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How to bleed a radiator

You should bleed your radiators every year, or whenever your radiator feels hot at the bottom but cold at the top. We look at how to go about it.

Does your radiator need bleeding?

Which? Trusted trader South London Heating Ltd suggests checking whether or not your radiator needs bleeding by 'going to the radiator and checking if it’s cold at the top. If it is, then this means there’s air in the system and your radiator needs bleeding.'

See our advice on what to do if your boiler breaks down or read on for more about bleeding your radiators.

Bleeding the radiator

Before you bleed the radiator, turn off your heating system and put a rug or old towel underneath the valve to catch any water. The South London Heating Ltd team advise 'locating the bleed key for your radiator as well as the valve, which is usually positioned on the side'.

Use the radiator key to 'turn the valve anti-clockwise for about a quarter of a turn, until the air stops hissing and the water starts running smoothly'. Once you're confident that all the air is out, you should 'turn the valve off (back to its original position)'.

Finishing up

Once you’ve finished, you can turn the heating system back on. Leave it a couple of hours then check the temperature of the radiator again. If the radiator is hot at the top as well as the bottom then, congratulations, you've successfully fixed the problem. 

However...

If it’s still cold at the top

If the radiator still feels cold at the top, then you need to call in a professional to investigate further. We wouldn't recommend undertaking further investigation yourself and remember all plumbers, gas or central heating engineers must be registered with Gas Safe before they are qualified to deal with a gas-fired boiler. 

Which? Trusted Traders has endorsed central-heating engineers throughout the UK that have passed our rigorous assessment process. Use our simple search facility to find a Which? recommended central heating engineer near you.

If it's cold at the bottom

If your radiator is cold at the bottom, this could mean that sludge has been building up in your radiator and heating system. If this is the case, your system may require a full power-flush. Contact your local heating engineer for more information, or find a central-heating engineer with Which? Trusted Traders.

Check your combi-boiler

Once you’ve bled your radiators, if you have a gas combi-boiler you'll need to check the system pressure.  Removing air can reduce the pressure and stop the boiler from running efficiently.

Check the pressure level on the gauge on the dashboard or side of the boiler. The ideal water level reading is normally 1.5 bars.

If the pressure is low – less than 1 bar, then you need to top up the pressure on your boiler.

Many, but not all models, will allow you to do this yourself. If you need specific tools to repressurize the boiler, or you have to remove the cover, or you are in any doubt, always call in a qualified professional. You can find a central-heating engineer you can trust with Which? Trusted Traders.

Check the specific instructions about repressuring your boiler and its system in your user manual - either in print or online. Then follow the instructions for your model. 

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