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Last updated: 23rd February, 2024


How To Use Central Heating Inhibitor

How To Use Central Heating Inhibitor

A central heating inhibitor can be a vital part of heating systems that many people forget, which dramatically drops their overall system efficiency and makes it a lot more likely for them to waste money on heating.

But what exactly is a central heating inhibitor, and how does it fit into the rest of your central heating system? Here is a very quick breakdown of what you need to know.

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What Does a Central Heating Inhibitor Do?

A central heating inhibitor is not a physical item, but a chemical compound you use to make sure that the other products and components in the 'chain' run smoothly.

Considering that every central heating system is basically a series of metal pipes that handle water, rust and corrosion are extremely common.

Trying to remove rust once it appears can sometimes cause even more damage.

Dirt, rust, limescale, and other debris can all form into a thick sludge that moves through radiators and pipes, creating blockages that prevent the water from passing by properly.

This sludge build-up can turn high-quality systems into inefficient messes until you are able to drain and bleed them and may even cause permanent damage such as cracks or ruined valves.

If you add inhibitor chemicals to the pipework, you can prevent this from happening. It can break down these minerals and rusted pockets before they become sludge, helping to protect the central heating system from major blockages.

How Do I Add Inhibitor to a Radiator?

Pouring central heating inhibitor into your pipes adds it to the heating system, where it mixes with the water. This stops sludge from forming and allows your boiler, radiators, and storage tank to work properly again.

There are two types of heating system to worry about here: open-vented or sealed. The difference is important because you need to add inhibitor in a slightly different way. Always make sure you turn the heating system off unless you are specifically told not to, since it can be dangerous.

Open-Vented Inhibitor

An open-vented system usually has more than one tank in the house's loft. The inhibitor goes into the expansion tank (the smaller one) and then has to be drained into the system as normal.

Then, you will have to clean the tank itself to make sure that there is not any sign of corrosion or damage.

This is important because you need to know that the tanks are also free from damage that could lead to adding more debris to the pipe layout. If you do see problems with expansion tanks, it means that you need may need to replace it entirely.

Sealed Inhibitor Boiler

Heating systems with single tanks only require you to add the inhibitor through the normal boiler filling loop. Different boiler designs may have slightly different ways of doing this, but the idea is always the same.

Combi Boiler Inhibitor

A combi boiler heating system is slightly different. Make sure that the radiators are cool and the system is turned off, then either add the inhibitor through a bleed valve or a plug (depending on whether you are using regular or towel radiators). Make sure all valves are closed, too.

Once the radiator is ready, plug the inhibitor bottle into the valve and pour it all in, then make sure you turn the heating back on. This should spread it throughout the pipework.

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How Much Do I Add to My Central Heating System?

One litre can often treat around eight to ten individual radiators depending on the exact brand and the radiator designs that you are using, although more than eight can sometimes require two bottles.

Never add more than you should, since more inhibitor does not ensure that you get better protection. It could damage filters (especially magnetic filters) and will basically lead to diminishing returns since there will not be any benefit to adding more inhibitor if you have already added enough to remove all of the limescale.

Trying to 'over-fill' your radiator and boiler system does not offer any major benefit or bonus, so using a full bottle on a single radiator just wastes the money you spend on most of the inhibitor.

How Often Do I Use It?

A professional engineer would generally recommend that you use a new wave of inhibitor annually. The chemical will get diluted as it spends more time in the system, as would all products, and the pressure can sometimes make this faster than you would expect. Different brands could dilute faster, but once a year is a good baseline to work from.

Do not 'top up' your system unless you need to, or the product's guide says this is recommended, since this might end up being a waste.

You can use an inhibitor testing kit to find out how strong the inhibitor still is. This can be a good option to get an idea of how badly you need to add more, and the kits are not always set at a high price. Single-use kits can be cheap, whereas reusable ones fetch a higher price for greater convenience.

What Problems Can It Prevent?

Some issues are best solved before they actually happen, and an inhibitor cleanse is a great way to break down anything that is starting to form at the top of a radiator or the bottom of a pipe.


Drops in boiler pressure are a common sign of a blockage, leak, or another type of damage. It could be in a tank, a pipe, or even the boiler itself. The inhibitor may not be able to stop physical damage on its own, but it can make it much harder for the scale of rusted patches or sludge formations to increase. You do not even need to find the problem area since the inhibitor should break down the debris quickly.


If your radiators, or showers, are not getting hot, they might have a blockage. Instead of getting an engineer to start working on the problem, you can remove it ahead of time by adding inhibitor to stop the blockage from forming, restoring heat to the radiators.


