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Last updated: 27th March, 2024


How To Bleed A Radiator

How To Bleed A Radiator

If you have not bled your radiator in a long time - or ever - then it is probably a good idea to try and get it done as soon as possible.

A lot of people do not know how to bleed a radiator, but doing it can make your heating system much more energy efficient and save you money in the long run. So, what is bleeding, and how can you tell if a radiator needs bleeding?

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What is Radiator Bleeding?

'Bleeding' your radiators involves releasing trapped air to make sure that the radiators in your home are not displacing hot water, making your central heating system much more efficient if you bleed the radiator effectively.

Air trapped inside a unit can start to displace and catch any water that is meant to be going through the heating system, meaning that the top of the radiator may become cooler and the performance will start to suffer.

In simpler terms, the water starts to have less space to flow, as trapped air fills the radiator.

The idea behind trying to bleed your radiator is simple: you find a way to release the air and let the hot water flow properly again.

To bleed them properly, you need a way to get the air escaping from the radiator itself, different models of radiator will handle the process in slightly different ways.

Can I Avoid the Trapped Air Build-Up?

Normally, you want to stop problems with the central heating system before they happen. However, almost all radiators need bleeding eventually: trapped air stops the radiator from performing at peak efficiency, and the air can enter from a range of different sources.

In an enclosed radiator, the air stops escaping since it has nowhere to go, and this air can even come from the movement of the pump itself - there does not have to be an open radiator valve or cracked pipe for air to start filling it.

Finding the Radiators that Need Bleeding

Not every radiator needs bleeding at the same time, so having to identify which radiators need bleeding can be tricky at first.

There is not much risk in trying to bleed a radiator that does not need it, but it can take a lot of time for no real benefit.

If you check the pressure of the radiator and notice a sudden drop while the boiler pressure gauge is still high, then there is probably an issue with one of the pipes or radiators in your home.

A normal boiler pressure gauge reading means that the boiler and central heating system are functioning properly, but something that delivers the heat is not.

You can also tell if you need to bleed your radiators by checking to see if the top of the radiator is cold. Check the pressure alongside this, and you will usually be able to tell if you should bleed your radiators.

Cold spots along the top can mean that pressure is too low, especially if there is only cold spots on the top and no cold spots closer to the bottom.

Getting Started

If multiple radiators need bleeding, start with the one furthest away from the boiler. You should ideally begin bleeding radiators on the ground floor if you can, then move upstairs afterwards.

Be sure to turn your heating system off before doing anything with your radiator: if you try to bleed the radiator with the central heating system still running, you could hurt/burn yourself and damage the system.

Keeping the system on could also suck even more air into the radiator you are trying to bleed, and may end up spraying you with hot water.

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How to Bleed your Radiators

Getting a Key

If you want to start the radiator bleed process, you will need a radiator key, as well as something to make sure that you can stop hot water from dripping onto your floor.

If you have lost your radiator key, you will need to go and buy another one, but they are fairly common and relatively cheap. This bleed key allows you to turn the valve inside the radiator, and start bleeding air out.

Turning Off the Heating System

As aforementioned, you will want to turn off the heating system before trying to bleed a radiator. Always turn off the system in your home a while before you start, giving it time to cool down and switch off.

Inserting the Key

There should be a slot for the key on your radiator, something that looks like a round hole with a square inside.

Insert and slowly turn the key anti-clockwise for about a quarter-turn: this will open the valve that you need, sometimes called a radiator bleed valve.

This radiator bleed valve will generally open as soon as you turn the bled key, so you will hear a hissing sound as the air starts to escape.


As long you hear a hissing sound, do not close the valve. Cold-water may drip out, but you can catch this with a cloth.

More water will begin to drip out as you bleed more air, so eventually, the hissing noise should stop completely. Be sure to stop once the hissing ends and cold water starts dripping out faster.

Shutting the Bleed Valve

Now you just need to twist the bleed key back to seal the radiator's valve. Make sure you do not turn it anti-clockwise, since this opens the valve further.

There is not anything special to do here, just be sure that you tighten the valve when you are done bleeding the radiator. It is okay to leave some trapped air inside if you are not completely sure how to bleed a radiator to the perfect level.

Cleaning Up

Finally, when you have the radiators sealed up, clean up any remaining water around your home. Radiators won't usually leak much during a bleed, but it is best to wipe them down since radiators can rust easily if they are made of certain materials.

Turning on the Heating

After all of that, you can turn your heating back on. Be sure to check the pressure gauge on your boiler afterwards, making sure that your radiators are properly sealed and working as desired.

