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How to bleed a radiator

Are your radiators not working as well as they should? If your central heating’s on but your radiators aren’t warming up properly, they may have air trapped inside them.

Here’s how to bleed a radiator and get your central heating running efficiently again.

Are your radiators not working as well as they should? If your central heating’s on but your radiators aren’t warming up properly, they may have air trapped inside them.

Here’s how to bleed a radiator and get your central heating running efficiently again.

Written by
Dan Tremain
Energy and business energy expert
Last Updated
7 JULY 2023
6 min read
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How do I bleed a radiator?

You don’t have to be a professional plumber to bleed a central heating system – anyone can do it. Just follow these simple steps:

  1. Turn off your central heating and let the radiators cool down sufficiently to avoid scalding yourself.
  2. Start with the radiators on the lowest floor first and work towards the highest. On each floor, start with the radiator furthest away from the boiler and move towards it. 
  3. Find the bleed valve on the radiator. It’s normally at the top corner of the radiator and looks like a circle with a square indentation.
  4. When you release air from the radiator, some dirty water will flow out too. Put an old cloth and bowl under the valve to catch any escaping water.
  5. Put the radiator key into the bleed valve and turn it anti-clockwise, about a quarter of a turn. You’ll hear a hiss as air starts to escape.
  6. Once the air’s out, water will start to drip. When the water starts running smoothly, all the trapped air is out and you can tighten the valve again. Don’t over tighten it as this can damage the valve.
  7. Once the screw valve is replaced, wipe down any water on the radiator to avoid it rusting. Move to the next radiator, repeating the steps above.
  8. Once you’ve bled all the radiators, turn your central heating back on.
  9. Check your radiators. Are they heating up evenly with no cold patches? Has any gurgling stopped? If so, you’ve fixed the problem.
  10. If your radiator isn’t working even after bleeding, try again. Sometimes you need to bleed a radiator more than once.

    Note that if the radiator is cold at the bottom rather than the top, it’s probably a build-up of sludge or limescale inside, not air. Although it’s possible to remove this yourself, it’s a bigger, messier job and might be one for the experts.
  11. Once you’ve finished all your radiators, you’ll need to check your boiler pressure and possibly repressurise the system.

What tools do I need to bleed a radiator?

To bleed a radiator, you need:

  • A radiator valve key, to open the valves
  • An old cloth or towel to catch any drips.

How to bleed a radiator without a key

If you’ve misplaced your radiator key, you may be able to use a flathead screwdriver instead – so long as the bleed valve has a similar shaped slot in the middle. If the slot is hexagonal-shaped, you might be able to use an allen key.

Don’t use pliers or a cross-head screwdriver as they can scratch or damage the valve, making it harder to open again in the future.

If you want to replace a missing radiator key, you can usually get a new one from your local DIY store. They typically cost just a couple of pounds.

How to repressurise your central heating system

When you’ve finished bleeding your radiators, you’ll need to check your boiler pressure to see if your system needs to be topped up. It should typically read between 1 and 2 bar – but check your boiler manual to be sure.

If your boiler pressure is too low, you have may lost water from the system while bleeding your radiators. Pressure is increased by adding water from your cold water main into your boiler.

Always make sure your boiler is off and cool before attempting to repressurise your central heating system.

If you have a filling loop on your boiler, that’s what you use to top up your system with water (but check your boiler manual before you start). Here’s how:

  1. Check that both ends of the filling loop are securely attached.
  2. Open the valve or valves on the filling loop (some only have one). They’re usually small handles, but if there isn't a handle you may need to use a screwdriver.
  3. Allow water to fill the system, while keeping a close eye on the pressure gauge.
  4. Turn off the valves as soon as the required pressure is achieved. Be careful not to over-pressurise the system.

Some boilers don’t have filling loops. Look for a pressure valve in the pipework close to the tank. Open this valve to get your central heating system back to the right pressure.

It’s a good idea to check your boiler pressure regularly and top up when needed.

What happens if you bleed a radiator with the heating on?

You should never attempt to bleed your radiators while the central heating is on because you could be scalded by the hot water.

And if the central heating is on and you open a bleed valve, the pump that circulates the water through your radiators will likely suck even more air into the system.

How do I know when to bleed radiators?

If you notice any of the following signs, it could be time to bleed your radiators:

  • Your radiators are cold at the top. There’s probably air in the system that needs removing.
  • Your radiators are gurgling or clanking. These are signs of air trapped in the system, which bleeding should fix.
  • Your radiators have damp patches, condensation or mould. These can be caused by uneven heating, so your radiators may need bleeding. If bleeding doesn’t fix the problem, it might need further investigation.
  • Your radiators are cold at the bottom. You might have a build-up of sludge. This could need a plumber or heating engineer call-out.

How often should you bleed radiators?

Even if you haven’t noticed anything wrong with your radiators, it’s a good idea to bleed your central heating system at least once a year – ideally before the arrival of the chilly winter months.

Why do radiators need bleeding?

Bleeding your radiators helps to keep your central heating working efficiently and your energy bills down.

If a radiator isn’t performing at its full capacity, your entire central heating system will have to work harder to pump water through the system and heat your home. That means you’ll end up having the heating on for longer or at a higher temperature – costing you more money.

Check out our energy-saving tips for more ways to cut your energy bills.

Frequently asked questions

How long does it take to bleed radiators?

It depends on how many radiators you need to bleed and if you have to wait for the system to cool. Once you’ve opened the bleed valve, it should only take around 30 seconds for the air to escape from one radiator.

How does air get into radiators?

Air can get into your central heating system in different ways. It could be introduced by the movement of the pump as it circulates water through your radiators.

If you have an older boiler, it might come into the system from the expansion tank. Or it could be down to routine maintenance.

Can I get an engineer to bleed my radiator? Will it be covered under my warranty?

Bleeding your radiators is considered part of your home’s routine maintenance, so it’s your responsibility as a homeowner or tenant to keep on top of it.

If you’ve tried bleeding the radiators but the problem hasn’t been fixed, it’s time to get a professional in to see what’s going on. You'll need to check your warranty to see if this type of investigation is covered.

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