Is your radiator cold at the bottom and warm at the top? Is your radiator cold at the bottom and warm at the top?

Is your radiator cold at the bottom and warm at the top?

The radiators in our home quietly go about doing their job of keeping us warm when we need them to. Occasionally, however, you may find that one or more of your radiators isn't behaving as it normally would. A radiator that is cold at the bottom is a relatively common problem. Thankfully Easy Boiler Comany have some quite easy fixes, and you probably won't need a new boiler.


How do radiators work?

Before we go into why your radiator might be cold at the bottom, let's have a quick review of how radiators work.

When the water in a radiator is heated. A radiator, hot water throughout, will heat the surrounding area through the process of convection.

The water is heated in the boiler and the boiler pressure pushes it to travel through the pipes into a radiator. Your radiators are connected in a chain with the water passing through each one in turn and then back to the boiler to be heated again.

To prevent the last radiators in the chain from only having colder water in them, your central heating system uses a process called "balancing". Water flow is restricted to certain radiators to balance how much of the hottest water each one gets and is controlled by the lockshield valve.


Why is my radiator cold at bottom and warm at top?

While we all know that heat rises, this shouldn't be the case with radiators. A radiator that is cold at the bottom and hot at the top indicates that there is some kind of blockage that is preventing the hot water from filling the radiator in its entirety.

You might also be able to feel cold patches or cold spots at different places in the radiator, and these cold patches suggest that the hot water isn't flowing around the radiator as it should.

The reason for a radiator that is cold at the bottom is almost always the same thing: radiator sludge. Sludge in your heating system isn't an indication that your home is dirty in any way. It is a common problem that every system will experience at one time or another.


How sludge forms in your central heating system

Your radiators are made from iron or steel. These materials are excellent for conducting heat, but they do have the potential to create deposits that can form a build-up of sludge.

As the water passes through the radiator, iron compounds are created. The most common compounds, magnetite and hematite, both form iron oxides. Over time, these compounds can start to form a sludge.

You might also find that water impurities such as limescale and other mineral deposits are also part of the sludge formation. This is especially common in hard water areas.

Once the sludge grows to be big enough, it can start to block off the flow channels to the radiator. When one of the flow channels is blocked, it prevents hot water from entering that part of the radiator, causing a cold patch. The water pressure in the radiator isn't high enough to clear the blockage.

Over time, the weight of the sludge will cause it to sink to the bottom of the radiator, blocking all of the flow channels and creating a radiator that is cold at the bottom.


How to remove sludge from your radiators

So now we know that the likely reason for a cold bottomed radiator is sludge. But what can you do about that?

Don't leave it

This isn't an issue that you should really procrastinate about. Turning the heat up on the thermostat isn't going to fix the problem and is only going to make your heating bills get higher.

And while you may not be too badly affected by cold radiators immediately, that sludge isn't going anywhere. And it is only going to build up even more.

This will have the effect of making all of your radiators stop working as they should but, more importantly, it can start to cause problems and damage to your boiler. Fixing a cold radiator is much easier, and much less costly, than fixing your boiler or having to buy a new boiler.

The good news if you need to fix radiators is that it is usually a relatively easy job and you shouldn't need to buy a new radiator or boiler. Let's take a look at how.

Manual flush

This is a great option if only one radiator is cold at the bottom and it is usually a job that you can do yourself without the help of a heating engineer, albeit a messy one.

Step 1 - get prepared

Did we mention that this was a messy job? You are going to want to make sure that you have everything in place to try to contain the mess as much as possible before you start.

It's a great idea to have lots of towels (an old towel would be best) laid down on the floor directly underneath the radiator nuts. You will also need quite a few buckets or bowls to catch the water when it starts to come out.

Step 2 - stop the water

Your next job is to stop the water from flowing into the radiator. You do this by turning the thermostatic radiator valve down to zero and closing the lockshield valve with a spanner. This valve is often covered with a plastic cap to keep it at the right setting so you will need to remove that first.

