Bleed the radiators: how to bleed a radiator in 5 easy steps

Before turning your boiler back on for the colder months, you’ll probably need to bleed the radiators. You’ll definitely want to do this if you have a radiator which makes funny noises, takes too long to heat up, or doesn’t get hot in certain places (usually the top).

Why do we need to bleed the radiators?

When your central heating is off, bubbles of air and other gases form inside the system. Eventually they form pockets of space inside the radiator, taking up areas which should be occupied by hot water. As a result, the radiator becomes less efficient. It takes more energy and more time to heat the room. A radiator which needs to be bled may also make strange noises. You may hear the sound of dripping water coming from inside, or more physical sounds, such as banging or clunking noises.

In an open vented central heating system – a system with a feed and expansion tank in the loft – unwanted gases which form inside the system usually exit via the expansion pipe. However these may not always be the case. Since air and other gases are less dense than water, they will rise as high as they can – and usually that means getting trapped at the top of the radiator.

What do I need?

Most importantly, you’ll need a radiator bleed key, which you can easily get from your local DIY store. Be sure to purchase one which has a ‘butterfly’ shape handle, and not two little metal notches, as these can be tricky to turn if the valve is stiff.

You can also pick them up dirt cheap off Amazon. We recommend the Bulk Hardware pack of 2. They’re really cheap and the larger grip makes it easier to open bleed valves which have gone stiff. Bulk Hardware also make an Assortment of Gas/Electricity meter and radiator bleed keys, with all four on a handy little tool, as well as two extra spare radiator keys.

If you have a radiator bleed valve which can only be accessed from the rear, Primaflow do a Rear Access Radiator Bleed Key which is cheap and has loads of extremely positive reviews.

You will also need one or two old towels to stop any water which comes out of the bleed valve from going on your carpets and walls. Hold one towel in your other hand, below the valve. Put another towel on the floor under the valve to help prevent central heating water from getting on your carpet.

Do I start upstairs or downstairs?

Assuming your boiler is downstairs, you should start downstairs with the radiator furthest from the boiler. After you’ve done the downstairs radiators, you can then repeat the process for the ones upstairs.

How to bleed the radiators

  1. Turn your central heating on high.
  2. Check which radiators need to be bleed. While being careful not to burn yourself, feel each radiator to see if there are any cold spots. Radiators which don’t get hot at the top are prime candidates.
  3. Turn off your central heating.
  4. Open the bleed valve. With the old towel or rag held underneath the valve, use the bleed key to open the valve by inserting it over the screw and turning it anti-clockwise. You should be able to hear the hissing sound of air escaping.
  5. When the hissing sound stops and/or water starts to come out, close the valve. Turn the key clockwise to close it, and use the old towel to mop up any drips of water.

Well done! Now simply repeat the process for the other radiators which need to be bled.

I’ve bled the radiators but now the central heating won’t come on

That’s most likely because so much air has been let out of the system, the boiler pressure is too low. Standard combi boilers require a pressure of 1 – 1.5 bar when cold. Anything lower, and they won’t work. You can easily solve this problem by topping the system up via the filling loop. Click here to read how to do this.

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