Homeowners Professionals

How to flush a central heating system

Your central heating system draws cold water in, which is then heated and pumped around your radiators to keep your home warm. However, over time, debris can enter your system, rust can build up from old pipes and, if you live in a hard-water area, limescale can develop. If you have found that your gas boiler is suddenly not working when the weather goes cold, it is highly possible that you have a frozen boiler condensate pipe. Read on or watch the video to learn more about how to diagnose, solve and prevent this very common problem.

All of these things get pushed around the system and, in bad cases, clog radiators and stop them from being heated all the way through. This in turn can have an impact on the efficiency of your system and you may find that you’re spending more money to keep your home warm.

But how do you know if your system has all of this debris inside it? There are some warning signs to look out for, including:

  • A noisy boiler
  • Radiators that are cold at the bottom or top
  • Radiators that need bleeding frequently
  • Radiators that take a long time to warm up.

If you’re experiencing at least one of the above, then it’s likely that your system contains more than just water. So the question is, how can you resolve the issue?

Important:  Please note this article is designed as a general guide for consumers awareness only. A heating engineer may adopt a different and far more methodical approach to ensure your system is flushed correctly. We would always recommend you have this work carried out by a qualified heating engineer.

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

There are a couple of ways that you can clean the sludge out of your heating system. The first is called a powerflush, which is both the name of the method and also a physical device that your heating engineer will use. Alternatively, you can take every radiator off one at a time and flush it yourself.


Powerflushing is the fastest and easiest way to remove sludge from your heating system, and possibly the one that will give you the best results. This is one example of using a particular device however other models and methods may exist.

First, you should lay down some sort of protection to keep the dirt or water from going on your floor. A dust sheet should do the trick. Next, you will need to remove one radiator so that the powerflush nozzle can be inserted. Just don’t forget to flush this radiator before reconnecting it later. At this point, if you have an open-vented system, cap off the cold feed and the expansion pipe.

The powerflush machine will likely have multiple hoses. Take the first one and connect one end to the correct valve on the machine and the other end to the radiator. Make sure the valve is in firmly and securely.

Next, you should connect the inlet hose. This allows water to enter the powerflush from the mains supply so that the system can be cleaned out. The overflow hose can then be connected to the device and taken outside to the nearest drain. Any excess water will flow out of here so make sure the liquid has somewhere to go. Finally, connect the dump hose to the last valve.

Once everything is connected, open all the radiator valves and turn the machine on, allowing it to run for at least 10 minutes. Be sure to reverse the flow of water regularly during this time by controlling the lever on the device.

After 10 minutes has passed, turn the ‘Dump’ lever upright. This will begin to allow any dirty water to exit the system. At the same time, open the inlet water valve. This means the dirty water can exit the system while new, clean water is circulated. Allow the system to continue running until the water coming out of the overflow hose is clear.

Then, close the dump valve and the water inlet valve and set the powerflush to ‘Circulation’. You should see a screw lid on the front of the machine. Unscrew this and pour in the recommended cleaning product. The manufacturer of the powerflush machine will be able to recommend a cleaning solution if you’re unsure what to use. Pour in the right amount and continue to run the device for a further 15 minutes. Don’t forget to reverse the flow of water every few minutes.

When the 15 minutes are up, turn all the radiator valves off completely except for one. Allow the powerflush to clean this device for five minutes. Turn this radiator off and move to the next one. Repeat this step until all of your radiators have been flushed.

As you’ve done previously, open the ‘Dump’ valve and turn the water inlet on. Now, it’s time to open all the radiator valves so that the water can circulate around the whole system. And that’s it - your system should now be full of fresh new water. Disconnect all of the valves from the machine. Don’t forget to manually flush the one radiator that was taken out of the system at the start. You can find out more about how to do this below.

Clean each radiator

Instead of performing a powerflush, you can clean each radiator individually. You will need to ensure the heating system is off and cold before attempting this, as you don’t want to burn yourself.

Choose any radiator to start and drain it. You can do this by turning off both valves and unscrewing the nut that attaches the radiator to the pipe. Water will start coming out, so use a bowl to catch as much as you can. Open the bleed valve as this will allow the radiator to drain faster. You may need a bucket handy to empty the bowl frequently.
Then, fully unscrew the nuts on either side of the radiator and it should now slide off the wall. Tilt the radiator to one side to drain off extra water into the bucket. If your radiator has TRVs, be sure to cap these off to ensure no water can escape from the system.

To clean inside the radiator, all you will need is a garden hose. Turn the hose on and attach it to one end of the device. This should flush out any rust or dirt inside it. Once the water coming out begins to run clear, you can put the radiator back on the wall, attaching it with the nuts and turning the valves back on.
Finally, bleed the radiator to make sure there’s no air inside it. Repeat this for every radiator. While this method isn’t as effective as a powerflush, it will still clear out the majority of the sludge in your heating system.

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