Central Heating Inhibitor: What It Does and How to Add It

What central heating inhibitor is, why it stops sludge and corrosion in radiators and the boiler, how to tell if yours has run low, and how to add it to a sealed or open-vented system.

Central heating inhibitor is a liquid chemical added to the water that circulates through your boiler and radiators. It slows the corrosion that turns steel radiators into black sludge, and it helps stop scale forming on the boiler's heat exchanger. Every system with radiators should have it in, it should be topped up whenever water is drained or lost, and on most homes it goes in through a radiator bleed valve or the tank in the loft. As the heating goes back on for winter, it's worth knowing whether yours is still protected.

What inhibitor does

The water in a central heating system goes round and round the same circuit for years. Most radiators are made of pressed steel, and the pipes, boiler and valves add copper, brass and sometimes aluminium to the mix. Put those metals in warm water with a little dissolved oxygen and they corrode. The steel rusts from the inside, and the rust becomes a fine black sludge called magnetite. Corrosion also gives off gas, which collects at the top of radiators.

Inhibitor forms a protective film on the metal surfaces and controls the chemistry of the water so that corrosion and scale build up far more slowly. It won't make a system last forever, but a well-dosed system stays clean for much longer than one running on plain tap water.

It's also part of how a boiler should be installed. Approved Document L, the building regulations guidance on heating efficiency, says that before a new boiler is fitted the heating circuits should be thoroughly cleaned and flushed, and a suitable chemical inhibitor added to protect against scale and corrosion, with systems prepared and commissioned to BS 7593. Check your boiler's warranty terms too, and keep the commissioning paperwork from the installation.

Signs your inhibitor has run low

You can't see inhibitor, but you can see what happens when it has gone. Watch for these, especially at the start of the heating season:

Black or dark brown water when you bleed a radiator. Clear or slightly grey water is normal. Inky black water is magnetite sludge; orange-brown water points to rust.
Radiators that need bleeding again and again. A little air after a drain-down is normal. Gas building up every few weeks usually means corrosion is producing it.
Radiators cold at the bottom and hot at the top. Sludge settles in the lower channels and blocks the flow. Our guide to signs your radiators need a power flush covers this in detail.
A boiler that bangs, rumbles or kettles. Scale and debris on the heat exchanger cause local boiling. See noisy boiler causes.
Pinhole leaks in radiators. Steel radiators that corrode from the inside eventually weep at the bottom or along a seam. That radiator usually needs replacing, and the rest of the system needs checking.

If you see two or more of these, topping up inhibitor alone won't fix it. The sludge already in the system has to come out first.

How to check the level

The simple home check is to run a little water from a radiator bleed valve into a clear glass. Clear water is a good sign. Black water means sludge is already circulating, whatever the inhibitor level.

Clear water doesn't prove the inhibitor is at the right strength, though. Inhibitor makers sell test kits, and a heating engineer can test the concentration on the spot or send a sample off during the annual service. BS 7593, the British Standard for preparing and maintaining heating water, treats water quality as an ongoing maintenance item rather than a one-off job at installation, so ask for the check when your boiler service is booked.

Adding inhibitor to a sealed system

Most London flats and houses with a combi or system boiler have a sealed system. There's a pressure gauge on or under the boiler and no small tank in the loft. Because the water is under pressure, you can't just pour inhibitor in. The common methods are:

Turn the heating off and let it cool

Radiators and water should be cold before you open anything.

Choose your entry point

Many retail inhibitors come in a pressurised canister or with an injector that screws onto a radiator bleed valve. If you have a magnetic filter under the boiler, some models have a dosing point on the cap. Follow the product and filter instructions.

Release a little pressure first

Bleed some water from an upstairs radiator so there is room for the inhibitor to go in.

Inject the full dose

Use the amount the label gives for the size of your system. Under-dosing gives little protection.

Repressurise and run the heating

Top the pressure back up to the level in your boiler manual, usually shown on the gauge, then run the heating so the inhibitor mixes round. Our guide on how to repressurise a boiler explains the filling loop.

