Radiator Bleed Valve

The winter is coming, and it is time to make sure your radiators are in top form for the colder months. But before you start, there are a few things you should know about bleeding radiators.

In this post, we will cover everything from how to bleed radiators, what you need to bleed them, how often they should be bled, and any potential problems that could arise.

Why do Radiators Need Bleeding?

Radiators are bled to remove the air from the central heating system. Air can enter the system in a few different ways, and it will often build up in one or more of your radiators.

Air is compressible and not subject to boiling so having air in the system can make radiators inefficient and you will often notice that a radiator containing air will heat up at the bottom and stay cold towards the top.

How Often Should Radiators be Bled?

Although each system is different, in general terms, we recommend that radiators should be bled a minimum of once per year, even if they appear to be working just fine. The frequency your radiators may need to be bled can also depend on how much they are used, where they are located in your home and the type of system that you have.

For example, the radiator at the highest elevation in your home is the one that is likely to collect any excess air in the system and will often need bleeding far more often than a radiator that is downstairs on the ground floor. This is because hot air rises.

What You Need to Bleed Radiators

You will need a couple of items before you can bleed your radiators. Make sure that you have the following:

Radiator Bleed Key

A radiator bleed key is like a small wrench with an offset head. The purpose of this tool is to be able to open up the bleed valve present on each of your radiators.

Towel or Cloth

You will need a towel or a cloth when bleeding your radiators, this is to catch the excess water that will escape when bleeding.

How to Bleed a Radiator

Once you have your radiator key and cloth, you are ready to begin bleeding your radiators. This is a simple task that anyone can do so you do not need to worry about calling out anyone to help.

Step 1 – Turn off the System

The first step is to ensure that the system is turned off and if the system is on a timer, you will need to be sure that it will not be coming on whilst you are bleeding your radiators.

This is purely for safety reasons as it is extremely unwise to attempt bleeding a system whilst it is hot. It may not only be too hot to touch but when you are bleeding each radiator, there will be small amounts of water escaping so you need the system off to prevent scalding!

Step 2 – Work out Your Order

Whilst your system is cooling down, work out in which order you are going to bleed your radiators. Although this is not the most important step, being a little more organised will help some people.

You will want to plan your route to start at the furthest, lowest point in the system and then work through each radiator until you get to the radiator at the highest elevation in your home.

Step 3 – Bleeding the Radiators

Locate the bleed valve at the top of your radiator, usually at one end but they can be hidden in the back of some.

Hold your cloth underneath the valve in preparation to catch any water that will escape and using your radiator key, slowly turn the valve anticlockwise until you either hear a gentle hissing sound or see water escaping.

If you see water escaping immediately, close the valve and move on to the next radiator.

When you hear the gentle hissing, that is the air escaping so hold the valve key in place until the hissing stops, and you get water escaping instead.

Again, once the water begins to escape, close the valve, and move on to the next radiator.

Step 4 – Repeat

Repeat step 3 for all of the radiators in your house starting from the lowest elevation right through to the radiator at the highest.

Step 5 – Re-pressurise the Boiler

One step that many people unintentionally skip is to re-pressurise the boiler after bleeding.

Check the pressure gauge and if it is too low, it will need re-pressurising. When cold, the pressure on the gauge should show just over 1 bar.

Although there are a lot of different boilers, they will all usually have a lever or tap that can be turned to increase the pressure – this is known as the filling loop.

Step 6 – Turn Your Heating System Back On

The next thing to do is to turn your heating system back on to ensure that all of your radiators are heating up evenly and that there are no cold spots remaining.

If there is still a cold spot or a particular radiator not heating up properly, repeat the process again – if following the second time of bleeding, you are still having cold spots, this could indicate a more serious problem such as a blockage somewhere.

Problems That Could Arise When Bleeding Radiators

There are several problems that could arise and to save you from standing there scratching your head, we have covered some of the more common ones below to try and help you get your system sorted without professional help.

No Bleed Valve on Radiator

It is a common occurrence that many people are unable to locate the bleed valve on a radiator leading them to believe that in fact there is no bleed valve on a radiator.

If you think that there is no bleed valve on your radiator, you have made a mistake and have just been unable to locate it. Some bleed valves are located on the back of radiators under small covers or caps, and I have also come across some towel rails that have the bleed valve extremely well hidden.

