Have you flushed your central heating system recently? It is not just the water that needs to be replaced, but also the sludge that has built up inside that needs to be removed. Sludge remover is a product that is used for removing the build-up of minerals and other sediments in your water system. There are many ways to use this product, so it can be confusing when trying to find out how long you should leave it in for. In this article, we will look at how to flush your central heating system, how long you can leave sludge remover in, and what happens if leave the sludge remover in too long.

Baxi Boiler. Flush central Heating

What is Radiator Sludge?

Radiator sludge is a combination of dirt, rust and minerals that have built up over time inside the heating system. While you might not be able to see it when radiator sludge builds up inside your heating system, it will start to clog any openings there may be. This causes the heat from your central heating system to be distributed unevenly and means that your system is not working as efficiently as it should.

What Causes Radiator Sludge?

There are many things that can cause radiator sludge. One of the main causes of sludge is rust build-up. This is made worse when your radiators are not bled often enough allowing too much air in the system. Another reason for build-up is if you have hard water in your home which contains high levels of calcium or magnesium. Hard water can also leave deposits on radiators and pipes which over time create a build-up called ‘scale’.

This too will clog openings within the central heating system causing uneven heat distribution and inefficient operation.

How Often Should I Flush My Central Heating System?

Along with other good practice techniques, it is recommended that you thoroughly flush your central heating system at least every once 5 years. This will help to remove any dirt, sludge and scale build-up inside the pipes, fittings and radiators and keep them working at peak efficiency.

How to Properly Flush and Clean Central Heating Systems

What many people do not realise is that to keep your heating system in tip-top condition, it will need regular maintenance. The radiators will need bleeding annually to remove any air that has built up and the whole system should be cleaned and flushed through approximately every 5 years or so. Flushing your system may seem like a daunting task and if you are not confident or are unsure of anything, it may be best to call a plumber.

If however, you are happy to crack on and give it a go, we have instructions for you here.

What You Need

You will need a few things to ensure that your maintenance goes smoothly without flooding your house.

  • Bucket
  • Towel/ Rag
  • 2 x Adjustable Spanner
  • Hose Pipe
  • Radiator Bleed Key
  • Lockshield Valve Key
  • PTFE tape
  • Rubber mallet
  • Sludge Remover (Sentinel X400)

Step 1 – Turn Off the System

Like with any maintenance task you carry out on your heating system, the first step is to turn it off. This is for obvious safety reasons because nobody fancies being scalded! If your system has a central heating timer, you must also be sure to set this so that no heating comes on while you’re doing your maintenance work.

Step 2 – Add Sludge Remover

Now you will want to add sludge remover to your system. Depending on what type of system you have, there are a couple of different methods of doing this.

Open System

  • With an open system, you will need to turn off the water feed to the small tank in the loft and then drain away the dirty water that is in the tank.
  • Then using the drain valve on one of your radiators, remove approximately 3 litres of water from the system (not forgetting to close the valve again) and then add your X400 sludge cleaner (amazon link – opens in a new tab) to the now empty tank in the loft.
  • Finally, open the water feed to the tank in the loft and it will fill normally.

Sealed System

If you have a sealed system with no F&E tank, the best way to get the sludge cleaner into the system is via one of the radiators themselves which can get a little messy. This is best done with the tallest radiator upstairs (a tall towel rail works best) or if your radiators are all the same size, choose the one nearest to the boiler. Now you will want to open the radiator bleed valve and bleed the water from the radiator until the water pressure drops to zero.

Next, the fun part, you will now want to tighten up the bleed valve and then using a spanner, remove the whole radiator bleed plug. Take your hose pipe and syphon approximately 3 litres of water from the system and then add 1 litre of X400 sludge remover (amazon link – opens in a new tab).

Note: When replacing the radiator bleed plug, wrap the threads with PTFE tape to ensure there are no leaks.

Step 3 – Turn the System Back On

Next up, you will want to repressurise the system, turn on the boiler and run the central heating.

Note: If you do not have an automatic air vent (AAV), radiators may need bleeding to ensure that all of the air has been purged from the system.

Step 4 – Wait for 3 Weeks

X400 is a fantastic product and does wonders but it needs a little time to do its job. It is said that X400 can be left in the system for up to 4 weeks, but I prefer to flush the system after 3.

During this period, even in warm weather, it is a good idea to run the central heating twice a week.

Step 5 – Turn Off and Isolate the System

Next up, you will want to drain the system. To do this, you will need to ensure that the heating system is off as before, and you will also need to isolate the system from the mains water.

There should be a water intake valve on the boiler but if you cannot find it, you can just turn off the water for the whole house.

Step 6 – Draining

Now you will want to drain the system by attaching a hosepipe to the radiator drain valve on the radiator furthest from the boiler. Draining down a system can take anywhere from 30 minutes to a few hours. Whilst draining, note the colour of the water. If there was heavy sludge in the system, it will be a deep murky brown colour and if it wasn’t too bad, the water will run reasonably clear.

To speed up the process, you can remove the bleed plug from the radiator nearest the boiler and the air intake will help the water drain away faster.

Step 7 – Flushing Individual Radiators (Recommended)

Although this step is optional, I certainly recommend flushing all of your radiators individually.

