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How to fix a radiator that's cold at the bottom

Learn how you can remove the sludge clogging your radiators

Radiators feeling cold at the bottom but warm at the top is a common issue. When your radiator isn’t operating at its full potential, your home will be colder so it’s important to fix it. In this guide, we’ll take you through how you can fix a radiator that’s cold at the bottom.

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Why is my radiator cold at the bottom?

When a radiator is cold at the bottom but warm at the top, it's usually caused by an accumulation of sludge at the bottom of the radiator. This sludge is composed of rust, dirt and impurities such as limescale which will accumulate inside your radiators. The sludge is heavier than water so it will always sink to the bottom of the radiator. This prevents the flow of hot water to these areas, which is why they feel cold.

The easiest way to fix a radiator that’s cold at the bottom is to remove it from the wall, flush the sludge out, then re-install it.

Equipment list:

How to fix a radiator that’s cold at the bottom:

  1. Turn off heating system
  2. Turn off the radiator valves
  3. Drain the radiator
  4. Disconnect and remove from the wall
  5. Flush the radiator with a hose
  6. Re-install the radiator

1. Turn off heating system

Before you can start working on your radiator you’ll first need to switch off your heating system. This step is important as you need to allow the water in your radiators to cool down before you start working, otherwise you risk coming into contact with scolding hot water.

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2. Turn off the radiator valves

Next up, you’ll need to switch off both of your radiator valves. For your thermostatic radiator valve (the valve used to change temperature), you can simply turn it to zero.

If you have a manual valve on the other side, you’ll need to turn this clockwise by hand and if you have a lockshield valve, take off the plastic cover and turn the square shaft clockwise using an adjustable wrench. Count the number of times you turn the valve as repeating the same number of turns in the opposite direction will help you re-balance your heating system when you re-install the radiator later.

Open up the bleed valve using a radiator bleed key to check the radiator has been isolated. A small amount of water and air might come out but once this stops, you've stopped the water supply to the radiator and you can leave the valve open.

3. Drain the radiator

Before you drain your radiator, lay down some towels or plastic sheeting to protect your flooring.

Place a bowl underneath the first valve you’re going to work on to catch the water that comes out of the radiator. Use your water pump pliers to hold the body of the radiator valve steady, whilst using your adjustable wrench to slowly slacken off the nut that joins the valve to the radiator. As you do this, water should start to flow, so don’t loosen the nut too far in case you need to empty your bowl before letting more water out.

Once the water stops flowing from the valve, you can move to the other side of the radiator and repeat the process to remove any remaining water.

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4. Disconnect and remove from the wall

Once water has stopped flowing from your radiator, you can loosen the nuts on both valves all the way and disconnect them from the radiator. You can also now close the bleed valve using your bleed key.

With the valves disconnected, you can now lift your radiator off its brackets. As you lift the radiator, tilt it to one side towards a bowl to drain any water or sludge that's left at the bottom.

Radiators, especially larger ones, can be very heavy. Make sure you ask for help when lifting it off the brackets.

5. Flush the radiator with a hose

Once you’ve removed your radiator and taken it outside, it’s time to flush out that sludge. To do this, you can connect a garden hose to one end of the radiator and turn the tap on. This will force water around the inside of your radiator to push the sludge out the other end.

Once the water starts to run clear, you can move the hose to the other side of the radiator to ensure you've definitely removed all the sludge.

6. Re-install the radiator

Once you’ve completely flushed out your radiator, you can re-install it back into your heating system. For help on how to do this, take a look at our guide to replacing a radiator.

How can I prevent radiator sludge?

Central heating inhibitor can help prevent the build up of sludge to keep your heating system running efficiently. You can also add a filter to your central heating. These contain magnetic filters that will collect debris to help reduce the build-up of sludge.

Can I flush radiator sludge without removing my radiator?

If you don't want to remove your radiators, you can employ a tradesperson to power-flush your central heating. A power-flush pushes a flow of liquid through your central heating system to dislodge and remove any sludge or debris.

Why is my radiator cold at the top but warm at the bottom?

If you’ve got a radiator that’s cold at the top but warm at the bottom, it’s likely you’ve got air trapped in the top of your radiator. To remedy this you’ll need to bleed the radiator to release that trapped air.

Radiator sludge is a common problem that will stop heating systems running efficiently. Luckily, it can be prevented in the future with some simple maintenance. If you’ve removed your radiator but decided it’s time for a new one, take a look at our radiator collection and our guide to replacing radiators.