Helping UK homeowners save money on their heating

What is Central Heating Inhibitor?

The ideal heating system is one which keeps us comfortable, works quickly, is cheap and keeps our carbon footprint to a minimum. To give your boiler the best chance of reaching those standards, the system needs an annual dosing of inhibitor alongside some other key maintenance.

Central heating inhibitor is a chemical which can be added to the water in a central heating system to prevent boiler breakdowns, keep heating bills low and extend a boiler’s life.

Do you really need to add inhibitor to your heating system?

The simple answer is yes, inhibitor should be added on an annual basis, but an even better solution is to get a professional heating engineer to do it for you as part of an annual service.

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Prevent costly breakdowns

When you combine metal heating pipes with the water running through them the natural consequence is the development of rust. When this rust and other dirt builds up as a sludge-like substance it can lead to blockages, corrosion and costly repair bills.

Keep heating bills low

In addition, blockages and sludge on key components can mean the boiler has to work harder to produce heat which will increase your bills and your carbon emissions.

Keep the manufacturer’s warranty valid

Lots of manufacturers stipulate that their warranty is only valid if the boiler has been serviced (including being dosed with inhibitor) every year. However, it’s important to realise that although it is important, it cannot do the job of protecting your heating system by itself.

Protecting your central heating system


A professional Powerflush treatment will push strong chemicals through the heating system to break down the dirt and sludge which has built up over time.

Magnetic boiler filter

Once the dirt and debris has been broken down by the inhibitor it needs to be filtered out by a magnetic filter. This prevents sludge from entering the boiler.

Central heating inhibitor

Now that you have a clear system you should add inhibitor to prevent corrosion in the future.

Scale reducer

If your home is in a hard water area you should be adding a scale reducer to your system to prevent the buildup of limescale which can lead to blockages and cause components to stop working. There are lots of scale reducer brands on the market but MagnaClean and ScaleMaster are often recommended by professional heating engineers.

These steps will help you to avoid costly repairs and breakdowns and help your heating system to work as effectively and efficiently as possible for as long as possible. In many cases it is also required to ensure that your manufacturer’s warranty remains valid. How often should you add central heating inhibitor?

Generally speaking, inhibitor only needs adding to a heating system once a year. The best time to add inhibitor is when the system is drained of water. If you have an annual service by a professional heating engineer each year they will usually add it as part of the service so it’s a good idea to ask what the engineer has done before they leave. You can also buy a simple central heating inhibitor test kit which will provide you with an accurate measurement of how much inhibitor in the system.

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Adding central heating inhibitor

The process of adding central heating inhibitor to your heating system will vary depending on the type of system you have, i.e. sealed, open vented or combi.

  • If your heating system is sealed (you have one large tank of water in your attic) you will need to add inhibitor via a boiler filling loop.
  • If your heating system is open vented (you have two tanks in the attic) you need to drain the system before adding the inhibitor to the smaller of the two tanks which is the expansion tank. Make sure the tank is clean and in good condition before you add the inhibitor and turn the water back on.
  • If you have a combi boiler (your hot water is produced on demand from a single boiler unit with no tanks airing cupboard or attic) the inhibitor can be added to one of your radiators via the bleed valve. Remember you need to turn off the heating and let the radiators cool down before you begin.
  • How much central heating inhibitor should be added?

    If you have 8 to 10 radiators in your home, a 1 litre bottle of inhibitor should be sufficient but it’s important to check the advice from the manufacturer and/or to check with a heating professional. Homes with more than 10 radiators may need to add 2 bottles of inhibitor.

    Choosing a central heating inhibitor

    The majority of heating engineers will recommend that you choose leading inhibitor brands such as MagnaClean MC1, Fernox F1. or Sentinel X100. However, it’s worth noting that each brand offers a range of different products which are designed to treat different issues in a heating system. If you are in any doubt, contact a professional heating engineer.

    Choosing a magnetic filter

    The best magnetic filter brands include BoilerMag, MagnaClean, Fernox and Sentinel. You will need to know the size (22mm or 28mm) of your boiler inlet pipe to ensure the filter will fit and also consider how often you want to change the filter. Smaller filters are cheaper but will become clogged up more regularly and so will need cleaning.

    Important note about topping up low boiler pressure

    When you add pressure to your heating system you are adding more water which will dilute the amount of inhibitor in the system. If you are topping up the pressure on a regular basis you will be reducing the effectiveness of the inhibitor and weakening the level of protection. If your boiler is often losing pressure, this is a problem which needs addressing by a Gas Safe engineer before you add inhibitor.

    Find a local heating engineer

    If you would like to improve your heating system’s performance and efficiency and are not confident adding inhibitor to your central heating system and/or would like some advice, get in touch with Boiler Guide today. They will put you in touch with up to 3 local Gas Safe heating engineers who will provide free no-obligation quotes to clean and protect your heating system.

    Get FREE Central Heating Quotes Now

    Find professional heating engineers in your local area

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