  1. Once you have drained down the system, or at least a good part of it, start with the radiator nearest the boiler and close off the TRV and lockshield valves (the ones located at either end at the bottom). Keep a note of how many turns it took to close the lockshield valve as they will need to be opened the same amount later – if not you may need to rebalance the whole system.
  2. Now you will need to undo the union nuts that join the pipework to the radiator – do these 1 at a time and be prepared for a little water to still be in the bottom.
  3. Once the radiator has been disconnected, lift it off its brackets and take it outside. Then, using a hosepipe, flush through the radiator with some clean water until the water runs clear. Do this from both sides and also give it a few gentle knocks with your rubber mallet.
  4. Replace the radiator in the reverse of how you removed it remembering to open the lockshield by the same amount you closed it.
  5. Repeat for all of the radiators in your house.

How Much Water is in a radiator?

How much water is in a radiator depends on the size and type of radiator in question. Older radiators tend to hold more water whereas modern radiators are more efficient and use less water.

Note: If you need to use the mains water, make sure the heating system has been isolated!

Step 8 – Turn the Water Back On

Once you are all done, you can close off the drain valve and turn the water back on. Refill and repressurise the system. Whilst the system is refilling, you may hear some knocking sounds coming from the boiler but that is just the air escaping from the AAV and should only last a minute or so.

Step 9 – Bleed Your Radiators

Finally, you will want to bleed the radiators to ensure that you remove all of the excess air in the system. Now your radiators should be good to go!

How to Stop Sludge Building Up in a Central Heating System

There are a couple of great preventative measures you can take to try and stop the build-up of sludge which are great for older systems.

Note: These measures are to be used to complement your maintenance schedule, not replace it completely!

Magnaclean Pro 2

The Magnaclean pro 2 is a magnetic filter (amazon link – opens in a new tab) designed to catch any metallic dirt and debris (rust) that may be floating around the system before it has the chance to settle and turn into sludge. These are fantastic pieces of kit that are worth investing in to help keep your system healthy and clean. They are easily fitted and can be fitted by a novice or DIY plumber although like anything else if you are not sure – call a professional to do it for you.

Sentinel X100 Inhibitor

X100 is a heavy-duty central heating cleaner you can leave in (amazon link – opens in a new tab) that helps protect against limescale and corrosion. It uses anti-corrosion technology, pH neutral formulation, and is suitable for all water hardness levels. This powerful protector also ensures optimum system efficiency by preventing the formation of hydrogen gas and assists in preventing corrosive interaction between the metal parts of your plumbing systems.

How Long Can You Leave Sludge Remover In?

It is good practice to follow the manufacturer’s guidelines with whatever product you use – I use X400 and this must be drained after a maximum of 28 days (4 weeks).

What Happens If You Leave the Sludge Remover in Too Long?

Sludge remover that has been left for too long will no longer be effective and serve any purpose. The sludge that had broken down with the introduction of the sludge remover will begin to settle again causing your system to work harder than it needs to.


Hopefully, by following our guide, you now know how long you can leave sludge remover in, and as we mentioned, sludge build-up occurs naturally but this can be mitigated by carrying out some regular best-practice maintenance techniques. With luck, you will not experience any problems with sludge build-up in the future and have a long-lasting boiler and central heating system.

How to Stop Sludge Building Up in a Central Heating System Infographic

Plumbing Wizard Tips

“Regular maintenance of your system is essential and will save you money in the long run!”

“Isolate the heating system from the mains if you can – this means that you can still use the cold water taps if needed!”

“Any preventative measures are to be used alongside your regular maintenance duties!”

“If you are unsure of any of the above – call a professional to come and do it for you!”

Frequently Asked Questions

Is there a central heating cleaner you can leave in?

Sentinel X100 is a heavy-duty inhibitor specially formulated for all systems metals, including hard water conditions. Its pH-neutral formula ensures optimum system efficiency, while also preventing the formation of hydrogen gas that can cause dangerous explosions.

How long does it take to drain the central heating system?

The time it takes to drain a central heating system depends on a few factors such as how many radiators are in the house, how many floors in the house and how much build-up is inside.

The process can be sped up a little by opening a bleed valve on the radiator nearest the boiler.

What is the best central heating system cleaner?

Sentinel X400! Sentinel X400 is a revolutionary system restorer that produces a non-acid formula that can be used in any type of heating system including aluminium. Its active ingredient performs the job of eliminating problems such as cold by breaking down sludge ready for draining.

Sentinel x400 how long to leave in system?

Sentinel x400 can be left in the system for up to 28 days before needing to be flushed.

How do I clean the sludge out of my central heating system?

The process of cleaning a central heating system is called a flush or power flush. This involves adding a sludge remover, leaving it in for a few weeks before draining the system and creating a flushing loop to remove all of the build-up.


Lee Pearce is not just a master plumber; he’s a veritable Plumbing Wizard. With over 30 years of experience in the trenches of pipes and drains, Lee has become the go-to sage for DIY plumbing, saving homeowners thousands in potential call-out charges. As the founder of Plumbing Wizard, he’s dedicated to demystifying the complexities of home plumbing, offering easy-to-follow advice that stands the test of time and pressure. His online blog is a treasure trove of tips, tricks, and tutorials that empower everyday individuals to take charge of their home’s plumbing health. Lee’s practical wisdom is not just about fixing leaks; it’s about imparting confidence and self-reliance. When he’s not writing or elbow-deep in a plumbing project, Lee is passionate about educating the next generation of DIYers, ensuring that practical skills are passed down and preserved.

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