Step 3 - drain the existing water out

Now you need to get rid of all of the water that is already in the radiator. Turn the radiator nuts slightly and open up the bleed valve at the top of the radiator using the bleed key. This releases any trapped air and allows the dirty water to flow out.

Step 4 - manually flush the radiator

Remove the radiator from its brackets and take it outside. Take your garden hose and use it to flush water through the radiator until it starts to run clear.

Step 5 - replace the radiator

Re-attach the radiator to its wall brackets and re-attach the pipes. Return the radiator valves back to their original positions. Water will begin to flow back into the radiator so be ready to close the bleed valve as soon as the water begins to come out.

Step 6 - pressurise the system (if needed)

If you have a pressurised system, you will need to add more water to the loop to get the pressure back to where it should be. If you have a conventional feed or an expansion system, it will re-pressurise on its own.


Chemical flush

A chemical flush (also known as a hot flush) is usually a job for a qualified heating engineer. You may need a chemical flush if other radiators in your house are affected, as this indicates that there is sludge throughout the entire heating system.

A chemical flush uses gravity to empty the central heating system. A chemical cleaning agent is added to help lift the sludge from your radiators so that it can be emptied from the system.

A final flush with clean water is often completed, after which corrosion inhibitor is added to the system to prevent more rust from forming (as much as possible) and often a scale reducer as well to reduce the amount of limescale that will form.

The whole process will take about an hour.


Power flush

A power flush is very similar to a chemical flush but it is more powerful. This should also be completed by a qualified heating engineer because it requires specialist equipment.

It can be more expensive than a chemical flush but it is also more effective. If you are having problems throughout the heating system, this might be a good option for you.

A power flush uses the same chemicals as a chemical flush but rather than using gravity to push these chemicals through the heating system, it uses a high pressured pump. The pressurised water is very effective at completely clearing the central heating system and boiler of sludge.



A MagnaCleanse is a new flushing technique that aims to slow down the build-up of sludge. It involves a chemical flush of the heating system and boiler to remove whatever sludge and debris are already in the system, as well as the usual application of a scale reducer and a corrosion inhibitor.

After this process is completed, however, a magnetic filter is added to the system boiler. This filter attracts any iron oxide or rust deposits preventing them from creating sludge. Using a magnetic boiler filter can help to reduce the chances of sludge becoming a problem again in the future.


How to prevent sludge build-up

Now your heating system is flushed and you have hot radiators instead of cold radiators again, you will likely be thinking about how you can stop the problem from reoccurring. There are a few techniques you can use that can slow the sludge and mean that you won't need to flush your system again for a long while.


Scale reducer

If you live in a hard water area, it can be a good idea to add a scale reducer agent to your water to prevent limescale from developing in your radiators.


Magnetic filter

As we have already mentioned, a magnetic boiler filter can help to catch any iron oxide deposits or rust to stop them from developing into sludge.


Central heating inhibitor

A central heating inhibitor is a chemical that coats your system and slows down the chemical reactions that form iron oxide. This will slow down the accumulation of sludge in your system.


What if the radiator is completely cold?

If your radiator is cold at the bottom and the top then a flush may not fix the problem. If it is just one radiator that is affected, you may need to buy a new radiator. If all of the radiators in your home are completely cold, then this can indicate that there is a problem with the boiler.

Your best bet is to call a heating engineer and have them assess the problem to see if your boiler is fixable or if you may need a new one.


How often should I flush my boiler system?

Your central heating system should be flushed around every 5-6 years. If you have a boiler filter and other protective measures installed, you may need to flush less often.

If you notice a cold radiator or other signs that there could be sludge in the system, then you should get it flushed as soon as possible. You should always flush the system after installing a new boiler.


The bottom line

A radiator, cold at the bottom, is usually an indication that there is a build-up of sludge in your central heating system. Thankfully, this is a problem that can usually be fixed without having to go down the road of new boilers or central heating systems. Flushing the system to get rid of the sludge will usually leave you with hot radiators and it will also help lower your heating bill.

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