how to bleed a radiator, radiator valve, heating system

How To Bleed A Radiator: 6 Easy Steps

How to bleed a radiator can be the answer to a lot of problems that come with a heating system. Your radiator might be cold in certain areas, or it could be part of a new combi boiler system installation. Knowing how to bleed a radiator is like changing a tyre – you never know when it’s needed!

radiator bleed how to

To bleed a radiator, you need to:

1. Find out which radiators need bleeding

2. Turn off Central Heating and let it cool

3. Turn the bleed valve using the radiator key

4. Keep open until the hissing stops and water starting coming out

5. Re-tighten the valve

6. Turn the heating back on

Do you find different parts of the radiator are more heated than others? Does the bottom feel well-heated but the top is ice cold? It’s probably because it has air trapped inside, which takes up the space that would otherwise be used by water to warm the radiator up.

The good news is that a radiator bleed is a quick and easy-to-perform solution. Doing this small task can help your radiators perform as efficiently as possible, saving you money on your energy bills. Your radiators work alongside your boiler to ensure your home is comfortable heated. However, if your radiators aren’t working properly, you’ll just be throwing money away, and who wants that?

If you need more information about boilers themselves, we’ve previously done articles that explain what is a combi boiler and how a combi boiler works.  Also take a look at our complete guide on condensing combi boilers to learn more.

So here it is: a step-by step breakdown of how to bleed a radiator!


How To Bleed A Radiator – What to do first

Before you start to look at what sort of supplies you might need, you’ll have to identify which type of boiler you have. Is it a system, combi or open vent boiler? It’s important to know based on how you’ll proceed with the radiator bleed. If it’s an open vent for instance, you won’t have a filling loop which would be used to top up the pressure afterwards. This step in particular will ensure you’ve done what you can to get the pressure back to the right level.

Supplies that you will need:

To perform a radiator bleed successfully and safely, you’ll need the following:

A radiator key – Pretty important for the whole process. You could try it with pliers but you may risk damaging the central heating radiator, which you don’t want to do. They’re issued with each radiator, but any regular DIY shop will keep them in stock as well.

A cloth – Have this around your hands when you turn the key to bleed the radiator. It’ll keep your skin safe from any nasty chemicals, and make sure everything is kept dry for the whole process.

A towel – The same reason that you’ll need the cloth. If you keep this underneath the end of the radiator as it’s being bled, it’ll prevent any spillages landing underneath. The same can also be achieved with any kind of basic container. Even more important if it’s in a room with carpet.

1. Turn on Central Heating to find out which radiators need bleeding

For you to know exactly where the problem is, you need to start from scratch. Start by turning the heating on all the radiators up to maximum – this shouldn’t take more than a few minutes.

Once they’re heated up, locate the cold patches by gently moving your hand over the surface – wear gloves for safety. If it’s cold near the top, then this will be the air that you’ll need to bleed out. If it’s cold at the bottom of the radiator, then it will probably be because of a build-up of sludge. This would demand a chemical or power flush – a method which is explained a bit more in this article from the Viessmann website.

Besides finding cold patches, there are other tell-tale signs that a radiator’s in need of a bleed. Try and listen for gurgling sounds, and pay attention to how long it takes for each radiator to heat up. If it’s taking a while or you can hear these noises, it’s because there’s too much water inside.



You might need to bleed more than one radiator, so make sure you’ve gone through all of them to save time later on. If they’re across multiple stories, start with the ground floor radiators, and then work your way up.


2. Turn off Central Heating and wait for it to cool

Now that you know where your cold patches are, you can turn the central heating back off. Don’t forget that your radiators will still be hot so wait as long as needed to until they’re completely cool. If you try it while the central heating is on, air will be sucked back into the system, creating more work.

Make sure that your radiators are cooled down completely, especially before you bleed any. If you don’t, then hot water may spurt out which can be dangerous if you’re working close to the valve. If you wait for the water inside to settle and cool, the bleed will be much more effective.

Taking your time with these steps is important in understanding how your heating system works. This is why a lot of people are forced into asking themselves the question ‘Should I replace my boiler?’. Before they know it, it’s too late!

3. Find the bleed valve and turn using the radiator key

radiator valve turn on


The bleed valve is a small square inside a round hole, found at the top of your radiator on either end. Using the cloth for extra grip, insert your radiator key over this opening, and turn anti-clockwise. You’ll barely need to turn it 45 degrees for it to do the job. If you keep turning it all the way, the water will reach you faster and it will be much more difficult to stop.

Once you’ve started turning it, you should hear a distinct hissing noise.

4. Wait for the hissing to stop

The length of time depends entirely on how much air is trapped inside the radiator, but it rarely goes beyond a minute. When the air is being purged, there’s a possibility it will have a foul smell. This might mean that there is a build-up of magnetic sludge which itself can create methane through a strong chemical reaction. This is potentially flammable which means that a chemical or power flush might be the best answer to ensure they’re safe to use. As soon as you hear the hissing start to disappear, be prepared to catch the water that will soon follow. Once all the air has been released, the valve will start to trickle water which can be mopped up by the cloth you’re holding underneath.

If you want to be certain there’s no more trapped air, wait until you get a steady stream of water out the end. Just be prepared with the towel if you do!


5. Re-tighten the valve


radiator valve turn off


Now that you know all the air has escaped from the radiator, it’s time to screw the valve shut once more. Funnily enough, it works the same way as opening it up except now it’s the other round! When turning the key clockwise, make sure not to over-tighten. The last thing you want is to damage the valve.

Once this is done, use the cloth and simply wipe away any excess water that dripped out during the bleed. Doing this not only tidies it up and prevents any slipping, but it helps to stop any rust grow over time.



6. Turn the heating on again

Now that you’ve gone through all your radiators and bled the air out, as long as the valve has been closed shut, it’s now safe to turn the central heating back on. This will be the best way to check that it’s worked. Gently place your hand over all parts of each radiator, and try to find any remaining cold patches. For the most part it will have done the job straight away, but don’t be afraid to try it more than once.

Once all your radiators have been bled, you can turn on the heating again to check your work’s been successful. The radiator panels will feel hot all over with no leaks or drips from the bleed valve.

Don’t forget, you’ll need to check your boiler to make sure it’s displaying a regular pressure level. This would typically be around 12-15 psi, but by bleeding air out of the radiators, you will have lowered the overall pressure. If the pressure’s fallen too low, you may struggle to get heat in some of the upper floors.

If this is the case and you’re working with a combi or system boiler, then you can simply top up the pressure by using the filling loop on your boiler. It looks like either a tap or a lever on the main water supply to your boiler. However, this is not something that you can perform with an open vent boiler.

combi boiler filling loop

You may find that you can add additional radiators to your central heating system if you’re replacing a non-combi boiler.

Always double check

Keep an eye out for any more cold patches that come up on your radiators. A radiator bleed will definitely do the job, but if you find that they’re still cooler nearer the top, then you may have had a build up of magnetic sludge. This isn’t the most unusual occurrence, as there’s a very simple response in the form of either a chemical flush or a power flush.


How To Bleed A Radiator – Success!

So there you have it, a basic guide on ‘How to bleed a radiator’.

It might be a daunting task for some, but it’s important to remember just how useful it can be. It can be unnerving to have to deal with issues like this related to your central heating system. If you’re considering an all-out replacement over a simple boiler service, it’s work checking out these other articles:

Any issues that you may have with your boiler are usually related to the water pressure, but as long as you have this knowledge at your disposal, you’ll be enjoying the warmth and comfort of your own home completely stress free.


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