Category - Boilers
Guide author photo

Will Scholfield, Engineer

18 May : Updated 16 Apr ● 7 min read

How to clear an airlock from your central heating system

Have you turned the hot tap on, only to find that there's no hot water coming out? Is the central heating on, but your property still feels rather chilly? If this sounds familiar, there may be an issue with your central heating system. 

One of the most common issues people find is that their system has an airlock. But what does this mean and how can you get your boiler up and running smoothly again? 

In this guide, we show you how to bleed your radiators, and also how to clear an airlock from the central heating system, without necessarily needing the help from a plumber. 

What is an airlock?

An airlock is a term used to diagnose excess air formation in your system. Your central heating system delivers hot water throughout your property via your boiler - however, there may be some instances where pockets of air either prevent your system working altogether or can affect its efficiency.

A common cause of airlock issues is the formation of trapped bubbles of air, resulting from excess water vapour in your central heating system or system pipework being configured incorrectly. An easy way to understand this is the air acting as a blockage in the system, which results in hot water being unable to pass through pipes as freely as usual.

An airlock within your system is generally nothing to worry about, however, and can be easily remedied. 

How do I know my central heating system has an airlock?

Diagnosing an airlock within your central heating system can be relatively easy and rectified quickly in most instances.

Symptoms of an airlock can be as follows:

  • Little to no hot water leaving your hot water taps, even when the boiler is on.

  • Some or all radiators are cool or cold to the touch at the top when the central heating is running

  • Your boiler is making banging, clunking, and/or tapping sounds when in operation. Take a look at our guide to the reasons why your boiler is making a noise to learn more.

How to remove an airlock from your central heating system

Removing an airlock in your central heating system can be a straightforward process with the assistance of basic tools. In most airlock occurrences, pockets of air become trapped within the system and could put unnecessary strain on your boiler. Bleeding your radiators is a cost-free and quick remedy to this problem which we’d recommend doing twice a year.

This is our quick overview, but for more information, check out our extensive guide on how to bleed a radiator

Tools you’ll need:

  • Some radiators may require a radiator key - these can be purchased from most hardware stores

  • A bucket/jug for collecting water

  • Towels on standby

If your radiators are not heating up or they’re making a gurgling sound, this could indicate that an airlock is present in the system. Bleeding your radiators is a relatively easy way to remedy your issue, and have your property warm and cosy in no time.

Top tip: Start with the radiator furthest away from your boiler, if you are bleeding more than one. If you live in a property with two or more floors, we suggest starting with the downstairs radiators first, before moving upstairs.

Step one:

Determine what type of system you have. If it is a sealed system, or you have a combination boiler, ensure you are capable of re-pressurising it after the bleeding process has been completed. If not, please contact us to arrange an engineer visit as there is a potential that if you bleed this type of system and you can’t, or don't know how to, repressurise it the boiler may be left fully inoperable. 

Step two:

Make sure your central heating system is turned off and the radiators are cold before you begin.

Step three:

Ensure the area underneath and around the radiator is protected against water spillage. Place your jug or bucket underneath the bleed valve. The bleed valve is typically located on the top of your radiator. It notably has a square that matches your radiator bleed key.

Step four:

Insert the radiator bleed key into the bleed valve, and turn anti-clockwise. You will hear a hissing sound from the air escaping the radiator- this is nothing to worry about.

Step five:

When water begins to escape from the bleed valve, simply turn the radiator bleed key clockwise to tighten and close the valve.

Step six:

Continue steps 1 to 4 on all radiators in the property, finishing on the radiator closest to your boiler.

Step seven:

Turn your heating on and check if your radiators are heating up as they should.

Top tip: When bleeding your radiators, if your system is sealed or you have a combination boiler the pressure within your system will drop, so it’s important to remember to check this and repressurise if necessary. If this isn't done, safety features within modern appliances may well stop them from working altogether

For more information check out our Youtube video on how to bleed your radiators with a key.

What if I don’t have a radiator key?

If for whatever reason you don’t have a radiator key, there are alternative methods of bleeding your radiator without one. For radiators with a slotted bleed screw, you may want to use a screwdriver to twist it off. 

Other bleed screws may be more rectangular or have a square deep within making it less simple for a screwdriver to work. In that case, you could use an allen key, a spanner or some pliers. Whichever tool you choose to use, you must be careful and slowly twist the bleed screw anti-clockwise until you hear a hissing noise. This sound will indicate that air is being let out. 

Once the noise stops, turn the screw clockwise and tighten up firmly.

For more information check out our Youtube video on how to bleed your radiators without a key.

What if an airlock is in my hot water system?

If your central heating system is functioning correctly, more so after bleeding your radiators but no water flows from your taps, there is likely an airlock still in your hot water system. In order to fix this problem, you will need to purge your system of water and then refill it.

How to purge your central heating system and refill with water

Step one:

Turning off your main supply is the first and most critical step in the procedure. In most properties, the stopcock - to shut off your supply- is positioned under the sink, and this tap should typically be twisted clockwise to shut off the water supply to your home.

Step two:

Once you have ensured that no water is entering your property via the water mains, it’s time to purge the water from your hot water system.

Ensure you locate and turn on ALL taps in and around your property. Remember to turn on any external taps you may have including those in garages or outhouses. Keep all of these taps ON and ensure that each one runs itself out of water.

Step three:

The next step is to flush all toilets until the tanks are empty. 

Step four:

Check on the taps and toilets upstairs in your property to ensure -at most- a trickle of water is running from each one.

Step five:

Return to the stopcock and reintroduce the supply of water to your property by turning it clockwise. Once water is flowing from all taps, adjust each faucet so that water is pouring out at about 50 percent capacity. After around five minutes, completely open all faucets.

After this time, the water should have pushed out any airlocks within your hot water system, and refilled with water.

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Get your heating system running smoothly with BOXT

At BOXT, we hope to provide you with the best advice for heating your home. If you’re still struggling to fix your radiators and central heating system, don’t be afraid to ask for help. 

An airlock may signal other underlying issues, so it’s important to get things checked with a certified plumber or qualified engineer if the steps we’ve provided have still left you with a problem.

Why not take a look at our boiler repair service with one of our qualified engineers, with prices starting at just £89.