12 September 2023

How to bleed a radiator - a complete guide

Bleeding a radiator involves releasing any trapped air which helps the radiator work more effectively. Knowing how to bleed a radiator of air is a really useful DIY skill.

This is typically done by opening the bleed valve at the top of the radiator with a bleed key or flathead screwdriver and allowing air to escape until water starts to flow, then closing the valve.

You don't need to get a professional to bleed your radiators as it's not dealing directly with a gas appliance, such as a boiler.

Once you learn this simple skill, you'll find that you can very easily make a huge difference to the smooth running and efficiency of your central heating system, without much time or effort.

In this in-depth guide, we'll look at why you might need to bleed your radiators, give you step by step instructions on how to do it and cover a whole range of questions and scenarios that you might encounter during the bleeding process.

If you need a new valve, you can always check our varied selection to find the right one for you.

Table of Contents

What does 'bleeding a radiator' mean?
When should you bleed a radiator?
How does air get trapped in radiators?
How to bleed a radiator step by step
How to bleed a radiator without a key
Which radiator should you bleed first?
How to bleed a heated towel rail
How to bleed a vertical radiator
How to bleed a radiator with a combi boiler
Is your radiator still cold after bleeding?
Heating system maintenance

What does 'bleeding a radiator' mean?

Heating expert, Nicholas Auckland, explains that the bleeding process is actually very simple to understand:

"Despite the rather dramatic wording, bleeding a radiator is a simple process of allowing air that has become trapped in the radiator to escape, usually by using a small brass radiator bleed key to open up the radiator bleed valve."

When air gets trapped in a radiator, it actually stops the radiator from heating up properly. This will cause more energy to be used as your boiler has to work harder to try and heat your home to a comfortable temperature. Releasing, or bleeding, the air out of the radiator will allow hot water to reach all parts of the radiator and heat up your room more efficiently.

When should you bleed a radiator?

The most common symptom of air being trapped in a radiator is if the rad feels hot at the bottom but is cooler the closer you get to the top.

Air is lighter than water so if it gets into your radiator it will rise up to the top of the radiator's chamber and create an air pocket. Because radiators work by filling up with hot water and emitting heat out into the room, an internal air pocket will stop hot water from reaching the top of the radiator. The more air that is trapped in a radiator, the less hot water is in the radiator to heat the room.

If you have a thermostat set to a particular temperature, or thermostatic radiator valves turned to a particular setting, your boiler and pump will be constantly sending hot water into your radiator in order to raise the room temperature up to whatever you've set. A lot of trapped air will severely hinder the efforts of your boiler because the radiator is not operating to its full capacity.

If you need to replace your valve, remember to check our radiator valves section. We keep a wide range of styles and types available.

How does air get trapped in radiators?

Air getting trapped in the top of radiators is definitely detrimental to the efficient running of your heating systems and there's a few reasons why it might happen:

Leaky pipes or valves

Over time, pipes and valves can develop small leaks due to corrosion or weakening of the joints. It doesn't necessarily have to be large leaks that involves water gushing everywhere as even a tiny leak is enough to allow air into the system. If you find that your boiler pressure regularly drops with the system requiring re-pressurising, you may have a leak in your system that requires further investigation.

Hydrogen build-up

If you don't keep your chemical inhibitor levels topped up within your central heating system, the internal components of your system are going to be more prone to corrosion and rust. Over time, tiny rusty parts of metal can break off and settle at the bottom of radiators in the form of a rusty, brown sludge.

This rusty sludge can release a lot of hydrogen, which will rise to the top of radiators and create air pockets. This can have a double-whammy effect of sludge preventing hot water from heating the bottom of the radiator and the trapped hydrogen (air) stopping hot water heating the top of the radiator - both of which are very bad for the efficiency of your heating.

Pump location

The pump is the component of your heating system that helps water to flow around your pipe network between the tank, your boiler and the radiators. If your pump is above your supply pipe, then it's possible for air to enter the system there. This is a common feature of older properties where the water tank may also be in the loft.

