Why is my radiator cold at the bottom?

Is your radiator cold at the bottom? In this guide we’ll show a range of things that could be causing your cold radiator issue, and what you can do to fix it.

Radiator cold at the bottom? What should you do?

If you feel cold patches on the bottom of your radiator when the heating is on, this means that the flow of water around the appliance is being restricted, and the most common cause is radiator sludge. 

Over time, radiator sludge can build up inside your radiator. 

Once enough sludge has accumulated, it creates blockages within the radiator’s flow channels. 

This means that water is unable to move around the entire radiator efficiently. 

Sludge is a thick, heavy substance. Therefore, it sinks and settles at the bottom of the radiator. 

This is why you will feel the cold patches along the bottom and in the middle. Heat rises, but the sludge prevents the hot water from reaching the congested areas inside the radiator.

This is likely to leave your radiator warm at the entry and exit points around your valves, and across the top, but quite cold along the bottom.

Why does radiator sludge build up?

Unfortunately, in situations where water and metal meet, other unwanted materials are inevitably formed. In terms of central heating, there is an abundance of both!

Most radiators are made from iron or steel. Water then flows from the boiler around the system. So the combination is a consistent occurrence. 

This constant chemical reaction causes iron oxide compounds like magnetite and haematite to form. Plus a damaging mixture of rust, dirt and scale.

You can imagine how easily more and more of this messy matter can collect over time! 

In extreme cases, the radiator can be filled with so much sludge that the entire bottom half of the radiator can feel cold.

But fear not, there are things you can do to fix and prevent this cold radiator problem!

How to fix radiators that are cold at the bottom

There are a couple of different methods you can use to fix radiators that are cold at the bottom. Each process has its benefits for different situations.

If one radiator is cold at the bottom

If you have noticed that only one radiator has cold patches at the bottom, you could attempt to fix it yourself. To do this, you will need to manually clean the radiator.

This is also known as flushing a radiator. 

To successfully flush your radiator and remove the sludge, first you need to bleed the radiator. 

Then you need to drain the radiator to remove as much of the dirty water as possible. Make sure to protect the floors as you do this and catch the water in a bucket. 

Next, you need to remove the radiator from the wall and take it outside to flush. Now it’s time to really get rid of that sludge! 

Attach the hose pipe to the radiator valve inlet and turn the tap on full blast.

As the clean water flows through the radiator and out of the other inlet, it should bring the dirt, sludge and grime with it. 

To ensure all of the sludge has been removed, try tapping the radiator inlet with a rubber hammer. 

The idea is to repeat this process by attaching the hose to the other end of the radiator and flushing clean water through it until it runs clear.

Then, once you’ve cleaned out all the gunk and sludge, put your radiator back on the wall and reconnect it by putting the nuts and pipes and valves back in their original positions.

Water should then reenter the system and you’ll need to bleed your radiator and maybe repressurise your heating system.

Once this is complete, turn your heating back on and wait for around half an hour.

Then, place your hand at the bottom of the radiator and (much like TikTok) swipe upwards.

If the sludge has been removed successfully, you should no longer have cold radiators and instead have a radiator that is evenly emitting heat.

For more information on this process, head over to our guide ‘How To Flush A Radiator In 10 Steps’.

If multiple radiators are cold at the bottom

In very severe cases, sludge can affect multiple areas of your heating system. This can result in more than one radiator feeling completely cold at the bottom.

If that sounds like your situation, you will need to enlist the help of a heating engineer and request a full system Power Flush.

This will usually take around 5 hours for a three-bedroom home with a vented heating system.

The great thing about Power Flushes is that the radiators don’t even have to be removed from the wall.

This is because heating engineers have special tools, chemicals and cleaning solutions to totally eliminate and dispose of the sludge and scale. 

For more information on this process, head over to our guide ‘What Is A Heating System Flush?’.

Other signs of radiator sludge

In addition to radiators feeling cooler at the bottom, there are a couple of other signals that tell you your radiator is full of sludge. 

If you notice any of the issues below, your radiator or radiators may require a flush:

• Radiators need bleeding often.
• Brown liquid when you bleed your radiator.
• Broken or leaking heat pump.
• Boiler makes loud noises.

How to prevent radiators getting cold at the bottom

Like most common heating issues, prevention is more effective than the solution.

The good news is, there are a couple of efficient methods to prevent sludge from building up in your heating system. 

Add Central Heating Inhibitor 

One of the easiest and most effective ways to prevent sludge and keep your system running smoothly is to add central heating inhibitor.

It is also a job you can get done yourself!

It’s pretty simple too. Just like you might pour a drain cleaner down a blocked plug hole, you simply need to pour inhibitor into your central heating system.

That sounds like it might be the most impractical way to stop your radiators from getting cold, but it really is a simple process.

For more info on how and why you should use inhibitor, head over to our guide ‘Why Is Central Heating Inhibitor Important?’. 

Add A Scale Reducer 

Similarly to inhibitor, a scale reducer can be added to your system to remove the build up of limescale. Less limescale means less chance of sludge building up over time, and a reduced chance of cold radiators. 

Install A Magnetic Filter 

Since sludge contains rust and other metallic debris, a good way to prevent the build up is to remove the metallic fragments. That is precisely what the magnetic filter does.

Most metallic filters are added to boilers during installation and they are fixed to the pipe that returns water to the boiler. If your system doesn’t have a filter, contact your heating engineer to fit one for you.

What NOT to do to a cold radiator

If you do notice any cold patches in your radiators, a common mistake you should try and avoid is to turn the thermostat up. 

Increasing the temperature on your thermostat will only make your boiler work overtime. But, it will not heat up the cold patches as the build up of sludge will still prevent this from happening.

All turning your heating up will achieve is to drive up your heating bills.

Are cold radiators a serious problem?

Cold patches on your radiator mean that you are not benefitting from the maximum heat output. Plus, this inefficiency also means that your heating bills will be too high for the amount of heat being provided. 

In the long term, the problem could lead to even more expensive issues.

Sludge and limescale can damage the internal workings of your central heating system, cause boiler breakdowns and even reduce the lifespan of your boiler because of the high pressure conditions.

Check out our ultimate guide on fixing cold radiators to help you diagnose more cold radiator problems!

Is Your Radiator Cold At The Top?

If you notice your radiator is cold at the top and hot at the bottom, you will need to bleed your radiator to remove the trapped air inside. We’ll take you through the process step by step in our ‘How To Bleed A Radiator’ guide.

Cold radiators – Frequently asked questions