How to bleed a radiator
24 October 2025 | OVO Energy
It’s important to bleed your radiators at the start of every winter. When radiators need bleeding, it’s because there’s trapped air inside, stopping warm water circulating around. This means your boiler and central heating system aren’t working as well as normal, so it can take longer to warm up your home. All of this means your home will be using more energy.
If you switch your radiators on after a long time being idle and you notice clanging, banging, or gurgling noises in the pipe, it’s a sign they need bleeding.
Never bled a radiator before? Our step-by-step guide is here to help.
We’ll help you find out if trapped air is the problem, and if so how to fix it. And we’ll give you ideas about what else might be making your radiator cold. You can also find out how to bleed your radiators and make them more efficient in this video guide from OVO engineer Rich Laniyan.
How to bleed a radiator in 7 easy steps
Radiator not hot enough, or making strange noises? Follow OVO engineer Rich Laniyan's step-by-step guide to bleeding your radiator. You’ll need:
A radiator key – you can get one from most DIY stores
A cloth or rag to catch any drips
A towel to protect your carpet from dirty water
How to check if your radiator needs bleeding
There are a couple of different reasons why your radiator might need bleeding. Once your heating is turned on, run your hands along the top of the radiator. If it’s cold at the top and hot at the bottom, there’s likely to be a build-up of air in the system.
If your radiator is hot at the top but cold at the bottom (or if it has cold patches), you could have a build-up of sludge. In that case, bleeding it might not fix the problem. There’s more information on this below.
Either way, it’s a good idea to bleed your radiators once a year – even if they seem to be working properly. The ideal time is when you turn the central heating back on after the summer. On top of that, it’s a good idea to check them now and again to make sure they’re staying hot.
If you think your old boiler is coming to the end of its life, you might want to take a look at getting a new one. A new, energy-efficient boiler could save you hundreds on your heating bill every year.
What you need when bleeding a radiator
Before you get started, you’ll need to gather the following:
A radiator key – you can get one from most DIY stores
A cloth or towel to catch any drips and protect your carpet from dirty water
While a dedicated radiator key is the standard tool, many radiators feature a slotted valve that allows you to use a flat-head screwdriver instead; however, you still need a tool unless your system has automatic air vents.
Now take a look at the radiator’s bleed valve, which has a square screw or bolt. You’ll find it at the top of the radiator, either on the side or at the back. If there’s a small slit in the middle, you can use a flathead screwdriver to open it.
Only use a flathead, though, not a crosshead screwdriver – as this can damage the valve. And don’t use pliers, for the same reason.
Which radiator should you bleed first?
To bleed radiators effectively, you should generally start with the downstairs radiator furthest from the boiler, then work your way around the ground floor before moving to the upper floors and repeating the process.
8 easy steps to bleed a radiator
Turn off the heating and wait for the radiators to cool down. This will stop you burning yourself if water escapes.
Find the bleed valve.
Put a cloth under the valve to catch any water that escapes.
Fit the radiator bleed key (or a flat head screwdriver, if it fits) in the screw. Slowly turn it anti-clockwise until you hear air hissing out.
Wait until all the air has escaped and water starts seeping out instead. Use the cloth to catch this.
Quickly tighten the bleed valve again. It needs to be tight enough to stop air getting in, but not so tight that you can’t open it the next time.
Turn the heating back on.
Check your boiler pressure using the instructions below and you’re all done.
Still not sure? Remember you can check our our step-by-step video showing you how to bleed a radiator, from our heating engineer Rich Laniyan. And don’t forget, it’s a good idea to bleed all your radiators at the same time.
Top tip: valve fall-out
When you’re bleeding your radiator, don’t turn the screw on the bleed valve more than one-and-a-half turns, or it could come out completely.
If that happens and you can’t get it back in right away, don’t panic. Turn the radiator valves off via the screw or dial valve at the bottom. That should stop more water from getting in while you re-attach the screw.
How to bleed radiators with a combi boiler
For combi boilers, and boiler systems that are pressurised, there’s an extra step. After you’ve released the air, check the pressure gauge on the front of your boiler. Usually these should be between 1 and 1.5 bar. But check your boiler manual to be sure.
With these boilers, you can actually let out too much air, making the pressure too low. If that happens, you usually just need to turn a valve to let in some more water – but again, check your boiler manual to find out the process for your model.
If you can’t find the boiler manual, check the boiler itself
Check the make and model on your boiler itself. Then head to the manufacturer’s website where you’ll be able to download a new manual.
If your boiler has stopped working, these guides can help
If the boiler isn’t working and you’re without hot water or heating, have a look at these guides:
How to bleed a radiator FAQs
A flathead screwdriver will work pretty well. If it’s too small, stick some duct tape over the end until it’s a snug fit.
The bleed valve is usually found at the top of the radiator. If you don’t have a radiator key handy, you can use a screwdriver in the slot across the front.
It’s very common to find a leaky bleed valve. If you have one, here’s what to do:
Drain the radiator of water to below where the leaking valve is
Switch off both the radiator’s supply valve (which lets water into the radiator) and the lockshield valve
Place a bowl or bucket under the valve
Undo the union nut, which connects the bleed valve to the radiator
Open the bleed valve, so all the remaining water leaks out
Wrap plumber’s tape around the “male” end of the valve to make a seal
Tighten up the nut again, and open the bleed and lockshield valves
Switch the radiator back on and close the bleed valve once more
Self-bleeding radiator valves automatically vent the air from your radiators, so you don’t have to.
Cold. Always bleed radiators once the heating has been off for a while and cooled down completely. The water can get very hot when the heating is on, and there’s a high chance you could scald yourself.
This is a common problem, as the valve can rust inside, which causes it to get stuck. You could try to very gently open the whole nut to let the air out, using a wrench or adjustable grips. Be sure to have the radiators turned off, because if the valve breaks off, hot water could spray out.
If it’s totally stuck, it’ll have to be drilled out and a new one fitted. It’s best to call a plumber to do this. They might be able to replace just the valve, but it could mean replacing the whole radiator.
At this point, you might have to call in an engineer. For as little as £17 a month, you can get peace of mind with our boiler and central heating cover.1
It’s a fairly quick job, even with big radiators. You should be able to bleed even the largest, most air-locked radiator in around 30 seconds.
Sources and references:
1 No claims can be made within the first 30 days. Terms, conditions, exclusions and cancellation charges apply. For boilers aged 7 years and over up to £2 a month additional premium will be charged. This product has a £60 excess.