How to bleed a radiator step by step

As previously stated, bleeding a radiator is actually a straightforward process that helps remove trapped air from your heating system. When the air has been released, your radiators will heat up evenly, distributing heat efficiently around the room. Here's a step-by-step guide on how to bleed a radiator:

Tools you'll need

A brass radiator key
An old towel to protect your floor
A container or recepticle to catch any water
A cloth to clean up afterwards
A kneeling pad (optional)

Step 1. Turn off your central heating system

Before you get started, make sure your central heating system is turned off. This is crucial to avoid hot water escaping while you bleed the radiator. It also means you can lean on the radiator without risk of being scalded. If your heating has been on, switch it off and wait 30 minutes before starting.

Step 2. Decide which radiators need bleeding

Walk around your house and check each radiator to see if they need bleeding. The most common sign of trapped air is when the top of the radiator is cooler than the bottom. If you find any radiators with this issue, they're the ones that likely need bleeding. There's generally no need to bleed radiators that are heating up evenly, so save yourself time by sticking to the obvious candidates.

Step 3. Locate the radiator bleed valve

At the top of each radiator, you'll find a small valve known as the "bleed valve." It's usually located on the side or the top of the radiator and looks like a small square or hexagonal nut. Your bleed key should slot onto the nut. Some bleed valves allow you to use a flathead screwdriver instead, though as a rule you will always be able to use a radiator bleed key, whatever the type of valve.

Step 4. Open the bleed valve to release air

Turn the radiator bleed key anticlockwise (left) by about a quarter of a turn. You should hear a hissing sound as the trapped air begins to escape. Be prepared for water to come out as well, so keep your container or old towel close to hand. Slowly keep turning the valve anticlockwise until the hissing stops and water flows steadily. It could take a few seconds for the hissing to stop, or up to a minute. It depends on how much air is trapped in the radiator.

Step 5. Close the bleed valve to stop the water flow

Once water flows without any hissing air, carefully close the bleed valve by turning it clockwise (right) with the radiator key. Do it up tight enough to stop the flow of water but nt so tight that you'll struggle to open it again if you need to repeat this task in the future.

Step 6. Check the radiator temperature

After closing the valve, switch on your central heating and allow the radiator time to warm up. Check if the top of the radiator now feels as warm as the bottom. If it does, you have successfully removed the trapped air.

Step 7. Repeat if necessary

If you have other radiators around your home that feel warm at the bottom but cooler at the top, simply repeat steps 3 - 6 until all radiators are heating up evenly.

How to bleed a radiator without a key

A white panel radiator being bleed of air using a flathead screwdriver

If you don't have a radiator key to hand then all is not lost as you might have something else in your toolbox that can help with bleeding your rad. This is important to know as being able to bleed your radiators can help to instantly solve the problems of a cold radiator.

The most popular alternative to a radiator key is by using a flat head screwdriver. Most modern radiators manufactured over the last 5 - 10 years will have a small indent allowing for a flat head screwdriver to be inserted into the bleed plug.

Grabbing a receptacle or a cloth to catch any drips, slot in the end of the screwdriver gently turn anti-clockwise. If you have air trapped in your radiators, you'll hear the hiss of the air escaping. Once the hissing stops and water starts coming out, you can quickly tighten up the bleed plug by turning your screwdriver clockwise.

What about old radiators?

Not all radiators will have the option for using a flathead screwdriver, particularly panel radiators that are quite old. In this situation, it is more difficult to bleed the radiator but not impossible - you can try some other tools such as an allen key, a small spanner or a pair of pliers.

However, do be very careful before using any of these implements as you may cause irreversible damage to the bleed plug. Even if you manage to open the bleed valve and allow the air to escape, you may struggle to close it again.

If you don't have a radiator bleed key and your radiator doesn't give you the option of using a flathead screwdriver, consider whether this issue can wait until your can get hold of a bleed key.

While frustrating not to be able to solve the issue immediately, a bleed key is designed specifically for this job and will help you clear the issue without causing damage to your radiator. It could really be worth the slightly longer wait rather than opting for a short-term, heavy handed solution.

Are radiator bleed keys universal?

Yes, 99% of the time. Radiators installed over the last 30 - 35 years will generally be of a standardised, metric size and you can buy a radiator key that will fit all radiators in your home. They usually have one end that is flat for you to hold between your thumb and forefinger and the other end will be square for you to slot over the bleed valve.

