What is Central Heating Inhibitor? Re September 22, 2021

What is Central Heating Inhibitor?

engineer fitting system filter

If you are a homeowner or a landlord, we know that you will appreciate any tips given to you when it comes to keeping your property warm, especially throughout the colder months.

Many of us are not aware of what a central heating inhibitor is or how they work. This blog will offer some guidance on simple ways that you can protect and preserve the radiators in your property so that your central heating system works as efficiently as possible and increase the lifespan of your boiler, your pipework and your radiators.

What is a central heating inhibitor?

Central heating inhibitor is a chemical liquid used to protect your central heating system and keep it working efficiently to prolong its lifespan.

Not many would be aware of the role of a central heating inhibitor, however, it is actually quite a vital component within your system that keeps the show rolling behind the scenes. Your central heating inhibitor is as important as your radiators or your boiler itself when it comes to keeping your central heating system going.

Many manufacturers will point out in their manual that without the correct maintenance for your inhibitor levels, your boiler warranty could become invalid.

What is inhibitor made of?

Central heating inhibitor is a liquid that contains a mixture of potassium tetraborate tetrahydrate, disodium molybdate, sodium nitrate, 2,2′,2”-nitrilotriethanol.

Inhibitor is usually clear or a pale yellow colour and must be stored in its original container in a cool well-ventilated place, ideally away from sunlight.

How does inhibitor work?

Naturally, as time goes on, erosion and rust build-up will begin to occur within your central heating system and pipes. There may also be a build-up of limescale and unwanted materials within your system, which can cause your central heating system to stop working as efficiently as it should, sometimes causing your boiler to make strange noises. 

You may find that a thick, gooey substance often referred to as ‘sludge’ can sit stubbornly at the foot of your radiators and pipes, stopping the warm water from flowing through each radiator and pipe properly. This stops your radiators from heating up efficiently which causes boiler and system faults, blockages, burst pipes and your boiler can be put under too much pressure, which can all lead to expensive repairs or even a system replacement, which is not what any of us need! If you are currently having troubles with cold spots on your radiator, we have a separate blog that can help you identify the issue and how to fix it. 

It is advised that homeowners and landlords use central heating inhibitors primarily to prevent the build-up of sludge in their systems. The mix of chemicals in an inhibitor can also help to break down dirt, rust and minerals and make them easy to extract.

Can inhibitor remove the sludge?

Not on its own. Inhibitors will certainly help towards breaking down the sludge inside the system and make it easier to remove. To be able to remove it, you need to install a magnetic filter. The filter catches all the particles and stores them until your next boiler service when you can get the filter emptied.

This is a great time to mention that when purchasing a new or replacement boiler with WarmZilla, your boiler package will include a free system cleanse to ensure that your central heating system is flushed out before your new boiler is installed. This will help to prevent any leftover sludge from your previous system, benign transmitted into your spanking new system so that it can keep you as toasty as you want this winter.

In more serious cases, where the sludge is tougher to remove, before a new boiler installation, your Gas Safe engineer may recommend a powerflush to help get rid of any unwanted material still in the system.

In an optimally running system, you would top up your inhibitor every year, which could potentially keep your system clean and serene for 8-10 years.

What are the benefits of central heating inhibitor?

  • It does what it says on the tin, preventing debris and corrosion throughout your system, helping to prolong its life.
  • In the long run, you’ll save money on expensive new boiler parts. Without using inhibitors, your system could experience more faults and blockages – or break down completely.
  • Radiators warm up much quicker and the whole system is more efficient.
  • It’s an especially good idea to use a central heating inhibitor in a brand new system to help prolong its life from day one.

How to add inhibitor to your central heating system

During your boiler installation, your Gas Safe Engineer will put the correct amount of inhibitor into the system. This will be recorded on your documentation, which will be handed to you once your installation and commissioning is completed. However, there are a few reasons why you may need to top up your inhibitor levels. Bearing in mind, that this will be done during your annual boiler service if required.

If for some reason, you need to add an inhibitor to your central heating system yourself, then it is a pretty simple task to do.

In order to do this, you’re going to add the inhibitor to one radiator in your system (you only need to choose one). If you haven’t done this before, we have added a step by step guide below to help guide you along the way. We’ll start with a general guide for combi boilers, which most of us have here in the UK.

Tools you’ll need:

  • Your chosen brand of inhibitor
  • Adjustable spanner
  • Hosepipe (will need to reach from your radiator to an outside drain)
  • Jubilee clip
  • Radiator key
  • Flathead screwdriver
  • Towels (to catch any drips or leaks)
  • Funnel and flexible tube to add inhibitor into radiator (although most inhibitor bottles come with a dosing adaptor so you probably won’t need this)

Adding central heating inhibitor for a combi boiler

The following instructions are for a conventional radiator. If you have a towel radiator, turn your heating off and let it cool, then remove the plug on the top of the radiator and pour your inhibitor in. You will need to turn your heating off and let your radiators cool down completely before moving onto step 2.


