Why Is My Radiator Cold at the Bottom?

Central heating systems can sometimes be a minefield, whilst some problems are easy to identify, others aren't so obvious. If you have discovered that your radiators are cold at the bottom, then you'll want to fix them as soon as possible to ensure you're getting the most out of your heating system.

What's more, it's easy to jump to conclusions - convincing yourself that you're going to need new radiators and a new boiler. To determine why your radiators are cold at the bottom, read our extensive guide to all the plausible causes behind a partially heated radiator. Combined with how-to instructions on making your home warm and cosy again.

Should my radiator be cold at the bottom?

Whilst it isn't standard practice for a radiator to be different temperatures, it can happen more often than not. If you are dealing with a radiator that is cold at the bottom but hot at the top, your hot water flow might be restricted.

If the hot water flow is redirected or restricted from flowing through your central heating system, it will leave your radiator cold. And, this typically happens at the bottom.

Note, cold radiators at the bottom are not usually caused by trapped air in your radiator. So bleeding your radiator won't fix the issue. If you want to check if the air is trapped in your radiator: the main symptom is typically a cold radiator at the top. This is because the trapped air is restricting the water from traveling around.

What causes sludge in radiators?

Nine times out of ten, most issues facing radiators becoming cold at the bottom is due to a build-up of sludge. Sludge results from steel or iron compounds gradually falling off inside radiators. After a while, these compounds form a mixture with the water running through it, before sinking and blocking the radiator.

This mixture, referred to in industry as sludge, is the main cause for the bottom of radiators being cold. This is because, once a little bit of this has accumulated, it is very easy for other materials to add to it. And most of the time, the water pressure is not strong enough to flush out these little particles by itself.

As hot water enters and exits the radiator at the top, sludge usually can be found around the middle and bottom area of your radiator. This radiator sludge stops the hot water from flowing evenly through the radiator - making it difficult to distribute heat evenly.

Please also note, if you have only one radiator out of your whole heating system showing these symptoms, then this is an issue that is easy to fix. Meaning, you may not have to get a specialist to check out your whole central heating system. If you feel confident to clear out radiator sludge yourself, then read on to learn how.

How do I fix radiators that are cold at the bottom?

Below is our extensive guide on how to fix the issue of a radiator that is completely cold at the bottom. If you've never drained a radiator before, then make sure you get help from someone who has.

As we already established, trapped air is not responsible for your radiator being cold at the bottom, so don't bleed your radiator.

Another measure that is not going to help you is cranking up your central heating all the way. This won't help your radiator's cold patches - it'll just skyrocket your heating bill.

But what should you do if your radiator is cold at the bottom?

Clean your radiator with a manual flush

Clean your radiator with a manual flush

The most common and easy way to fix your radiator being cold at the bottom is by cleaning it manually. That is if you do not wish to get an engineer or a chemical flush involved.

Below are our suggested instructions for cleaning a radiator with a manual flush. Please note, if you're not 100% confident then you should contact a Gas Safe-registered professional or ask for help from someone with experience.

To manually flush a radiator, you won't necessarily need specialist equipment but definitely a few household tools. Here is exactly what you will need:

Several buckets

  • Old towel or more

  • Spanner

  • Bleed key

  • Water hose

    1. Isolate your radiator

    If you have thermostatic radiator valves, then turn them down to 0.

    On the other ends, there will be other valves. These are called lockshield valves and are likely covered in a plastic cap. Make sure this valve is also closed. You will probably need a spanner to turn it for about a quarter and a half to close it completely. Remember how much you have turned it. This is exactly how much you will have to turn it again to open it back up.

    Wait for half an hour after you have closed your thermostatic radiator valve. This ensures that instead of having your radiator hot and potentially burning you, your radiator is cold. This prevents boiling hot water running all across your room.

    2. Prepare for leaking water

    Since you are going to empty your whole radiator, you should expect a lot of water. Make sure you have a couple of buckets or containers as well as old towels handy. This way you can capture the water and clean up any spillage or overflow.

    Also, place one of your old towels under the connector nuts. Now, take a spanner and turn those radiator nuts slightly. A little bit of water should come out. But don't expect a lot of water to pour out just yet.