Ensure the radiator bleed valve is closed properly with the radiator key before you do anything else: if you open the valve but do not shut it, then turn your heating back on, you can cause a lot of issues.

Bleeding a Radiator More Than Once

Knowing how to bleed a radiator gives you everything you need to try bleeding a radiator multiple times. Sometimes, some of the trapped air may not come out with the first attempt at trying to bleed a radiator, so you will need to bleed it a second time.

Some radiator models may also need you to bleed them several times if they have a lot of trapped air.

You do not need a special step-by-step guide to bleed a radiator more than once: just follow the same easy steps all over again.

If you still have pressure problems, there may be an issue with the boiler or filling loop.

How Does Bleeding Help?

Choosing to bleed your radiator can save you money on heating energy bills since you are getting more efficient use of your boiler through the radiators themselves.

To be sure that there is more room for hot water in a radiator, bleed them regularly and check them if they get colder or do not take in as much water as they used to.

Of course, both your boiler and your radiators could be suffering performance problems, but boiler troubles usually require a gas-safe engineer to fix. Radiator bleeding is much easier.

If you need to bleed multiple radiators, turn off the heating and boiler, then get one radiator done at a time.

Even if your boiler and heating are turned off, you can still get problems with water left in the system, and opening multiple radiators can cause big leaks even while the boiler is off.

Get to each radiator's bleed valve, follow the step-by-step guide, then close that bleed valve and move on the next radiator's bleed valve until they are all fully bled.

Is Radiator Bleeding Dangerous?

Always check your boiler and heating system in your home before you bleed radiators, just in case.

Most of the time, bleeding a radiator won't be dangerous at all, as long as the boiler (and heating) is not still running, since the heating will be pumping hot water from the boiler straight into the pipes you are using.

If the heating is turned off, there is nothing to really put you in danger.

Always check how serious the heating issue is before you bleed the radiators in your home since there might be another issue causing the loss of heat that requires a gas-safe professional.

Bleed options are an effective tool for getting better radiator efficiency, but if the problem lies in a deeper part of your heating system, there might not be much that you can do on your own to fix it.

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Frequently Asked Questions

Can You Bleed a Radiator When the Heating is On?

It is generally not recommended to bleed radiators when the heating is on. Doing so can result in hot water escaping and an increased risk of burns. Furthermore, with the system active, it may not effectively release all the trapped air. For best results and safety, turn off your heating system and wait for the radiators to cool down before bleeding them.

Do You Empty Radiator When Bleeding?

No, you do not empty the radiator when bleeding. Bleeding a radiator involves releasing trapped air from within the system, not draining the water out. Only a small amount of water should come out once all the trapped air has been released, indicating that the bleeding process is complete.

How Much Water Should Come Out When Bleeding Radiator?

When bleeding a radiator, only a small amount of water should come out, indicating that all the trapped air has been released. This could be just a few drops to a small trickle. Once water starts coming out in a steady stream without air bubbles, you can close the bleed valve as this indicates that the air has been fully expelled.

How Do You Bleed a Radiator Without a Bleed Valve?

Bleeding a radiator without a traditional bleed valve is more challenging and may not be possible in the conventional sense. Some older systems or specific radiator designs might not have a bleed valve, in which case you may need to slightly loosen a fitting to let air escape, but this should be done with caution and potentially by a professional to avoid damaging the system or causing leaks.

How Do You Get Rid of an Airlock in a Radiator?

To remove an airlock in a radiator, you typically need to bleed the radiator. This involves: - Turning off the heating system. - Using a radiator key or a flat-blade screwdriver (for radiators with a bleed screw), turn the bleed valve counterclockwise to open it slightly. - Wait for the hissing sound of escaping air to stop and for a small amount of water to appear before closing the valve tightly.

How Do You Bleed an Old Fashioned Radiator?

Bleeding an old-fashioned radiator involves a similar process to modern radiators. Use a radiator key to turn the bleed valve, usually located at the top of the radiator, counterclockwise. Be prepared with a cloth to catch any drips. Once water starts to come out without air, close the valve. Old-fashioned radiators may require more caution due to their age and the potential for rust or corrosion around the valve.

How Often Should I Bleed My Radiators?

The frequency of bleeding radiators can vary depending on your heating system's performance and signs that air is trapped, such as cold spots on radiators or gurgling noises. As a general guideline, it's a good idea to check and potentially bleed your radiators at the start of the heating season or whenever you notice these issues, which might be once a year or more frequently if necessary. Regular maintenance checks can help ensure your heating system operates efficiently and effectively.

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Article by
Stephen Day | Co-founder
Gas Safe registered and FGAS certified engineer with over 20 years experience in the heating and cooling industry.