Airlocks in Central Heating Systems

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This article will hopefully explain the how to identify airlocks within your system by identifying the signs and symptoms and explaining the different types of airlocks. It will describe the common and most probable causes, examine the effects and suggest some potential ways to cure airlocks in your central heating system. An airlock in your system can cause several issues ranging from a small loss of output in 1 radiator to a total loss of central heating. Inevitably these issues always arise when cold weather kicks in as the system has usually been underutilised during the warmer weather. Although airlocks in your central heating system can be hard to prevent, fixing them may not be as complex as you think.  Knowing how to perform some relatively simple maintenance on your central heating system is important knowledge, and if you are confident doing so then you can not only ensure that your property is well heated and maintained but can also save money in several ways. Obviously if you are doing this yourself then you are saving yourself significant costs on employing professional tradesmen but you will also have a far more efficient heating system which will not only save you money on your regular bills but will also contribute to reducing environmental pressures. Consider adding the checking and bleeding of your central heating system as a regular proactive task rather than wait for the issues to show themselves in the depths of Winter when you are woken up by a strange noise coming from the pipes and a cold bedroom !! Maybe pencil it in when you are putting the lawnmower away for the year or notice the nights drawing in and the days getting shorter. Do not be afraid of these procedures, like any process around the home if you follow the steps in a logical manner then the resolution of these issues should be relatively simple. However, as with many projects involving water or electricity if you are not totally confident then do not hesitate to employ the services of a qualified professional. It can sometimes be useful to follow them around and watch what they are doing so you will know for next time!

Signs and Symptoms of an Airlock

The most common way that you will know that you have an airlock in your central heating system is when you have air trapped in individual radiators. This issue typically presents itself by your radiators having cold spots at the top with hotter temperatures at the bottom and a resultant reduction in the amount of heat produced. This occurs because any air in the system will rise to the highest level which is why it frequently gets trapped in radiators and causes these typical cold spots which are usually indicative of an airlock issue.  This can also occasionally present itself by tapping, gurgling, or banging sounds and a decrease in boiler pressure. Boiler pressure can be monitored on the dial and should ideally be between 1 and 1.5 bar, usually indicated by a green section on the dial. If the pressure falls below this, then the system has lost water and this will need to be replaced. Refer to your manufacturer’s instructions for this step.

Please be aware that there are other reasons that the radiator may be cold so please ensure that these are checked before assuming that the issue is down to an airlock. Other causes may be a stuck valve in the individual thermostatic valve or a jammed wheelhead or lockshield, this however presents itself as a totally cold radiator, rather than the classical half hot half cold radiator that is caused by an airlock.  You may also experience an airlock in the heating system itself, not in individual radiators which could result in either no heating whatsoever or no heat from the radiators from the airlocked point onwards.   Before you can confidently identify your issue as airlock related ensure that you have ruled out other possibilities. Start with checking that your boiler is working correctly, is not showing any fault codes and is set for both hot water and heating. Once the boiler has been checked out and is seen to be working correctly then perform a quick check of the heat pump. You should be able to hear this or feel the vibrations however if this is not possible then a helpful tip is to place the metal end of a flat bladed screwdriver gently against the pump and the other end to your ear, this will amplify the noise and vibrations and leave you in no doubt whether the pump is functional or not. If the issue is with the boiler or the pump then seek the advice of a professional tradesman.

What causes an Airlock

Airlocks in your radiators can occur when air / vapour becomes trapped in your central heating system and this subsequently prevents hot water from moving into the radiator, leaving them cold. In your property your central heating system is responsible for distributing hot water throughout the building into the radiators, which will then heat your home or business. Sometimes however this process can create excess water vapour and as this has a lower density than water it will rise to the highest point in your system and create what is known as an airlock. When your central heating system is initially filled with water, this water will contain dissolved air that could constitute up to one sixth of the volume, if this is not released after a few days then this too could cause airlock issues. It is worth mentioning that sometimes the ‘air’ that is causing these problems may not be air at all. Sometimes chemical corrosion can form gases as part of the corrosion process if your system has not had any corrosion inhibitor chemicals added. These airlocks can be responsible for all sorts of problems within your central heating system from a lack of heat coming from individual radiators to a total loss of heating throughout your property. You may also experience an airlock in the heating system itself as opposed to in individual radiators, which could potentially result in a total lack of all heating. This can sometimes happen when you have had your system refilled in the wrong order, usually caused by filling downstairs radiators when the upstairs ones have not been closed off. The piping (flow and return) for your central heating system can sometimes run vertically on walls, or in cupboards and the flow in these pipes can go upwards or downwards. This is not an issue when the normal flow direction is upwards as the air will normally be flushed out of the vertical pipe. However, when the water flows downwards the air cannot be flushed quite as easily. Combine this and a lower pressure from the pump and you have the perfect situation for airlocks to form, leading to some, or all of your radiators failing to heat up, depending on how many radiators the affected pipe serves.

