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How To Flush Sludge In Radiators

Sludge in your radiators, which is an accumulation of dirt and rust, will limit the flow of water around your radiator and reduce its performance. In this guide, we outline how you can flush the sludge in your radiators to get them working at their best. If you're not sure if sludge is your problem, take a look at 'diagnosing radiator sludge' below.

How To Flush a Radiator:

  1. Turn off your heating
  2. Spread towels
  3. Turn off valves
  4. Drain the radiator
  5. Remove & flush radiator
  6. Reconnect your radiator

You Will Need:

  • Old towels/sheets
  • Radiator bleed key
  • 2 adjustable wrenches
  • Bucket
  • PTFE tape
  • Garden hose

Step 1: Turn Off Your Heating

Before you can take your radiator off the wall, you'll need to turn off your heating system and let it cool down for at least an hour. Make sure you leave it to cool down for long enough as if you come into contact with water, you don't want it to be hot!


Step 2: Spread Out Your Towels

Next up you need to protect your floors by spreading out your towels around the radiator. In particular, make sure the areas under the valves are protected.


Step 3: Turn Off The Valves

Next up, you'll need to cut the radiator off from the rest of the system:

  • First turn off the thermostatic radiator valve (TRV) by twisting it to the '0' or 'off' position.
  • Move to the other side of the radiator to the lockshield valve. Remove the plastic cap then use a wrench to turn the valve off clockwise. When turning off the lockshield valve, count the number of turns it takes and note that number down. This will help you re-balance your radiators later on.


Step 4: Drain The Radiator

  • Place your bucket underneath the TRV. Grip one wrench onto the body of the valve while using the other to loosen the nut that joins the radiator to the valve.
  • Use your bleed key to open the bleed valve, this allows air into the radiator which will help the water flow out.
  • Collect everything that comes out of your radiator into the bucket.
  • Loosen the lockshield valve the same way as above to let more water out. You can also now tilt the radiator to ensure you've emptied everything out.
  • Close the bleed valve with the key.

changing radiator-valve

Step 5: Remove & Flush The Radiator

  • Once the water has stopped flowing out, you can now completely disconnect the radiator and remove it from the brackets. Tilt the radiator to one side as you lift to tip any remaining water into your containers.
  • You can now take the radiator outside to flush it out. Place your hose onto the pipe inlet and turn on the water to flush out any sludge inside. You can move the hose to the inlet on the other side and continue rinsing until the water coming out the other end is clear.


Step 6: Reconnect Your Radiator

  • Once you've finished flushing out your radiator, you can reconnect it using the above instructions in reverse order.
  • When reconnecting your radiator, turn the lockshield valve by the same number of turns you noted down earlier.
  • Open the bleed valve at the top of the radiator to release any trapped air.

If all went well, you should have now successfully flushed your radiator.

Preventing Radiator Sludge Issues


If you're still having problems after flushing a radiator, then a power flush may be needed to clear not only your radiators but your pipes as well.

Magnetic Boiler Filters

A magnetic boiler filter before your boiler will remove sludge, rust or magnetic particles before they can get into your system.

Diagnosing Radiator Sludge

If you're not sure whether you need to flush your radiators, here are a few simple checks.

Check the colour of the water when you bleed your radiators:

  • Clear: Radiator has no or very little sludge.
  • Slightly brown: Sludge is starting to build up, so flushing is recommended.
  • Very dark brown: Flush your radiators as soon as possible.

If you have any of the following symptoms:

If you've flushed your radiator and you're still having problems with your heating system, take a look at our guides on how to bleed a radiator, diagnosing a radiator not working and replacing a radiator.

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