    It might be wise to turn off the water supply to your home whilst executing this for peace of mind.

    3. Open the bleed valve

    Take the bleed key and open the bleed valve on top. The air will now flow into the radiator, which will cause the water to start pouring out where you loosened the nuts earlier.

    4. Remove and clean your radiator

    After the water has stopped running out of your radiator, disconnect it from the valves. Next, lift the radiator up and off the brackets.

    Please note: Do not fully undo the valves! You do not want the content of your heating system to be emptied as well.

    Now, take the radiator outside. Take the water hose and blast water through the radiator for a few minutes. Check whether the flow is clean. If not, try the hose in different openings of your radiator, to make sure all the radiator sludge is being flushed out.

    5. Replace your radiator

    Take your radiator back inside and re-hang it on the brackets. Then, re-attach the nuts and pipes. Now, turn the valves back into their original position.

    The water will slowly start refilling the radiator. So get the bleed key and close the bleed valve as well, as soon as the water passes through it and tries to escape.

    6. Test your system

    Turn the heating back on to test your radiator.

    It's best to wait for about 20 minutes so that the water begins to heat up. After which, you can check whether your radiator is hot and running.

    To test whether the heat rises equally through your radiator and no cold patches are left, place your hand on different parts of the radiator. Start at the bottom and swipe your hands up.

    If your radiator is giving off heat evenly without any cold patches, congratulations - you have solved the problem and there is no major issue with your central heating!

    Please note, as you have taken your radiator off of the sealed system, then your central heating system will require repressurising. Once this is done, add more water to the central heating loop in order to get it back to the required pressure.

    How do I prevent my radiator from getting cold at the bottom?

    As with most instances, preventing the problem is much better than treating it later. But how can you prevent radiator sludge, i.e. your radiator from having cold patches?

    The easiest way to prevent sludge is to add a central heating inhibitor to your system. Attaching a corrosion inhibitor to your heating system stops iron compounds building up in your radiator. It slows down chemical reactions that can cause iron oxides and sludge build-up. This will extend the life of your radiator significantly.

    Using an inhibitor is often required to keep the warranty of your radiators and comply with any industry benchmark best practice. We highly recommend adding this to your system as soon as you can.

    Here are some other ways to prevent sludge and cold radiators:

    Add scale reducer

    A scale reducer is a similar agent that will reduce limescale in your water. Limescale can speed up the process of sludge build-up. Therefore it is a great option for homes that are located in hard water areas.

    Add a system filter

    Another thing to consider is the installation of a magnetic filter for your boiler. Such a boiler filter will take out any grime in the loop before it can settle or accumulate in certain areas. It achieves this by attracting sludge, rust, and other metallic debris in your radiators.

    With a boiler filter, it will be easy to remove any debris. It is neatly collected inside the boiler filter and you will just need to exchange or clean it every now and then.

    Looking after your heating system and preventing cold radiators will be the easiest way to keep your system running smoothly. This is the best way to prevent cold spots on your radiators for a long time.

    Whether you want to renew your entire heating system in the wake of soaring gas prices or are just looking for new radiators, PlumbNation offers high-quality service and plumbing supplies for your home.


    How do I power flush my radiator?

    If you cannot remove the entire radiator, perform a power flush to clean the radiator sludge. However, you will need to hire a professional plumber for this.

    The professional will pump high-pressure water mixed with special chemicals through your whole system to clean it out.

    Note that power flushes are used to clean a whole system and not just a single radiator. For that reason, if you have noticed more than one radiator is cold at the bottom, this is the right procedure to choose. A power flush is quite a tricky process; therefore, contact a professional.

    Can I add a chemical flush to my radiator?

    Yes, you can if you want to remove metallic debris from your radiators quickly.

    Chemicals can be added to your radiator system in order to clean out any sludge build-up. It works similarly to how drain cleaner would work and is a one-off job.

    For a chemical flush, chemical cleaner is added to your system. After which, your heating is then run for about an hour to warm up the chemical flush before flushing everything out and replacing the water in your system. In this case, we also recommend leaving this job to a heating engineer as dealing with chemicals can be quite tricky as well.