Central heating inhibitor: what it does and how to add it

What inhibitor actually does

A heating system is a loop of water running through steel radiators, copper pipes and a boiler heat exchanger made of aluminium or stainless steel. Mixed metals and oxygen in water corrode. The steel rusts into a fine black magnetic sludge called magnetite, and the reaction gives off hydrogen gas. Hard water adds limescale on the hottest surfaces, which in a boiler means the heat exchanger.

Inhibitor is a blend of chemicals that coats metal surfaces and keeps the water chemistry stable, so corrosion and scale happen far more slowly. It is not a cleaner and it is not a leak sealer. Those are separate products with different jobs.

Signs your system is short of inhibitor

You cannot see inhibitor, so the clues come from what corrosion does to the system. Any one of these on its own could have another cause, but two or three together point strongly at poor water treatment.

• Radiators need bleeding often, sometimes every few weeks. The gas is usually hydrogen from corrosion, not air.
• Water from a bleed valve is black or dark brown rather than clear or slightly tinted.
• Radiators are warm at the top but cold along the bottom, where sludge settles.
• The boiler kettles, bangs or rumbles as scale and debris build up in the heat exchanger.
• A magnetic filter, if you have one, fills with black sludge between services.
• Pinhole leaks appear in the bottom of steel radiators.

Inhibitor is a chemical. Wear gloves, keep it away from children and pets, and do not drain treated heating water into a garden or a surface water drain. Follow the product label.

How to add inhibitor to a sealed system

Most homes with a combi or system boiler have a sealed, pressurised system, with a pressure gauge on or under the boiler. Check the label on the bottle for how much your system needs: most brands size a standard bottle for an average home and tell you to use more for a larger system.

Many inhibitors come in a pressurised canister with a nozzle that screws into a radiator bleed valve or a filter dosing point, which is the easiest method. If yours is a liquid in a bottle, you need an inhibitor injector or dosing tool.

• Turn the heating off and let the system cool completely.
• Note the pressure on the boiler gauge.
• Pick a radiator, ideally upstairs, and close both of its valves if you want to limit mess.
• Release some pressure by opening the bleed valve with a cloth underneath, then close it.
• Attach the canister or injector to the bleed valve as the product instructions show and push the inhibitor in.
• Reopen the radiator valves, top the pressure back up through the filling loop to the level your boiler manual gives (usually around 1 to 1.5 bar cold), and check for air.
• Run the heating for an hour or so to circulate it, then bleed any air that has collected.

How to add inhibitor to an open vented system

Older systems with a regular boiler usually have a small feed and expansion tank in the loft, separate from the large cold water tank. Inhibitor goes into this small tank.

Turn the heating off. Tie up the ball valve in the small tank so fresh water does not rush in, then drain a little water from a radiator or drain-off valve downstairs so the level in the tank drops. Pour the inhibitor into the tank, release the ball valve so the level refills and draws the inhibitor down into the system, then run the heating to mix it through.

Make sure you have the right tank. Inhibitor must never go into the large cold water storage tank that feeds your taps.

How often to top it up

Inhibitor does not evaporate, but it gets diluted every time water is added. That happens when a radiator is changed, a leak is repaired, the system is drained, or a sealed system is repeatedly topped up because it keeps losing pressure.

Manufacturers recommend testing the concentration once a year, which a good boiler service can include. Simple test kits are available, and some brands offer a lab test. Treat the system again after any drain-down, and at a full dose after a new boiler, a power flush or a system clean. Boiler manufacturers generally make system cleaning and inhibitor part of the conditions of their warranty, so keep a note of what was added and when.

If you are topping up the pressure every week or two, adding more inhibitor is not the answer. The system is losing water somewhere, and the fresh water brings in oxygen that drives more corrosion. Find the leak first.

When inhibitor alone is not enough

Inhibitor protects a clean system. Pouring it into a system full of sludge is like painting over rust: it slows things down but leaves the blockage where it is. If radiators are cold at the bottom, the bleed water is black, or the boiler is noisy and slow to heat, the system usually needs cleaning first.

Depending on how bad it is, that means a chemical clean with the system running, a mains flush, or a power flush that pushes cleaning chemicals through each radiator in turn. After any of them, the system is refilled, treated with inhibitor at full strength, and ideally fitted with a magnetic filter to catch what still circulates.

Getting it done properly

Adding inhibitor is a reasonable job for a confident homeowner. Diagnosing why a system keeps corroding, finding the leak behind constant top-ups, or cleaning a system that is already full of sludge is not. Loyal Plumbers has 10 vans covering London and everywhere inside the M25, 24 hours a day, and we service boilers, repair heating systems and carry out power flushes.

If you get in touch, tell us how often you bleed the radiators, what colour the water is, whether you top up the pressure, and the age of the boiler. That tells us whether you need a simple dose of inhibitor at the next service, or a clean first.