t optional in many situations:

• After a power flush

• With a new boiler installation

• Following radiator replacements

• In hard water areas

• On older systems (most UK homes built before 2000)

Only very new, untouched systems might temporarily get away without one — and even then, not for long.

Final Thought

Central heating inhibitors don’t improve comfort in a dramatic, obvious way — and that’s why they’re easy to ignore. Their value shows up quietly, over years, in lower bills, fewer repairs, and boilers that just keep going.

For the cost of a takeaway each year, they’re one of the smartest bits of protection you can add to a heating system.