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How to remove an airlock from a radiator

It may be necessary to bleed a radiator if you find that a portion of it is cold. It generally means that air is trapped inside.

If you need help from a professional, try our radiator installation service here in London.


Disclaimer: The information contained in this page is for information purposes only. We cannot be held responsible for any problems that arise as a result of following the guidance contained on this page. We strongly recommend that plumbing work of this nature only be attempted by a qualified expert plumber. Performing this type of work as DIY can lead to other plumbing issues such as leaks and central heating failure.


  • Adjustable grips
  • Adjustable spanner or wrench
  • Radiator key
  • Bucket
  • Cloth

Shut the radiator valves off

First thing to do whether its a pressurised system or a tank fed from the loft is to turn off the valve at each end of the radiator.

Bleed the radiator

Then get your a radiator key and dissipate the pressure from the radiator. If there is any air in the radiator you should be able to hear the air coming out. This is usually a hissing or spurting sound. Eventually after the air has been drained the water should follow. If water comes out straight away there may be another reason that your radiator is cold.

Basically you want to allow the water come through (drain) cleanly without spurting so that there is a constant flow. Once this is the case do the radiator key up again.

On all radiators you should find that you have a bleed nipple on the side and there will be a little tiny water outlet at the bottom of it like in the diagram below. Many radiators have a slit in the nipple so you can use a screwdriver or a radiator key.

Radiator Bleed Nipple

You’ll need a bucket to collect the water once the air comes out and also a cloth to clean up any water. Firstly hold the bucket in place up to the bleed nipple and undo the nipple, while holding the bucket underneath.

If you prefer to drain the radiator at speed you can use a normal 15mm hose pipe with 1/2” male thread on the end. Some radiators have built in, and welded in, thread just for the bleed nipple. If thats the case you won’t be able to perform this next activity and you’ll have to endure the wait while the water drains via the tiny hole in the bleed nipple.

Remove the bleed nipple and screw in the hose. It’s best to have enough length on the hose so you can put the other end of hose outside. If it’s shorter just get a big bucket and pop the end of the hose into that.

Get a wrench and unscrew the bleed nipple. Put a towel on the floor to catch any small amount of water. As soon as the bleed nipple comes out, make a quick change over and screw in the hose.

If you don’t think you have a good enough connection between your hose and your spigot, or bleed nipple as we call it, get a little jubilee clip, wrap it round and tighten it up.

Turn your central heating system off

Before you move onto the next bit its wise to turn all your heating system off. Go to any 2 port of 3 port valves, and if you know how, latch them open.

Also if you have a pressurised system, make sure that you have a reasonable amount of pressure. In a normal house that would be 1 to 1.5 bar. If you want to know how to do that up look at our pressurised heating systems article.

Also if you have a tank fed system, from the loft, make sure that the ball valve drops down nicely and isn’t stuck up. Ball valves can sometimes get stuck up and that won’t allow any more water into the system.

Purge air from the system

Once you’ve done that you are ready to move onto the next bit which is purging air from the flow and return side of the radiator.

Radiator valve

First the flow side, which is very easy to do. All you have to do is open the radiator up. You’ll hear air and water rushing out of the large hole that we’ve now got here with the hose, instead of the tiny hole through the bleed nipple, and that will let any air out.

Sometimes, if you’ve had the heating system running already, you can open up the valve (the one on the right in the side – under the bleed nipple hole with the hose) and feel hot water coming through.

As soon as you feel hot water coming through we can pretty much guarantee that you’ve probably removed the airlock. So you can hear the air coming back into the radiator and you’ll also hear distinctive bubbling/gurgling/hissing noise which will be the air.

So we’ve shut the TRV (thermostatic radiator valve) at the other end at bled that bit of air.

Now to do the lock shield side. Open the valve. You may hear a coupe of blobs or more of air there and then water should flow through into the radiator.

So now we’ve purged the flow and the return side of the radiator and we’ve got (warm) water coming through both of those ends.

What you need to do now is shut both of those valves. If you are using the technique the pressure will dissipate automatically out of your hose, so you don’t have to worry about that. Remove your spigot and then put back in your air bleed key.

Once you’ve done that make sure that the system has topped itself up again, so the feed and expansion tank (often referred to as the F&E tank) has stopped running, or top it up with your pressurised filling loop.

How did air get into the heating system

Before we finish, you should think about how or why air got in the system and into that radiator. Some possible reasons could be:

1. There’s not enough inhibitor in the system and the metal is reacting with the water inside the radiators and creating hydrogen.

2. You’ve got an automatic air vent that is too close to the suction side of the the pump and sometimes they can tend to suck air in through the air vent

3. Your expansion pipe is on the wrong side of the pump. sometimes you’ll be able to put your thumb over the expansion pipe and actually feel it sucking on your thumb.

4. The pump speed is set too high. It may sound crazy but if there’s not enough water being delivered to the pump impeller (i.e. there’s not enough suction) then the differing pressures between the suction and the discharge side of the pump impeller are such that it causes the seater to cavitate.

If you want to know more about cavitation then you can easily search for a comprehensive explanation in any of the search engines, but all you need to know at this stage is that it does create air and does create airlocks – so make sure that the pump speed is at the right setting.

How to remove an airlock from a radiator

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