How to bleed a radiator in 7 easy steps

30 November 2020 | Matt Mostyn

kids playing at home

Have you turned your heating system on yet? When you do, if you can hear pipes banging or radiators gurgling, it could be a sign that it’s time to bleed your radiators. It’s good form to do it annually at the start of the cold season anyway. 

So without further ado, here’s our easy step-by-step guide to bleeding radiators, without causing you to swear!

We’ll show you how to check if trapped air is the problem, and how to take the right steps to fix it. We’ll also give you ideas about what else might be making your radiator cold. And here's a step-by-step video guide, with one of our expert engineers.

So why bleed radiators? 

The problem is caused by trapped air in your radiator, 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 which means higher bills – and that costs both you and the planet.

By the way, if your boiler has actually stopped working altogether, and you’re without hot water or heating, check out our guides to:

How to check if your radiator needs bleeding

There are a couple of different reasons why your radiator might need to be bled. Here’s how to check for the culprit. With the heating turned on, feel along the top of the radiator. If it’s cold at the top and hot at the bottom, it’s likely a build-up of air in the system. 

If, on the other hand, your radiator’s 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. Then, it’s a good idea to check them once in a while to make sure they’re staying hot.

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How often should you bleed radiators? 

You should bleed your radiators at least once a year, as the cold season starts. 

Plus, you should also bleed them any time your radiators feel cold at the top, but warm at the bottom. This is because air has risen to the top, forming a pocket, and is blocking the flow of water around your radiator.

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

What you need to begin

radiator key in hand


Before you begin, 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

Some more modern radiators can be bled without a key. But if yours can’t be, we recommend the key option, or a screwdriver as an alternative. 

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, not a crosshead screwdriver, as this can damage the valve. 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, so you won’t burn yourself if water escapes.
  2. Find the bleed valve. 
  3. 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 at hand!
  5. Wait until all the air has escaped. When that happens, 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.

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. It should usually be between 1 and 1.5 bars – but check your boiler manual to find out for 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.

Top tip: can’t find the boiler manual!

To be honest, who can, right? If this applies to you, check the make and model on your boiler itself, then just hop online to their website 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, and save more money? With a smart thermostat, you can do just that. 

Basically, 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:

To find out even more about smart thermostats, read our comprehensive guide. And remember, all OVO members get £79.992 off a wireless tadoº Smart Thermostat, which can help you reduce your heating bills by up to 31%3.

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. With OVO HomePlan, you get complete peace of mind with boiler and heating cover, for as little as £13 a month1.

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:

1No 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 £99 excess.

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

3An independent study has shown that installing and correctly using a tadoº smart thermostat can reduce your energy use from heating by up to 31%. The study found that the tadoº device can reduce heating energy requirements by 14-26% through controlling the heat source (e.g. boiler or heat pump). You could also save another 7% by turning down the heating based on the weather forecast, depending on the window sizes. If the window surfaces are relatively large, the energy saving will increase.

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