Heating expert, Nic Auckland, has this to say about the other 1% of the time:

"On rare occasions, some houses with very old central heating systems will have radiators that have imperial radiator bleed plugs that modern radiator keys won't fit. These older radiators can still be bled by using imperial clock keys (keys traditionally associated with winding up clocks). A clock key size 12 (4.8mm) will likely be the right size for a very old radiator, though it's worth getting keys in sizes 8 - 12 just to make sure you've got yourself covered."

Whichever type of radiator you've got, it is worth investing in one or two radiator bleed keys. They're not expensive but can be an absolutely vital tool in getting your radiators working properly and heating up your room satisfactorily again.

Which radiator should you bleed first?

It's generally best to start bleeding the radiators that are furthest from your gas boiler. You should then work your way through back until you get to the closest radiator to the boiler.
The radiators that are furthest away are often upstairs (in a standard two-storey dwelling). These radiators are more likely to have trapped air since air tends to rise and collect in the upper portions of the system.
The radiators closest to the boiler are typically the last in the sequence since they are closer to the boiler and often have less trapped air insdie them.
By bleeding the radiators in this order, you ensure that any trapped air in the system is pushed towards the radiators that are closest to the boiler. This allows for more efficient removal of air from the entire heating system.

How to bleed a heated towel rail

Bleeding a heated towel rail is a very similar process to bleeding a standard radiator. Heated towel rails, also known as towel radiators, provide both warmth and a convenient place to hang towels and have become a common feature in homes across the UK.

In many cases (though certainly not all) a towel rail may be located in an upstairs bathroom, with the boiler being a long distance away downstairs. This makes the towel rail a prime candidate for getting air trapped in it, so it's worth knowing the simple process of how to bleed a towel radiator of air.

Tools you'll need

Brass radiator key or flathead screwdriver
An old towel to protect your floor or carpet
A container to catch any escaping water
A cloth to clean up afterwards

Step 1. Turn off your heating system

Before you start, ensure your central heating system is turned off. This is essential to avoid hot water spilling out while you bleed the towel rail. It also means it doesn't matter if you accidentally touch the towel rail as it won't be hot.

Step 2. Locate the bleed valve

Find the bleed valve on the heated towel rail. It's usually a small nut located at one end of the rail, most commonly on top of one of the vertical side bars.

Step 3. Open the bleed valve

Turn the radiator key or screwdriver anticlockwise (left) by about a quarter turn. As you do this, you should hear a hissing sound as the trapped air begins to escape. Be prepared for water to come out as well, so have the container at the ready. Keep turning the valve slowly until water starts to flow steadily, and no more air is coming out. This may take a few seconds to a minute, depending on the amount of trapped air in your towel rail.

Step 4. Close the bleed valve

Once water flows out without the sound of hissing air, carefully close the bleed valve by turning it clockwise (right) with the radiator key or screwdriver.

Step 5. Check the temperature of the towel rail

After bleeding the towel rail and closing the valve, turn your central heating system back on and wait for the rail to warm up. Check if all bars or sections of the towel rail are now heating evenly. If so, you've released the trapped air and sucessfully bled your heated towel radiator.

How to bleed a vertical radiator

Vertical radiators are commonly installed to maximise vertical space, but this can add an extra challenge the bleeding process. Often the bleed valve is located at the top of a tall vertical radiator, which means reaching it safely becomes a priority.

The additional height of vertical radiators often requires the use of a step ladder to reach up to the bleed valve. While a ladder (or stool) can make the valve easier to access, it introduces an element of complexity and potential risk to the task. Safety precautions, such as ensuring the step ladder is stable and positioned on a level surface, are important to prevent accidents. Additionally, caution should be exercised when leaning over the radiator to reach the bleed valve, as any slip or imbalance could lead to injuries or damage.

As long as you are careful when accessing the bleed valve high up on your vertical radiator, the actual process of letting the air out is the same as described earlier in this guide.

How to bleed a radiator with a combi boiler

If you've got a radiator that needs bleeding, but you're concerned because you've got a combi boiler and a pressurised heating system, then read on. Below, we will explain how a combi boiler works and explain what a pressurised system is. We'll also take you through the process of bleeding your radiator so that you don't need to call in a professional to do it for you.

