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Why is my radiator cold at the top/bottom?

A hot and cold radiator

It’s rare that we discover problems with our heating systems at the height of summer. In most cases, it’s when the temperatures are at their coldest that we discover our central heating isn’t operating as efficiently as it should be.

One of the most common problems that we all face at one time or another is that of a radiator not getting hot evenly across the whole surface area. Sometimes it can be hot at the bottom and cold at the top or vice versa.

This article will explain the most likely reasons why this happens and what you can do to fix it.

  • Air in your radiator
  • Sludge in your radiator

Air in your radiator

If you’ve got a radiator that is hot at the bottom but cold at the top, the most probable explanation is that you have air trapped at the top of the radiator.

The good news is that this is a simple issue to fix as all you need to do is to ‘bleed’ the air out of the radiator.

When bleeding a radiator, it’s a good idea to switch your central heating system off, just to avoid any problems such as boiling hot water spilling out of your radiator onto you.

Using a radiator key (or a flat head screwdriver if your rad has that option), gently loosen the bleed valve on the top or side of the radiator.

As you slacken the bleed valve, air will start to escape with a large hissing sound. Once the air stops hissing, you should start to get a little bit of water spurting out.

It’s at this point, you can be sure that the air is out of your system and you can tighten up the bleed valve before turning on the central heating system again to check the radiator is getting hot fully.

One of the most common reasons for air getting trapped in your radiators is due to a lack of chemical inhibitor in your heating system. If you do need to bleed air out of your radiators, it’s a good opportunity to top up your inhibitor levels as well.

The video below has Jimmy the plumber explain in detail how to bleed a radiator and how to stop air building up in your system again.


Sludge in your radiator

If you have a radiator that is getting hot at the top but cold at the bottom, it isn’t a problem with air being trapped in that rad.

This is much more likely to be caused by so called ‘radiator sludge’ accumulating at the bottom of the radiator and preventing it from being heated by the hot water in your system.

Radiator sludge is actually lots of little bits of rusted metal that has flaked off from the inside of your system and come to rest in the bottom of your radiator.

When water comes into contact with metal, there is a chemical reaction that can cause the metal to corrode. Again, in order to stop this process of corrosion from occurring, it’s important that your chemical inhibitor levels are topped up.

The best way to remove sludge from the bottom of a radiator is to give it a power flush. However, this is something that is beyond the abilities of a casual DIYer and normally requires calling in a professional plumber.

An alternative is to remove the radiator from it’s pipework, take it outside and flush it out using a hosepipe.

Jimmy the plumber gives an excellent demonstration of this process in his detail video below.

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