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How to bleed a radiator

Updated: Feb 24

Bleeding your radiators may sound like a big job, but it’s one of those home maintenance tasks which really pays off. Replacing a radiator can be expensive, but you’ll often find bleeding it is a quick fix you can try before deciding to fork out for a new one. We've prepared a step-by-step guide on how and when to bleed your radiators, to help keep your home warm and your heating system working properly all year round.

Decorative Radiator installation
Decorative Radiator

Do you need to bleed your radiator?

How do you know if your radiators need bleeding? If they’re cold at the top but still hot at the bottom – or maybe cold all the way through – air could be trapped inside. Or you might hear gurgling and clanking noises in the pipes when you turn on your heating at the start of the winter, again because of trapped air. Bleeding the radiators lets out this air, meaning that water can flow freely through them again. The upshot? Your radiators will work more efficiently and give out more heat.

How to bleed a radiator

We've split the process into 8 short steps to make sure it's as straightforward as possible for you. Before you start, you'll need to get a few things together:

  • Radiator bleed key – if you don't have one of these, you can find them in most DIY shops.

  • A dry cloth or towel to use as a grip when using your radiator key or to absorb any escaping water.

  1. The first thing to do is turn the central heating system on. This will make it easier to find the problem areas.

  2. Then work out which radiators need bleeding. The usual signs are cold patches, unusual gurgling noises or radiators that are taking a long time to heat up. These all mean that there’s air trapped inside the radiator. Safety tip: Remember the radiators will be hot, so be careful when checking for cold spots. Try not to touch the radiator; you’ll still be able to feel whether it’s releasing heat from a short distance away. Perhaps wear a pair of thin gloves to avoid burning your hands.

  3. Now turn off the central heating entirely and wait a while for all the radiators to cool down. This is to avoid the risk of burning yourself.

  4. The fourth step is finding the bleed valve. The bleed valve or bleed tap is often found at the top of the radiator, on the side. It looks like a small metal screw or square. This is where you’ll release any trapped air or water.

  5. Prepare the area by placing down a towel beneath the bleed valve in case any water leaks. The water might even be discoloured when you’re bleeding an older radiator, so this will save you having to clean up later. Tip: We suggest starting with the ground floor/downstairs radiators first and working your way up. As air’s lighter than water, the best place to start is at the bottom due to gravity.

  6. Take the radiator bleed key and insert it into the valve. Once you insert the radiator key, you’ll feel them lock together. Carefully turn the valve anti-clockwise. A quarter to a half turn will be enough (never open the valve fully). A hissing sound is a good sign that the air’s escaping from the radiator .Be careful – the air might be hot – and use your towel to keep a firm grip.

  7. Keep doing this until the hissing sound stops, and there’s some water dripping at a steady pace. When there’s only water left dripping from your radiator, it means it’s fully bled. Now you can tighten the bleed screw by turning it clockwise. Be careful not to tighten it too much – this could damage the radiator bleed valve.

  8. Once you've finished bleeding your radiators, you might need to repressurise your system. This is because the pressure in your system might have dropped when the water and air was released.

Repressurising your boiler means letting enough water enter the system from the water mains supply. Check the pressure gauge in your boiler to make sure it’s at the right level (around 1.5 bar) – if it’s too low, you can follow the instructions below.

How to top up your boiler pressure

Boiler systems vary, so make sure you check your user manual before attempting to repressurise it yourself. if you have a small feed & expansion tank (usually in the loft) rather than a filling loop the water will top up automatically. Check out our "how to to up your boiler pressure" blog with links to videos that should help. As always, contact a professional if you’re not sure about doing this yourself.

  • Switch off your boiler and let it cool

  • You’ll need a filling loop. Attach this and check both ends are secure.

  • Open the valves on your system (you’ll often find these just underneath your boiler) to allow cold mains water into the system

  • Wait for the gauge to read 1.5 bar

  • Close the valves, one at a time

  • Switch the boiler back on and reset it if you need to

  • Undo both ends of the filling loop and remove it

When’s the best time to bleed a radiator?

It’s best to bleed your radiators just before you switch your heating on in the autumn, so that they’re ready to heat your home when you really need them.

What to do if your heating’s still not working

You might need to bleed your radiators more than once – this often happens if it's never been done before. If you still have no luck after two attempts, contact a Gas Safe engineer to identify the root cause of the problem.

How to efficiently heat your home

Regularly bleeding your radiators will prevent any trapped air or sludge from building up. When there’s a build-up, your boiler has to work extra hard to produce enough heat, and your energy bills could be more expensive. There are other ways of making sure your boiler and central heating system keep running smoothly:

  1. Have your heating system serviced by a professional engineer at least once a year. An annual service will help prevent those minor problems from developing into anything more serious. All our plans include an annual boiler service so you have peace of mind.

  2. Don't turn your thermostat up too high when you don't need to; this can save you money on your bills.

  3. Test your system before winter to spot any issues before you really need the heat. Simply switch on your heating for a short amount of time – even if it's just for a few hours. Spotting any issues early on will help you fix them so your system isn’t using more energy than it needs to.

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