Central Heating Inhibitor Explained

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Last updated: October 31, 2023

A healthy, well-maintained central heating system equals a happy – and warm – home. And this means a system with radiators that heat up quickly to give out sufficient warmth, while being efficient and cost-effective to run. But over time, a build-up of sludge inside the pipes and radiators can leave you with a central heating system that struggles to work as well as it once did.

You can, however, help to prevent sludge build-up protect your central heating system and keep it working at its optimum with a central heating inhibitor. Here’s all you need to know about this heating system ‘lifesaver’.

What is a Central Heating Inhibitor & Why Do You Need It?

Time can be unkind to your central heating system, as the pipes gradually erode, leaving a build-up of a sludgy substance, made up from internal rust and limescale plus general dirt and debris. This thick sludge will then sit in the bottom of your pipes and radiators, slowing down the flow of warm water through the system and affecting your radiators’ ability to effectively heat your home. If left untreated, this build-up will manifest itself in a host of ways, including radiator cold spots, pipe blockages, metal corrosion, reduced heating performance, poor heating efficiency, increased boiler pressure and eventually you could experience a full system breakdown.

One way to prevent this build-up and any potentially expensive repair bills is to use a central heating inhibitor. This chemical solution protects your central heating by not only helping to prevent sludge from building up but also by breaking down existing dirt, rust and other minerals to keep your heating efficient and prolong the life of your boiler.

Central heating inhibitor is made up from a mix of sodium nitrate, disodium molybdate, potassium tetraborate tetrahydrate, and nitrilotriethanol and is pale yellow in colour. You should add inhibitor to the water in your central heating system, with many boiler manufacturers making the use a heating inhibitor a condition of their warranty.

How to Protect Your Central Heating System

As well as using a central heating inhibitor, there are other things you should also be doing to ensure your heating system is protected. By using a combination of an inhibitor and the following, you can keep your boiler and central heating working efficiently for as long as possible. The following can also help to extend the life and effectiveness of a central heating inhibitor:

  • Get a magnetic filter installed

An inhibitor is effective in breaking down dirt and debris in your heating system which will then need to be removed. A magnetic filter is the device for the job as it will catch this broken-down material before it has the chance to enter your boiler, where it could cause serious damage. The magnetic filter can then be cleaned out as part of your annual boiler service.

  • Add in a scale reducer

This is particularly effective if you live in a hard water area and suffer from limescale build-up, caused by the natural minerals in the water. Limescale build-up can be particularly problematic for your boiler’s heat exchanger and thermostat if allowed to develop. And, as a magnetic filter is only able to remove metal, then the addition of a scale reducer means you have all the sludge bases covered!

  • Organise a powerflush

To get rid of all the broken-down sludge that may go on to clog your system, a professionally conducted powerflush will give your heating system a final ‘deep clean’. Your heating engineer will use a high-speed flush to power blast all that goo out in one go. A powerflush should also be performed when having a brand-new boiler installed for extra protection.

Heating Radiator

How to Add Central Heating Inhibitor

Central heating inhibitor fluid will become diluted over time so if you are using an inhibitor in your system, it is best to top it up annually to maintain protection against, sludge, limescale and rust build-up.

Adding a central heating inhibitor is a relatively straightforward process but how you add it, will depend on the type of boiler and central heating system you have. Your heating engineer can add the inhibitor during your annual boiler service, or you can opt to do it yourself, ideally when your system has been drained first.

While it is essential to always follow the manufacturer’s instructions for your chosen central heating inhibitor product, here’s a general ‘how to’ guide:

Open vented or sealed system

Whether your central heating system is open vented or sealed will indicate where you need to add the inhibitor.

Open vented

If you have two water tanks (typically in your loft), then you have an open vented system. With this type of system, the inhibitor will need to be added to the smaller of the water tanks, also known as the expansion tank.

To add the inhibitor into an open vented system you need to turn off the water to the tank, so that it drains. Check the drained tank for any signs of serious corrosion as this will need to be fixed before you add the inhibitor fluid. If all is ok with the inside of the tank, you can simply pour in the inhibitor before turning the water back on.

Sealed system

If you have one large tank in your loft, then this is a sealed central heating system. With a sealed system, the central heating inhibitor can be poured into the system via the boiler filling loop. Follow your boiler manufacturer’s instructions to use the filling loop on your specific boiler model correctly.

Combi boiler system

With a combi boiler system, you need to add inhibitor directly into the radiators. To do this, first turn off your heating system and wait until all the radiators are fully cooled down. You only have to add inhibitor to one radiator for it to infiltrate the whole radiator and heating system.

Once cooled, you will add the central heating inhibitor via the radiator’s bleed valve. To do so, simply shut both valves on your chosen radiator, then attach the inhibitor bottle to the valve and pour it in. Once all the inhibitor has been released into the radiator, open the valves back up before turning your combi heating back on.

If you are in anyway unsure how to add inhibitor to your particular central heating system, arrange for a qualified heating engineer to do it for you. The best way is to arrange for central heating inhibitor to be part of your boiler’s annual service as this way you know that your system is adequately topped up and ready for another year.

How Much Central Heating Inhibitor Should You Add?

If you are looking to add inhibitor to your central heating system, it is essential that you get the quantity just right as it needs to be appropriate to the size of the system you have.

The first step, as always, is to check the bottle and read the manufacturer’s instruction. The amount you need will be based on the number of radiators you have and, as a guide, a typical litre bottle of inhibitor will be enough to treat around eight radiators. So, if you have more than eight radiators, you will need to use two bottles.

As a word of reassurance, too much central heating inhibitor won’t damage your system but it is wise to keep within the manufacturer’s guidance so you can be sure you have the ideal amount for your central heating system. You can also use a radiator water sample test kit to periodically measure the level of central heating inhibitor you have in your system, as it will get diluted over time. This way you can ensure your system is topped up if needed and keep that inhibitor doing its sludge prevention job.