Tools you'll need

A radiator key or flathead screwdriver
An old towel to protect your floor or carpet
A container to catch any escaping water
A pair of grips and/or spanner for the radiator valves
A cloth to clean up afterwards

Step 1. Find the bleed valve

At the top of each of your radiators will be a bleed valve. Depending on the type of bleed valve, you may need a radiator bleed key or a flat head screwdriver to undo it.

Step 2. Turn off your heating

The next thing to do is switch your heating system off completely so that no problems are created by bleeding the air out while it's still running.

Step 3. Open the radiator valves

Use a pair of grips to ensure that your valves at the bottom of your radiator are open so that water is able to flow into your radiator.

Step 4. Loosen the bleed valve

You can then slacken off your radiator's bleed valve with your radiator key or flathead screwdriver. If there is air in your radiator, as you slacken off the bleed valve, you will hear a hissing sound as the air escapes. Have a cloth ready to catch any drips that come out after the air escapes.

Step 5. Tighten the bleed valve

Once water starts coming out, you know that all of the air has been released from that radiator. Then tighten the bleed valve back up.

Step 6. Repeat the process

Go round each of your radiators and repeat the process as necessary and

Step 7. Check your boiler pressure

Switch your heating back on and check your radiators are getting warm all over. Also also check the pressure gauge on your combi boiler to see if it has returned to around 1 bar.

Is your radiator still cold after bleeding?

While bleeding your radiator will solve a lot of heating problems, it won't always solve them all. If you've bled your radiator of air but it still won't get hot then read on for some additional solutions.

Check your thermostatic radiator valve

Once you start digging deeper into the issue, you might find that all you need to do is repair your thermostatic radiator valve (TRV). A common issue, particularly after summer, is the pin inside the valve getting stuck. A stuck pin can prevent the flow of hot water into the radiator. Fortunately, it's an easy fix that won't take long to sort out. For guidance on this please read our following post, how to repair a thermostatic radiator valve (TRV)

If the pin is not the problem then it might be time to replace the valve. This is also a straightforward process that doesn't require you to have a great deal of in-depth DIY knowledge to complete. You can read our guide on how to replace a thermostatic radiator valve here.

Flush out your radiator in case of an air lock

If you've established that your TRV is not the cause of the radiator staying cold, then there could be an airlock somewhere in your heating system that is preventing the water from flowing into the radiator. Flushing your radiator through with water is a really effective way of getting rid of any airlocks.

Although challenging, you'll be able to do this yourself by reading our guide on how to remove an airlock in your heating system.

Balance your heating system

An unbalanced heating system means that water will not flow into all of your radiators evenly. Your radiator might not be warming up because hot water from your boiler is simply not reaching it. By making some tweaks to the valves on each radiator, you can easily get the water flowing evenly again.

You can find out how to do this by following our guide on how to balance a radiator.

Heating system maintenance

If all these suggestions still haven't solved the issue of your cold radiator, then it's time to call in a Gas Safe Registered plumber to check over your system and see if there is a more serious underlying problem.

Central heating specialist, Julie Boyle, highlights the importance of regular heating system maintenance:

"Getting in a professional once a year to inspect your boiler, pump and other parts of your system is a good habit to get into as it can save you from having a costly breakdown when you most need it. A lot of boiler warranties and insurance terms and conditions actually insist on customers' having maintenance carried out on their systems, so it's worth doing so that you don't invalidate your warranty or fail to get an insurance payout should the worst happen."

Chemical inhibitor

To stop rusty flakes of metal from forming a horrible brown sludge in the bottom of your radiators, you need to make sure that you add a chemical inhibitor into your heating system. Radiator sludge can stop your rads getting hot at the bottom, so adding a corrosion-preventing solution should definitely not be overlooked.

You can find out how to stop internal rust building up by reading our quick guide to central heating inhibitor.

System pressure

The pressure of the water that flows from your boiler around your system must be within the recommended range. You can usually find the correct pressure range in your boiler's instruction manual. Hot water in a home often needs to flow upwards, against the force of gravity. If the boiler pressure is too low, radiators upstairs may struggle to get hot. If the pressure is too high, your boiler is likely to shutdown for safety reasons, resulting in no heating or hot water at all.