Water can cause rusting in most metals, and you can't usually change rusted metal back into a safe, usable material. An inhibitor product can't protect the entire pipe, but it can stop the rusted parts from trying to build up through the rest of the metal.

Impure Water

While water purity does not always matter that much in a central heating system, you can sometimes see problems if you compare performance. If debris can build up in the boiler tank, less water can be heated at any one time.

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What Else Can I Do?

If you are finding that the inhibitor is not doing enough, then there are other ways you can care for your system. Some may come with a high price, while others are a fairly cheap service, but it is important to know what you are doing before you start messing around with a filter or your boiler system.

Install a Magnetic Filter

A magnetic boiler filter can get more debris and dirt out of the system, making it easier to ensure blockage-free operation.

They catch a lot more, especially metal shards from rusted sections of the pipework, which can be a good way to avoid the slow accumulation of sludge. Even the best regular filters can let small particles through now and then.

Adding a new magnetic filter can require some help, so you might need a registered engineer to handle it if you are not able to use a guide or do not trust your own skills. Keep in mind that this can often come with an added price, so set aside extra cash if you think you might need it.

Use a Scale Reducer

A scale reducer removes limescale from your system, which can save you from the problem of letting it clump up. These are similar to inhibitors, but are specifically aimed at limescale, which means that you are free to use other products that target other problem areas.

A scale reducer can be the best option for dealing with problems that are purely scale-related, giving them a very effective niche use that you should keep in mind if you find piles of scale in your boiler system.

Be Careful With Your Central Heating Inhibitor

Common sense is free, and it can be important when dealing with boilers or other pressure-focused appliances. Whether it is mixing different brands of inhibitor together or trying to make your own sort of inhibitor from various chemicals, there are just some things that you should not attempt unless you are aware of the risks and how to do it safely.

For example, topping up the boiler when it has low pressure can mean that it is leaking, so trying to add more water can dilute the chemical even further. It is better to patch up the problem first, especially if you have already added the inhibitor.

There is also the obvious thing to mention: no matter what sort of inhibitor you have or the brand it came from, do not drink it, and be sure to wash your hands and clothes if you spill any on yourself.

Remember Your Warranty

The warranty of your boiler or heating system, no matter which brand (or brands) they came from, can be very useful. They might reveal specific details, like having access to free component replacements if you suffer certain faults or types of damage. They might also make it easier to get a registered engineer for a lower price.

Each brand can have its own warranty options. Some brands might offer warranties that extend up to ten years, another brand might have warranties that only go up for one or two years. The more you understand about the appliances you bought, the easier it becomes to use the rules to your advantage and save cash or get better support.

Inhibitors can also come with warranties of their own, but they will usually be short-term. These might offer things like refunds if the mixture does not work or the container will not connect to a radiator properly, so read through them carefully before you actually use it.

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Frequently Asked Questions

Is Central Heating Inhibitor Worth It?

Yes, using a central heating inhibitor is worth it. It protects your heating system from corrosion, limescale build-up, and sludge, ensuring it operates efficiently and extends the lifespan of the system. An inhibitor can also improve heat distribution and reduce energy costs.

How Often Do You Need to Change the Inhibitor in a Central Heating System?

It's recommended to check the inhibitor levels annually and top up if necessary. A complete change is advisable every few years or following any significant work on the heating system, such as installing new radiators or a boiler.

Which Is Best Inhibitor for Central Heating?

The best inhibitor for a central heating system depends on the specific needs of your system and water hardness in your area. Popular brands like Fernox, Sentinel, and Adey offer highly regarded products that cater to a variety of systems and requirements.

How Do I Add Inhibitor to My Central Heating?

To add inhibitor to your central heating, you can typically pour it directly into a radiator via a bleed valve or into the system via a filling loop on the boiler. Ensure the system is off and cool before adding the inhibitor and follow the manufacturer's instructions for the correct dosage.

Can You Add Inhibitor Without Draining the System?

Yes, you can add an inhibitor to your central heating system without fully draining it. You can introduce the inhibitor into a radiator or via a specific dosing point in the system, minimizing disruption and avoiding the need to drain the system entirely.

Does Inhibitor Get Rid of Sludge?

While an inhibitor is designed to prevent the formation of sludge and corrosion, it is not a cleaner and might not remove existing sludge. For systems with significant sludge build-up, a specialist cleaning agent and professional power flush may be necessary before adding an inhibitor.

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Article by
Stephen Day | Co-founder
Gas Safe registered and FGAS certified engineer with over 20 years experience in the heating and cooling industry.