How to flush sludge out of your radiator

How to flush sludge out of your radiator

A woman checking the temperature of the radiator by putting her hands on it

Radiator sludge is a common issue and it can cause your radiators to struggle to heat up the room efficiently. The sludge is simply as result of corrosion inside your heating system which has caused tiny particles of metal to become rusty and fall off into the water.

If your radiator is hot at the top and cold at the bottom, it's very likely that rusty, dirty sludge has settled inside and is stopping the hot water from circulating properly. Below we'll look at how you can get rid of this brown sludge and get your rads back to working to optimal performance.

Steps to flush a radiator

  • Switch off your central heating system
  • Lay down towels or sheets
  • Turn off radiator valves
  • Drain the radiator
  • Remove radiator from the wall and flush
  • Connect radiator back up

Tools you'll need

  • Radiator bleed key
  • PTFE tape
  • Garden hosepipe
  • Bucket
  • 2 x adjustable spanners
  • Old towels or sheets

1. Switch off your central heating

This is an important step because it's important to let your radiator cool down before attempting any work on it. Water inside a radiator can get very hot so to avoid scalding yourself, turn off your heating and wait 60 - 90mins before starting any work to flush out your rad.

2. Lay down towels or sheets

The sludge at the bottom of your radiator will cause the water to be very dirty. In order to protect the floor underneath your radiator, lay out some old towels, sheets or blankets to catch any brown water that might squirt out. This is obviously a particularly important step if your floor is carpeted.

3. Turn off radiator valves

Your radiator likely has a valve at each end which are connected to the in and out flow pipes. You need to isolate your radiator from the rest of your system by turning off both of these valves. Firstly go to the thermostatic valve and turn it off or to the position indicated by a 0.

Secondly, go to the other valve, which is known as the lockshield. Remove the white plastic cap and use a pair of spanners to twist the valve clockwise until it is off and wont twist any further It's helpful to make a note of how many turns it took to switch the valve off so you can repeat the process the other way when you reconnect the radiator.

4. Drain the radiator

Take you bucket and put it under the thermostatic radiator valve. The bucket will catch any water the leaks out when you undo the valves. Take both of your adjustable spanners and use one to grip the valve while you loosen the nut that connects the radiator to the valve with the other.

Take your bleed key and open up the bleed valve on your radiator. This lets air into the radiator and allows the water to flow out more easily.

Loosen the lockshield valve in the same way and you can slightly tip the radiator to make sure that all the water has come out and gone into the bucket. Once you're confident all the water has been removed, close the bleed valve with your radiator key.

A plumber removing a radiator from the wall

5. Remove radiator from the wall and flush

After you've collected all the water in the bucket, completely undo the nuts connecting both valves to the radiator and gently lift it off the wall brackets. Keep the bucket close by in case there is any remaining water in the radiator that need need to be tipped in.

Take the radiator outside and put your garden hosepipe in the inlet pipe and switch on. The water should flow out the other side of the radiator, so make sure you are doing this in an area where it's ok to make a bit of a mess. The watery sludge will be very dirty. After a while switch your hose to the outlet pipe and keep running the water until the water coming out is clear. Clear water means you have removed the internal sludge.

6. Connect radiator back up

Once you're happy the radiator is fully flushed and you've removed as much sludge as you can, you need to reconnect your radiator in exactly the opposite way you did previously. If you noted how many turns it took when turning off the lockshield valve, open it back up using the same amount of turns. This will help to balance your radiator. Open up the bleed valve to allow any air to escape and then close it again. You can switch your heating system back on to see if the radiator is getting hot more evenly.

Prevent sludge with inhibitor

A great way of minimising the amount of corrosion that occurs inside your radiators is by ensuring your chemical inhibitor levels are regularly topped up in your heating system. Inhibitor stops the chemical reaction from happening so that the water doesn't react with the metal to cause rust.

The video below shows a great example of what happens when you don't use chemical inhibitor.


How much does it cost to replace a radiator?

How to add inhibitor via a heated towel rail

How to replace a broken radiator valve

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