Radiator Hot At Top, Cold At Bottom Causes & Fixes

James Elston
Written by James Elston
Updated on 4th April 2024
Posted on 3rd April 2024
Radiator Hot At Top, Cold At Bottom Causes & Fixes

Radiator hot at the top, cold at the bottom? – Here are the reasons why?

Radiators have a job of keeping our houses warm, so when a radiator is cold at the bottom, it is not functioning properly. Asides from that, having a radiator cold at the bottom could lead to more complicated issues like your radiators not heating up at all or having to replace your boiler or radiators.

This article aims to dive deeper into the details to ascertain why a radiator is cold at the bottom. You will also discover practical fixes to the problems.

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How Do Radiators Work and Get Hot?

The first thing you need to understand before trying to fix a radiator that is cold at the bottom is how a radiator works in the first place. If you are unfortunate enough to recently experience how much a new boiler costs and had one installed or had a new boiler replacement, you shouldn’t really be having this problem, but here we go:

The process begins with heated water travelling from the boiler through a double loop of pipework to every radiator in your home. The double-loop design allows the hot water to circulate around the radiators before taking the return loop back to the boiler.

Most heating systems operate this way; however, recent systems come with a manifold design where the return and flow connections are independently directed to each radiator in the home.

Inside the radiator, the hot water gets directed through channels that ensure it keeps flowing upwards and sideways before going back down to exit through the exit piping. This design ensures that the entire surface of the radiator is covered with flowing hot water while your boiler system is on.

The moment you turn your boiler off or reach your desired temperature, the flow of hot water stops, causing the radiators to cool. However, as soon as the boiler begins operation again, the radiators will warm up.

Why Your Radiator Might Be Cold at Bottom

technician checking radiator from bottom

If you discover that your radiator has cold patches while the heating is on, rather than need your central heating boiler replacing, it could just be that you have sludge in your radiator.

The common cause of cold radiator bottoms is sludge, as sludge tends to accumulate inside the radiator over time. Once enough sludge is in the radiator, it blocks the flow channels. The water is unable to move around the radiator effectively.

Sludge is a thick and heavy substance that settles at the bottom half of the radiator, which is the reason why the radiator is cold at the bottom. While heat rises, the sludge doesn’t let the hot water get to the parts at the bottom.

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What Causes Radiator Sludge?

In situations where metal and water meet, other materials are, unfortunately, created. The process of central heating has an abundance of both elements. Most home radiators are steel or iron, and water flows through the system from the boiler, so an occurrence where water and metal meet is common.

This constant amalgamation causes iron compounds such as haematite and magnetite to form. These compounds are then joined by a scale, dirt, and rust mixture. Over time, this can get quite messy as these materials accumulate.

In very extreme cases, you can find radiators with quite a lot of sludge to the point that the bottom part of the radiator is cold. Nevertheless, this isn’t something you should worry about as there are things to be done to fix this issue.

How to Fix It a Radiator That’s Only Hot at the Top

There are various ways you can fix a radiator cold at the bottom. You should note that each method has its benefits and is suitable for unique situations.

If only one radiator is cold at the bottom or has this issue

radiator placed in hall

If you discover that just one radiator in your home has a cold patch at the bottom, you could try to solve this problem yourself. The radiator will have to be manually cleaned. It is a process known as flushing the radiator.

To successfully flush the offending radiator and get rid of the central heating sludge, you first have to bleed the radiator. Once you are done with this, you will have to drain the radiator as much as possible. This process helps you get rid of the dirty water.

When doing this, ensure that you place towels down to protect the floor as much as possible. You can also have a bucket nearby to catch the water.

The next step might require you to take the radiator off the wall. This process lets you take it outside where you can flush it, eliminating the sludge.

Connect a hose to the radiator valve inlet and switch the tap fully on, causing clean water to flow through the radiator exiting at the other inlet. The force of the water flow should cause grime, sludge, and dirt to exit the radiator.

You need to ensure that all the sludge is removed from the radiator. Do this by using a rubber hammer to tap the radiator inlet lightly.

Give the radiator a once-over by connecting the hose to the other inlet and then flush water through it till the water is clear.

If you have more than one radiator cold at the bottom

In extreme cases, sludge and grime can affect various branches of your central heating system, resulting in more than just a single radiator cold at the bottom. Should this be your situation, you will need to hire the services of a heating and boiler engineer to commence a Power Flush on your heating system.