To get your system pressure right, consult our helpful guide on how to repressurise your boiler.

Check all radiators

If you've noticed a problem with one of your radiators, it's best to check if it's an isolated issue or affecting multiple radiators. Narrowing down the problem to one radiator can help you decide if it's something you can tackle yourself. If the fault appears to be more widespread, unless you're particularly experienced, it's advisable to get in a heating engineer to resolve the problem.

For guidance on employing a professional, take a look at our guide on who you should call to fix a radiator.

Inspect for leaks

Heating can be negatively affected by water loss caused by a leak. Water leaks almost always start small and get steadily bigger, so it's vital to deal with them quickly before larger, more expensive issues appear. Carefully feel around the valves and pipework around your radiators to see if you can feel any moisture. Leaks never go away on their own, so if you can't fix the problem yourself, call in professional help immediately.

For further assistance, consult our article on how to fix a pinhole leak in a radiator.

A complete guide to radiator leaks

If you've got a radiator leak, this guide explains the most common causes, how to quickly identify the source, and which repairs you can safely tackle yourself. From valve issues to pinhole leaks, we cover everything, plus advice on preventing future problems and replacing old, corroded radiators.

Why do I have hot water but no heating?

Hot water but no heating? This article explores causes like radiator or boiler issues, and whether it affects specific types or radiators. Learn signs to watch for and easy workarounds to try before calling in a pro.

VIDEO - Why won't my radiator get hot?

Do you have a radiator that won't get hot? There are several things that could be causing the issue. James the plumber shows you some things to look for and how to solve it!

How to fix a leaking towel radiator or rail

Heated towel rails look great and are very practical, but sometimes things go worng. In this article, we'll look at how to fix a leaky towel rail, which is an important job to be dealt with as soon as possible.

How to remove an airlock in your central heating system

Radiator still cold after belleding? Read how to clear an airlock in your system and what other problems might be causing your radiator to stay cold.

Are your radiators too hot?

How to fix a pinhole leak in your radiator and copper pipes

Pinholes or micro holes are tiny holes that can form in radiators or pipes and allow water to leak out of them. They're often formed by internal corrsoion in your heating system, effectively rusting from the inside out. In this article we'll look at how you can locate a pinhole leak the the steps you can take to fix the problem.

Can radiators leak gas?

Worried about gas leaks from your radiators? Rest assured, radiators can’t leak gas, including the dangerous carbon monoxide associated with faulty heating systems. This article explains why radiators only leak water, how to ensure your boiler is safe, and the importance of regular boiler maintenance to prevent carbon monoxide risks. Keep your home safe and your heating system running smoothly with these essential tips.

3 December 2025

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What does 'bleeding a radiator' mean?

When should you bleed a radiator?

How does air get trapped in radiators?

Leaky pipes or valves

Hydrogen build-up

Pump location

How to bleed a radiator step by step

Tools you'll need

Step 1. Turn off your central heating system

Step 2. Decide which radiators need bleeding

Step 3. Locate the radiator bleed valve

Step 4. Open the bleed valve to release air

Step 5. Close the bleed valve to stop the water flow

Step 6. Check the radiator temperature

Step 7. Repeat if necessary

How to bleed a radiator without a key

What about old radiators?

Are radiator bleed keys universal?

Which radiator should you bleed first?

How to bleed a heated towel rail

Tools you'll need

Step 1. Turn off your heating system

Step 2. Locate the bleed valve

Step 3. Open the bleed valve

Step 4. Close the bleed valve

Step 5. Check the temperature of the towel rail

How to bleed a vertical radiator

How to bleed a radiator with a combi boiler

Tools you'll need

Step 1. Find the bleed valve

Step 2. Turn off your heating

Step 3. Open the radiator valves

Step 4. Loosen the bleed valve

Step 5. Tighten the bleed valve

Step 6. Repeat the process

Step 7. Check your boiler pressure

Is your radiator still cold after bleeding?

Check your thermostatic radiator valve

Flush out your radiator in case of an air lock

Balance your heating system

Heating system maintenance

Chemical inhibitor

System pressure

Check all radiators

Inspect for leaks

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