If you are thinking there is no bleed valve on your radiator, have another look, it will be there.

Radiator Key Doesn’t Fit

This is another common problem that people come across when they finally get around to bleeding their system – the radiator key doesn’t fit.

This could be down to the age of the radiators or simply the design. Although many bleed valves will be the same from different manufacturers, there are some out there that like to keep us on our toes.

In this case, if you have a radiator where the key doesn’t fit, you are likely to need to find one that does. Any good plumbers’ merchant should have a replacement for the valve you are trying to access.

No Water in the Radiator When Bleeding

You may have come across a radiator that appears to have no water in it. You may have heard the hissing sound and then it has stopped, and no water has tried to escape. Having no water in a radiator when bleeding is almost always down to the pressure in the system.

What to do if no Water Comes out When Bleeding a Radiator?

If the hissing sound has stopped and there is no water coming out, this means that the system lacks the pressure to push the water high enough to remove the trapped air.

We recommend re-pressurising your boiler using the filling loop to ensure that the pressure is sufficient enough to let the air escape when bleeding.


Bleeding your radiators is a regular maintenance task that needs to be done on a yearly basis and is one of the most important things you can do besides ensuring your system is functioning efficiently and cleanly.

When we bleed our radiators, we are removing trapped air which causes cold spots in specific areas or can make some radiators heat up at the bottom only or not heat up at all.

Plumbing Wizard Tips

“Ensure you have a cloth, towel or rag handy to catch any escaping water – you don’t want to have wet carpets!”

“Ensure the heating is turned off before bleeding the system – heating systems are HOT, and scalding does occur!”

“Remember to check the boiler pressure after bleeding and ensure that it is at or just above 1 bar!”

“If your radiator key doesn’t fit, do not panic, your local plumber’s merchant should have one for you!”

Frequently Asked Questions

Which radiators do you bleed first?

You should start at the furthest, lowest point in the heating system and bleed your radiators in order of elevation finishing with the highest.

How do I know if my radiator needs bleeding?

There are various ways to know when a radiator needs bleeding. A radiator may heat up slowly, it may heat up at the bottom only or it may not heat up at all.

You may also hear gurgling, creaking noises from the radiators as the water is forcing its way past the trapped air.

If you practice good maintenance of your radiator system, you should bleed them annually regardless of whether they appear okay or not.

Should water come out when you bleed a radiator?

Yes, when you are bleeding your radiators, you should expect a little water to escape. It is the water pressure inside the system that is forcing the air up and out – as soon as you get water emerging from the bleed valve – turn it off!

If the air stops hissing and you are yet to see water, it is likely that the boiler needs repressurising.

Can I bleed a radiator without a key?

You can use a flathead screwdriver to bleed a radiator if you don’t have a key, but a radiator key is recommended for proper and easy bleeding.

What does it mean when my radiator is cold at the top?

A cold radiator at the top may indicate that it needs to be bled to remove excess air.

How often should I bleed my radiators?

It’s generally recommended to bleed your radiators once a year, or as needed if they are not heating up properly.

What do I do if the bleed valve is stuck?

If the bleed valve is stuck, try gently tapping it with a hammer to loosen it. If it still won’t budge, you may need to shut off the water and replace the bleed valve.

Can bleeding a radiator fix low pressure in my heating system?

Bleeding a radiator can help with low pressure if the problem is caused by air in the system, but it may not be the only solution.

Can bleeding a radiator cause damage to the system?

If done properly, bleeding a radiator should not cause any damage to the system. However, it is important to make sure not to over-tighten the bleed valve or let it leak.

What should I do if I have a lot of rust coming out of my radiator when bleeding it?

If you see a lot of rust coming out of the valve when you are bleeding, it would indicate corrosion somewhere in the system. Your system may need flushing or in worst-case scenarios, the radiator may need replacing.

About Me

I’m Lee the Plumbing Wizard. I’ve been a plumber for over 40 years and have seen it all. People used to call me a magician because I could fix things so easily – hence the name Plumbing Wizard. I’m always happy to help out, so if you need any advice or just someone to talk about plumbing with, don’t hesitate to get in touch!

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