The entire process takes about 5 hours; however, the actual time depends on the size of your home and the type of heating system. An advantage that power flushes have over the hose method is you don’t have to take the radiator off the wall. The process can be conducted with all the components connected in their working design.

This is because repair engineers tend to have special chemicals, cleaning solutions, and tools that eradicate and eliminate limescale and sludge this in turn should balance your radiators. If not you can learn how to balance your radiators here.

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Cleaning Your Radiator – How to get rid of the cold spot at the bottom

This part of the article provides a step-by-step guide on how to clean your radiator.

1. Isolating your radiator.

If your affected radiator has a thermostatic radiator valve, you must turn this down to 0. On the other side of the radiator is a valve known as the lockshield valve. It is the valve covered with a plastic hood.

Close the lockshield valve using a spanner – note that you might need to turn it slightly, either a quarter or half a turn. Remember this, as this is how much you will need to turn it open once you are done.

If your radiator doesn’t have a TRV, you simply need to then turn both valves off using a spanner. After this, wait for at least an hour before you do anything. This period will allow the water in the radiator to become cold. You don’t want hot water spilling across the room.

2. Set up for leaking water.

Since you are emptying your entire radiator, you will need to expect a lot of water. The best thing to do is to have some containers or buckets on hand to ensure that you catch all the water that escapes from the radiator without overflow or spillage.

A good tip is to place rags or old towels under the connector nuts with buckets. Doing this ensures that should any leaks happen, you can seamlessly clean them.

The next step is to turn the radiator nut slightly with a spanner. At this point, just a bit of water will drip out, so you shouldn’t expect a lot of water to begin pouring out.

3. Open the bleeding valve.

Open the bleed valve right at the top of the affected radiator using your bleed key. At this point, you do the opposite of what you would normally do by letting air in and not out. This will cause water to begin pouring out from the loosened nuts.

4. Removing and cleaning the radiator.

man cleaning radiator

When the water stops coming out of the radiator, disconnect the valves and take the radiator off its bracket. Ensure that you don’t completely undo the valves. Doing this could cause the contents of your heating system to empty.

Take your radiator outside to clean. You can then attach a hose to one end of the radiator and blast water through the end for a couple of minutes until the water begins to flow clean. To ensure that the radiator is completely clean, you can connect the hose to several different openings.

5. Placing your radiator back in its brackets.

Once done, you should replace your radiator on its bracket. Ensure the pipes are attached to the nuts while turning the valves in either end to their original positions.

Water will begin refilling the radiator, so you must have the bleed key on hand. Close the bleed valve when water starts to escape from it.

6. Test out the system.

At this point, you should test the radiator out by switching the heating back on. Once on, you should wait for about 20 to 30 minutes for the system to heat up. Check the radiator by putting your hand at the bottom of the radiator. If you cleaned the radiator thoroughly, it should be heating evenly.

You should note that once you remove a radiator from a sealed heating system, you must re-pressurise it. If your heating system is pressurised, add more water to the loop to level out the pressure.

One thing people sometimes do that can stop people realising that their radiators aren’t working is that they put on a radiator cover that can block heat.

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How to Prevent Cold Spots Sludge in Your Radiator

Like many common boiler and heating issues, prevention is more effective and inexpensive than a fix. Fortunately, you can do a few things to stop sludge from becoming a problem in your central heating system especially if you are buying a new boiler.

Using a magnetic filter.

Since central heating sludge has metal debris like rust, a great method to stop the build-up is to have a filter that removes the metallic fragments. You can add most of the metallic filters to your boiler during installation.

They get attached to the return pipe that makes its way to the boiler. If you are interested in installing a magnetic filter, you should speak to a licensed boiler engineer to install one for you.

Use a scale reducer.

A scale reducer’s job is to eliminate any limescale build-up. Less limescale means that the likelihood of sludge building up reduces.

Use a central heating inhibitor.

An effective way to stop sludge build-up and ensure that your system continues running is to use a central heating inhibitor. This inhibitor can stop sludge from building up by breaking it down.

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Written by
James Elston
James Elston Director Of Boiler Central
Posted on: 3rd April 2024

James Elston is our resident boiler replacement and heating expert here at Boiler Central. With over 20 years experience in the boiler installation industry, James ensures that he knows everything there is about our Gas Safe boiler installations, energy saving and home heating solutions. This can be from simply procuring the latest best combi boilers, to reviewing and ensuring that Boiler Central maintains the highest standards across our boiler installation company.

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