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How to bleed a central heating system

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 your radiators and get your central heating running smoothly 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 your radiators and get your central heating running smoothly again. 

Sofia Hutson
Utilities expert
minute read
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Posted 7 JULY 2021

Why do I need to bleed my central heating system?

If yours is one of the 86% of UK homes with gas central heating, you’ll need to bleed your radiators every so often. It’s what keeps your central heating working efficiently and your energy bills down.

  • Radiators are filled with water that, once warmed, delivers heat to our rooms. Over time, air bubbles can get trapped in the radiators, preventing the hot water from circulating properly. If your radiator isn’t performing at its full capacity, your entire central heating system will have to work harder to heat your home.
  • Bleeding a radiator lets the trapped air out of your central heating system so it can work at full efficiency again, keeping your home warm and your energy bills down. 
  • Experts recommend bleeding your radiators once a year, even if you haven’t noticed a problem. So, especially if you find that your radiators aren’t getting hot, it could be time to give them a little TLC. 

How do I know when to bleed my central heating system?

Is your room taking longer to heat than normal? If so, turn on your central heating and then check the following: 

  • Feel your radiators. Are there cold patches? If it’s cold at the top, there’s air in the system. If your radiator’s cold at the bottom, you may have a build-up of sludge. This is slightly more complicated and might involve a plumber or heating engineer call-out. 
  • Listen to your radiators. Is your radiator making gurgling, clucking or clanking noises? These are signs of air trapped in the system, which bleeding should fix. 
  • Look for damp patches, condensation and mould. These can be caused by uneven heating – your radiators may well need bleeding. 

Even if you haven’t noticed anything wrong with your radiators, it’s a good idea to bleed your central heating system as part of its regular maintenance and to stay ahead of any problems. At least once a year is a good rule of thumb – ideally before the arrival of the chilly winter months.  

How do I bleed a radiator?

You don’t have to be a pro to bleed a central heating system – anyone can do it. All you’ll need is a radiator valve key, to open the valves, and an old cloth or towel (one you don’t mind getting mucky) to catch any drips. It could take a while though, depending on how many radiators you have, so make sure you give yourself plenty of time.  
To bleed your heating system, follow these simple steps: 

  • First, turn on your central heating to warm up your radiators. Check your radiators by carefully feeling for cold spots and listen out for any gurgling or other noises. 
  • Next, turn off your central heating and let the radiators cool down sufficiently to avoid getting scalded. This is a crucial step – otherwise you could end up with boiling water bursting out of the pipes!  
  • 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.  
  • Find the bleed valve on the radiator. They’re normally at the top corner of the radiator and look like a circle with a square indentation.  
  • When you release the air from the radiator, some water will come out too, so to protect your floor, place an old tea towel/cloth and bowl under the valve to catch any escaping water. The water will be dirty, so don’t use anything you’d mind getting ruined. 
  • Place the radiator key into the bleed screw valve and turn it anti-clockwise, about a quarter of a turn. You’ll hear a hiss as air starts to escape. 
  • Once the air’s out, it’ll start to drip water. When the water starts running smoothly, you’ll know that all the trapped air’s out and you can tighten the valve again. Don’t over tighten it, as this can damage the valve. 
  • Once the screw valve is replaced, wipe down any water on the radiator to avoid it rusting and then move onto the next radiator, repeating the steps above. 
  • Once you’ve bled all the radiators, turn your central heating back on. 
  • Check your radiators. Are they heating up evenly throughout? Has the gurgling stopped? If so, job’s a good’un, you’ve fixed the problem! 
  • If it didn’t work, try again. Sometimes you need to bleed a radiator more than once. Remember 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 it yourself, it’s a bigger, messier job and may be one for the experts. 
  • Once you’ve finished all your radiators, you’ll need to check your boiler pressure and possibly repressurise the system.  

Repressurise your central heating system 

Some water will come out with the air when you bleed your radiators, and that can cause your boiler pressure to drop. When you’ve finished bleeding, 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 1.5 bar – but double-check your boiler manual to be sure.  
Top your system up with water, if necessary. Pressure is increased by adding water from your cold water main. The boiler will be connected to the main. You have to find what is called the filling loop in the system. It’s usually close to the boiler or may even be part of the boiler. If in doubt, check your boiler instructions or contact your installer. 
Open the valve or valves on the filling loop – they’re usually small handles, but if there isn't a handle you may need to use a screwdriver. Allow water to fill the system, while keeping a close eye on the pressure gauge, and turn off the valves as soon as the required pressure is achieved. Be careful not to over-pressurise the system. 
It’s a good idea to check the pressure regularly and top up when needed. 

Why will bleeding my radiators save me money?

If there’s air trapped inside your radiators, your boiler has to work harder to pump heat throughout your home. That means you’ll end up having the heating on for longer, or at a higher temperature – which will cost you more money. 

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

Frequently asked questions

What if I don’t have a radiator key?

Don’t worry, you can pick one up from any DIY store for a few quid, max. You may be able to use a flat-edged screwdriver for some radiators but be careful not to damage any of the valves, just to avoid a quick trip to the shops.  

Can I bleed my radiator if the central heating is on?

No, because you’ll probably find that it’s too hot to touch. You’ll also be at risk from being showered with very hot, dirty water. And if the risk of burns and ruining your carpets isn’t enough, if the central heating is on and you open the valve, the pump that circulates the water through your radiators will likely suck even more air into the system.  

Where does the air come from?

Air can get into your central heating system in a few different ways. It could be introduced into the system by the movement of the pump as it circulates water to your radiators. Or, if you have an older, ‘regular-type’ boiler, it might come into the system from the expansion tank. Or it might just be from routine maintenance. It’s not anything to worry about, but it does need to be taken care of.

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.  
As well as checking your radiators regularly, it’s a good idea to get regular check-ups for your boiler. And make sure you turn on the heating for a few minutes every now and then over the summer to prevent the mechanics from seizing up.