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When to Bleed a Radiator?

It is quite easy to tell when to bleed a radiator as the top section will remain a lot cooler than the bottom section, or in severe cases the entire radiator will stay cold when the heating system is turned on. This happens because trapped air displaces the hot water that normally heats the radiator.

This air gets released when you bleed your radiator. The hot water will be able to flow freely when the air is released.

Sounding a bit too technical already? Boiler Guide has a network of Gas Safe engineers all over the country who can help. Complete our form and get quotes from engineers in your area who can test your heating system and bleed your radiators.

Where Does the Air Come From?

Air can be introduced into a central heating system in several ways.  This could happen when new water enters the system from the expansion tank or as routine maintenance is carried out.  It could also be ‘created’ by the movement of the pump as it turns.

Where to Start When Bleeding a Radiator

If your home has 2 floors you should begin bleeding the downstairs radiators first.  It’s also advisable to start with the radiator which is furthest away from the boiler.  Once you’ve bled all the downstairs radiators you move on to the upstairs, again beginning with the radiator which is furthest from the boiler.

Don’t forget to make sure you have the central heating system switched off before you start the process of bleeding a radiator. This is very important because some water pumps  – depending on where in the system they are fitted – will actually suck more air into the radiator and consequently the heating system if they are turned on while you open the bleed valve.

How to Bleed a Radiator



    1. You will need a radiator key, dry cloth or towel and a container to catch any water that comes out of the radiator. If you don’t have a radiator key, you can find them in any DIY shop and tend to cost between 50p and £3. Pliers may also work but there’s a chance you could damage the valve so a proper key is highly recommended.


    1. Before you begin, make sure that your central heating is turned off. Having the central heating on while bleeding radiators is very dangerous; the last thing you want is boiling water bursting out of the pipes.


    1. You should see a square ‘bleed screw’ at the top of the radiator. This is the part you’ll need to turn in order to release the air and water from the radiator. You’ll want to put your container on the floor beneath this area to catch any water.


    1. Use the key to turn the bleed screw anti-clockwise, the cloth will help with gripping. You should hear a hissing sound as the air escapes, use the cloth to catch any water.


    1. When the hissing air stops and there’s a steady trickle of water, the radiator is fully bled. Use the key to tighten the bleed screw but don’t do this too tightly as you could damage the valve.


    1. Wipe down any water on the radiator to avoid rusting then move on to the next radiator, repeating the previous steps.


  1. Once all of the radiators have been bled, you can turn the heating back on. It’s a good idea to check the pressure gauge on the boiler to make sure it’s at the optimum level (around 1.5), that the radiators are heating evenly and there’s no sign of leaking.

It may be necessary to bleed some radiators more than once. If this still doesn’t fix the problem you may need a professional engineer to inspect the system.

For further information on our plumbing and heating services go to Optical Facilities or Optical Heating and Plumbing.