Step 1: Connect your hosepipe to your chosen radiator’s drain valve
You’re going to drain your central heating system by connecting the hosepipe to the drain valve on one of your radiators (it doesn’t matter which).


Step 2: Tighten up with the jubilee clip


Step 3: Place a towel underneath to catch any drips


Step 4: Open the drain valve
Drain any amount of water at least equivalent to the amount of inhibitor to be added.


Step 5: Shut off the drain valve
The drain valve is usually found on the lowest part of the pipes attached to your radiator, or on an outside wall.


Step 6: Unscrew the radiator bleed plug
Use your adjustable spanner.


Step 7: Attach the inhibitor dosing adaptor
If your inhibitor comes with a dosing adaptor to attach to the radiator bleed plug, attach it now. If not, use a funnel and a flexible tube.


Step 8: Pour in the inhibitor in
Use your dosing adaptor or funnel.


Step 9: Remove the adaptor or funnel


Step 10: Tighten the bleed plug back on with your adjustable spanner
Don’t tighten the valve too much – this can damage the ‘O’ ring.


Step 11: Turn your radiator valves back on


Step 12: Check your boiler pressure and top it up if needed


Step 13: Now bleed your radiators
Bleeding your radiators will make things much more efficient while the inhibitor circulates around your central heating system.

Adding central heating inhibitor for a system or regular boiler

If you don’t have a combi system, but instead have a conventional boiler and heating system, you’ll need to pour your inhibitor into your heating tank, which is usually in your loft.

If you have a large water tank and a small expansion tank (again, in the loft), or you have an open-vented system – you’ll need to add the inhibitor to the smaller tank.

Step 1: Prior to doing this, you need to stop the flow of water into the system by turning off the valve on the tank or, if you don’t have a valve, shutting the water off at the mains.

Step 2: Now the tank’s empty, check its condition. If it’s clean and corrosion-free, add your inhibitor.

Step 3: If the inside of your tank is covered with limescale or rust, give it a good clean before adding an inhibitor.

Step 4: Now pour the required amount of inhibitor into the tank. You’ll need to follow the instructions on the bottle and make a calculation relative to the number of radiators you have in your home.

Step 5: Once you’ve added your inhibitor, switch the valve or the mains water back on.

How often should central heating inhibitor be changed?

As mentioned above, your central heating inhibitor levels should be checked by your Gas Safe Engineer during your annual boiler service. If you do not get your boiler annually serviced (which you should do to maintain your boiler warranty), you will still need to have your central heating inhibitor changed at least once a year. This is due to the active chemicals breaking down over time, making the inhibitor less effective.

Whenever your system is drained, you should also always remember to top up the inhibitor again, again this should be done by your Gas Safe Engineer. During this, they should also drain the inhibitor liquid along with all the water.

How long does central heating inhibitor last?

Inhibitors do dilute over time, and there are many factors involved in how fast this happens, depending on the size of your system, etc. Your best bet is to regularly test the levels in your system and add more inhibitors if needed. You can buy fairly cheap test kits online to sample your radiator water with. If your radiators start taking a long time to heat up, use one of these test kits as it’s possible your inhibitor has diluted and isn’t doing its job properly anymore.

Always use the same kind of inhibitor, unless you’ve drained your entire system and are starting from scratch. If you mix inhibitor brands in your radiators, you may be potentially mixing extra, unnecessary chemicals and conducting your own little science experiment in your central heating system. We strongly advise against this!

Ways to make your inhibitor go further

We hope that this blog has helped you understand what a central heating inhibitor is, how it works and how essential it is to keep your central heating system performing efficiently.

To help you out a little more, we have added in steps that you can do to prolong the life of your system:


Power flush your radiators
Remember, the inhibitor can only break down the sludge in your system, giving your radiators a power flush will help to completely clear your system. Your Gas Safe Engineer may recommend that you do this before your new boiler is installed, to ensure that all the sludge and built-up debris is definitely removed from the system.


Install a magnetic boiler filter
Magnetic filters protect your boiler by catching all the bits of sludge that the inhibitor breaks down, before it gets to the boiler to do some damage. Magnetic filters need to be maintained once a year – this should be covered in your annual boiler service.


Add a limescale reducer
Magnetic filters aren’t able to remove limescale, because it isn’t magnetic! So it’s a good idea to get a limescale reducer as a build-up is potentially very damaging. Adding a limescale reducer saves you money in the long term, so it’s a great investment.

Even with the best of intentions, your boiler, radiators and pipes can develop faults and give you something else to pay for just when you least expect it.


Central heating inhibitor is a vital component within your system that keeps the show rolling behind the scenes. Your central heating inhibitor is as important as your radiators or your boiler itself when it comes to keeping your central heating system going.

It is advised that homeowners and landlords use central heating inhibitors primarily to prevent the build-up of sludge in their systems. The mix of chemicals in an inhibitor can also help to break down dirt, rust and minerals and make them easy to extract.

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