The older method is to drain one radiator, take it off the wall, pour inhibitor into it and refit it. It works, but it's messy and involves draining and reconnecting valves. If you'd rather not, it takes an engineer a few minutes during a visit.

Adding inhibitor to an open-vented system

If you have a regular boiler with a hot water cylinder and two tanks in the loft, the small one is the feed and expansion tank for the heating. That's where inhibitor goes.

Turn the heating off and let it cool.
Tie up or turn off the float valve on the small tank so it can't refill.
Drain a few litres from a drain-off valve downstairs so the level in the small tank drops.
Pour the inhibitor into the small tank.
Close the drain-off, release the float valve and let the tank refill. The water drawn into the system carries the inhibitor with it.
Run the heating for an hour to mix it through, then bleed any radiators that need it.

Make sure you're using the small tank, not the large cold water storage tank that feeds your taps. If you're not sure which system you have, see types of central heating systems.

How often it needs topping up

Inhibitor doesn't evaporate, but it does get diluted. Any time water leaves the system and fresh mains water replaces it, protection drops. That happens when:

a radiator is removed or replaced, or a valve is changed
the system is drained down for work. Our guide to draining a central heating system ends with the inhibitor step for this reason.
there's a leak, or a pressure relief valve has been discharging
you regularly top up a sealed system's pressure, which is itself a sign of a leak worth finding
a power flush or chemical clean has been done, which always finishes with fresh inhibitor

Beyond that, the sensible routine is a test at each annual service and a top-up when the test says so.

When to get an engineer in

A top-up is a reasonable DIY job on a system that is already clean. Call a heating engineer when:

bleed water comes out black, or several radiators are cold at the bottom. The system needs cleaning first, either a chemical cleanse or a power flush. Our comparison of power flush vs chemical flush explains the difference.
you're topping up pressure more than once every few weeks
you'd like a magnetic filter fitted to catch sludge before it reaches the boiler
you can't tell whether the system is sealed or vented, or the boiler shows a fault code

Don't confuse central heating inhibitor with a scale inhibitor. That's a small device fitted on the cold mains feed to a combi to reduce limescale in the hot water side, and it's worth having in hard water areas like London. Nor is standard inhibitor antifreeze: if your boiler or pipes are in a cold garage or loft, see our guide to frost stats and boiler frost protection.

Our labour rate is £95 an hour, and the £60 call-out is waived when the job runs over 2 hours. We confirm the price before any work starts.

Worried about sludge in your radiators?

Tell us what the bleed water looks like and which radiators are cold. We'll tell you whether it needs a top-up or a proper clean, and confirm the price first.

📞 Call 020 7870 3200

Or see our heating repairs and pricing.

Frequently Asked Questions

Does central heating inhibitor really work?

Yes, as long as it's dosed correctly and goes into a reasonably clean system. It slows the corrosion that turns steel radiators into black sludge and cuts scale on the boiler heat exchanger. It can't remove sludge that has already formed, so a dirty system needs cleaning before fresh inhibitor goes in.

How often should you add inhibitor to central heating?

There's no fixed calendar interval. Add it whenever water has been drained or lost, for example after a radiator is changed, the system is drained down or a leak is repaired, and have the concentration tested at the annual boiler service. If you top up a sealed system's pressure often, fresh water is diluting the inhibitor each time.

Can you put too much inhibitor in a central heating system?

Overdosing with the same product is generally less of a worry than underdosing, but it wastes money and doesn't add protection. Follow the dose on the label for the size of your system, and don't mix different brands or cleaner with inhibitor unless the manufacturer says they are compatible.

Is central heating inhibitor the same as antifreeze?

No. Standard inhibitor protects against corrosion and scale but does not stop water freezing. Combined antifreeze and inhibitor products exist for systems with pipework in unheated outbuildings or for some heat pump systems. Most London homes with an indoor gas boiler don't need antifreeze, because the boiler's frost protection and a low heating setting do that job.

Is scale inhibitor the same as central heating inhibitor?

No. A scale inhibitor is a device fitted on the cold mains supply to a combi boiler or water heater to reduce limescale in your hot water. Central heating inhibitor is a liquid that goes into the sealed water in the radiators. Homes in hard water areas like London often benefit from both.