How to Fix & Flush Blocked Radiators

Blocked Radiators? How to Tell If You have Blocked Radiators and How to Fix Them.

The cooler months are right around the corner. The radiators haven’t been used all summer. September is your last chance to ensure your central heating systems are in prime condition ready for the long winter months ahead. You’ll want to ensure they are ready to go when the cold sets in. Nobody wants the endure emergency call outs, the cost of repair or replacement on top of the already expensive utility bills. One of the more common issues you will face after a long hot summer is a build-up of debris and sludge in your radiators. But what is sludge and how can you clear a blocked radiator?


What is Sludge?

Sludge is a result of built-up dirt and debris inside your radiator. It limits the flow of water throughout you central heating system making both heat pumps and radiators work harder. This can have a knock-on effect for your boiler as if left unchecked, can cause boiler breakdowns. This will in turn - rapidly age the whole central heating system which will mean you’ll need to replace sooner than later. It is important to carry out radiator maintenance at least once a year. Summer is the best time to do this.  


Symptoms of Blocked Radiators

You have may have noticed that your radiators aren’t heating up as quickly or they have cold spots. This can be a tell-tale sign of a blocked radiator. We’ve got a list of things for you to diagnose the issue:

  • Noisy boiler

  • Broken or leaking heat pump

  • Radiators are slow to heat up

  • Cold spots at the bottom of the radiator

  • Brown water when bleeding your radiator


You will be able to tell by the colour of the water that comes out when you bleed the radiator, just how bad the sludge is. If the water is clear, you will have little or no sludge. If it is slightly brown, sludge is beginning to build up. This would be the best time to flush it out before it does any damage. If, however, the water is a very dark almost black colour, you will need to flush your radiator immediately. You may also need to speak to a professional to check potential damage caused to the rest of the central heating system. This will help prevent a complete breakdown when you can least afford it.  


What Will I Need to Remove Sludge from Blocked Radiators?

As with anything related to your central heating system, you will need the right tools. Now you’ve diagnosed the issue, you will need:

  • An old towel or two to clean up any water spillages

  • A large bucket or bowl to catch water

  • A spanner or adjustable wrench

  • Your radiator bleed key

  • PTFE tape

  • A hosepipe

  • A rubber hammer (this is optional)


How to Remove Sludge from Blocked Radiators

The first thing you will need to do is look at which radiators need clearing. This could be one, a few or worst-case scenario, all of them. Here is a handy step by step guide on clearing a blocked radiator or towel rail.

  1. Turn it off

At this stage, it is best to turn off the whole central heating system to prevent any hot water being pushed through the radiator you are working on. Give it at least an hour to cool down before you begin.

  1. Put down your towels

Ensure a good amount of floor is covered with your towels and/or old sheets. You don’t want to damage the flooring. You will most likely experience spillage under and around the valves so be sure to get the towels all around this area.

  1. Turn off the valves

Set your Thermostatic Radiator Valve (TRV) to off or 0. To turn off the lock shield valve (at the side of the radiator), you will need to remove the cap and using a spanner or wrench, turn the valve clockwise all the way. Make a note of how many times you need to turn it to loosen it as you will need to tighten up the same.

  1. Drain the radiator

Place a bucket or bowl under the TRV at the bottom of your radiator and loosen the TRV at the nut joint with a spanner. Open the bleed valve using the bleed key to remove any air in the system. Sludge and dirty water will begin to drain out. Then, loosen the lockshield valve and repeat. Once the radiator has been drained, close the bleed valve.

  1. Remove the radiator from the wall

Once the water has stopped flowing freely, carefully remove the radiator from the wall. Be mindful of any wall brackets, radiator feet and excess water when moving the radiator. We highly recommend you empty the radiator outside as this next bit will get messy.

  1. Power Flush the radiator

Push the hosepipe through the inlet and turn the water on. This will help flush out any sludge inside. Move the hosepipe around a bit to loosen some of the debris. If you have a rubber hammer, place a soft dust cloth and gentle tap the bottom of the radiator to dislodge any stock on sludge. Repeat this process by attaching your hosepipe to the other inlet and let it run until the water is clear again.

  1. Reconnect your radiator

Carefully hang your radiator back on the wall brackets, reattach any feet you may have taken off. Ensure you tighten the valves back up and that the lock shield also opened as before. Use the bleed valve to remove any trapped air again.  


How to Prevent Sludge Build-Up

There are a few things you can do to prevent sludge build up from getting out of hand.

  • Regular maintenance is key – keep up with maintenance internally and externally

  • Use an inhibitor to help break down unwanted particles in the system and keep it topped up

  • Install a magnetic filter such as the Adey MagnaClean. This will filter out any magnetic material that can build up in the system and stop it reaching the boiler and radiators

  • Schedule an annual radiator and boiler service with a local tradesperson to ensure the whole system remains healthy for when you need it most


We have plenty of products for radiator protection that could also help with your blocked radiators. If you are still having some issues with the efficiency of your radiator, it might be time to call in the professionals for some expert troubleshooting. They can power flushing machines to flush sludge out of the radiator system. Sometime sludge is made worse by corrosion in the radiator, boiler or pipework. While it can be a DIY job to clear out the build-up of sludge in these situations you should consider getting a plumber or heating engineer involved, especially if you do need a new boiler (which is not a DIY job).  


Did you know? You can have a completely custom radiator style, unique to you and your requirements at any time from Column Rads. Take a look at our Instagram or Pinterest for more inspiration and get in touch if you have any questions.