How to bleed a radiator

25 November 2021 | Matt Mostyn

Bleeding your radiators at the start of every winter is one of those rituals that always pays off. Because stuffed up, inefficient radiators use a lot of energy to run. Tell-tale signs that your radiators need bleeding are if you switch them on after a long time being idle and you notice clanging, banging, or gurgling noises in the pipes.

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. But if you’re in a hurry you can find out how to bleed your radiators, and make them more energy-efficient, in this simple video guide. Featuring OVO Senior Engineer, Rich Laniyan.

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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 you’ll be using more energy – and, unfortunately, this comes at a cost to both you and the planet.

If your boiler has stopped working altogether, and you’re without hot water or heating, these guides might help:

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, on the other hand, 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. More on that 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 then 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 a deal for a new one. A new, more energy-efficient boiler could save you hundreds off your heating bill every year. 

How do you bleed a radiator: a step-by-step guide

What you need to begin when bleeding a radiator

radiator key in hand

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

Modern radiators can usually be bled without a key. But if yours can’t be, don’t worry, you can always use a screwdriver as an alternative. 

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 could use a flathead screwdriver to open it. 

Only use a flathead, though, not a crosshead screwdriver – as this can damage the valve. And definitely don’t use pliers, for the same reason.

Which radiator to bleed first?

Generally speaking, you should bleed the radiator that’s furthest away from your boiler first. If you live in a 2-storey (or more) home, then this will probably be downstairs. 

Work your way around your home, until you reach the radiator closest to your boiler. 

7 easy steps to bleed a radiator

  1. Start by turning the heating off, and most importantly, wait for the system to cool down. This will save you from burning yourself if water escapes.
  2. Next, find the bleed valve. 
  3. Then, put your cloth under the valve, ready to catch any water that escapes.
  4. Fit the radiator bleed key (or your flat head screwdriver, if that works) in the screw. Turn it anti-clockwise about a quarter of a turn, or until you hear a hissing noise (that’s the air coming out). You might also get a few drops of water, so have a cloth to hand!
  5. Wait until all the air has escaped. You’ll know when that happens, because water will start seeping out instead.
  6. Next, tighten the 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.
  7. Turn the heating back on and your radiator should soon be working fine.
  8. Just give your boiler pressure a quick check too, and you’re good to go! 

Still not sure? Remember you can check our our step-by-step video showing you how to bleed a radiator, courtesy of one of our heating engineers, 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 one 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 bars. 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, it’s usually just a question of turning a valve to let in some more water – but again, check your boiler manual to find out the process for your model.

Can’t find the boiler manual?

To be honest, who can? If this applies to you, check the make and model on your boiler itself. Once you have this info, you can head online to the manufacturer’s website where you’ll be able to download a new manual.

Take control of your heating with a smart thermostat

Now your radiators are all performing as they should, wouldn’t it be great to make your home more energy-efficient? With a smart thermostat, you can do just that. 

Smart thermostats help you control your heating from wherever you are. You can change the temperature, or even turn your heating on or off – all from your phone, tablet, or other device. 

Here’s a quick summary of all the main benefits of having a smart thermostat:

You can even add value to your home. To find out even more about smart thermostats, our comprehensive guide is here to help. It’s also worth knowing that all OVO members can get £79.992 off a wireless tadoº Smart Thermostat, which can help you reduce your energy use.

How to bleed a radiator FAQs

If you’ve mislaid your key, 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 the level of where the leaking valve is located
  • Switch off both the radiator’s supply valve (which allows 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

In an ideal world, we would all have self-bleeding radiator valves! As the name suggests, self-bleeding radiator valves automatically vent the air from your radiators, so you don’t have to.

Always bleed radiators once the heating has been off for a while and cooled down completely. The water can get extremely hot when the heating is on, and there’s a high chance you could scald yourself.

This is another 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, or it might have to be the whole radiator.

At this point, it looks like you may have to call in an engineer. For as little as £15 a month, you can get peace of mind with our boiler and central heating cover1.

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.

2 Saving based on manufacturer's recommended retail price of £199.99.