How can I fix an Airlock?

So hopefully from the above you have identified that it is pretty likely that you have an airlock in your central heating system. The good news is that this can usually be resolved with the minimum amount of plumbing knowledge. However, as with all issues around the home, especially those involving water or electricity if you are not totally comfortable and confident in what you are about to attempt then it is probably better to employ the services of a professional If you feel that this is something that you wish to attempt, then the steps for releasing an airlock are as below. As previously mentioned, the airlock can be present in one or more radiators or in the system itself. We will initially deal with the process for removing the air from individual radiators, this process is referred to as ‘bleeding’ and can be very simple. Bleeding is when you remove the air from various points in your central heating system, and allow the water in the system to fill this gap. Bleeding your radiators Is often a quick fix to dealing with you airlock problem, and is a fairly simple process to do yourself. The good news is that at this point you do not need a large selection of specialist plumbing tools, a couple of old towels or a bowl and a radiator key are all that you really need at this point to perform this task and look like a true professional! On more modern radiators the bleed valve can also be rotated using a flat headed screwdriver. Make sure you check what tool you will need before beginning to bleed your radiators.

Removing an airlock from radiators Once you have the towels, bowl, and radiator key ready then the following steps should resolve any issues that are caused by air being present in the radiators.

  • Ensure that all radiator thermostats are set to full and run the heating system for at least 10-15 minutes to give the radiators a chance to warm up
  • Try and identify which radiators require bleeding, these can be identified by those that are producing gurgling noises or have cold spots at the top whilst remaining hot at the bottom
  • Ensure that your central heating is turned off until the radiators have cooled down. Allow between 30 minutes and an hour for this
  • If your home has more than 1 floor, then you should begin bleeding the downstairs radiators first.  It’s also advisable to start with the radiator which is furthest away from the boiler.  Once you have bled all the downstairs radiators you move on to the upstairs, again beginning with the radiator which is furthest from the boiler.
  • For each affected radiator being bled, position the towels on the floor below the radiator to catch any excess water that may escape and to protect your floor from staining
  •  Identify the radiator bleed valve, this is normally found at the top of the radiator and is a round hole with a square inside, double check that your towels are directly below this valve as this is where any excess water will drain to
  • Be aware that the radiators will be hot, so please take extra care at this point as there is a danger of burning yourself
  • Using the radiator bleed key turn the square slot anti clockwise, you do not need to turn this more than half a turn at this point. Having done this you should hear a hissing sound, this is entirely normal, and is a sign that the trapped air is escaping the radiator
  • Keep the radiator key in place until you start to see or feel a little water coming out, let the water splutter and wait until there is a constant stream, this will ensure that you have released all of the trapped air. At this point take note of the colour of the water, if the water is black then this could be indicative of a dirt build up and a power flush may also be required, this will usually require the additional services of a qualified tradesman
  • Re tighten the bleed valve by turning it clockwise, this will reseal the radiator. Take a few extra seconds to ensure that there is no more water escaping the radiator at this point and be careful not to overtighten
  • Work your way around your property repeating this process for all of the radiators that you had previously identified as having an issue
  • Turn your heating back on and repeat the checking of the problematic radiators, if this has resolved the issues then sit back and reflect on a job well done. If the issues still remain then you may need to seek further professional advice

There are plenty of tutorial videos online if you require additional instruction. If you have any questions or concerns, you should always contact a professional for help. Removing an airlock from the central heating system If your issue is with an airlock in the heating system itself, then there are several other methods that could help to resolve this.

  • The first problem may be a lack of pressure from the pump, these pumps usually have a variable setting for pressure so try to turn up the pressure which may force the airlock out of the system

If the adjustment of the pump fails to resolve the issue, then there may be a faulty radiator or a locked pipe. This can be identified by following these steps :

  • Take a note, or a quick picture, of the setting on the boiler thermostat and set this to the minimum. If you have an older boiler, which allows you to switch the thermostat off completely and leave the pump running then you can switch the thermostat off completely.
  • Ensure that the pump speed is set to the highest setting possible.
  • If your boiler heats both a water tank and the heating then turn the heating setting to ‘Off’, this should now ensure that you will have the maximum pressure running to your radiators.
  • Turn off all radiators except for the one closest to the boiler, this will reduce the risk of your boiler overheating. Radiators can usually be turned off at either end. If you are turning off your radiator using the thermostatic return valve ( TRV ) then take a note of the present setting. If you are using the lockshield to close the radiator then remember how many turns this has taken to turn off the radiator. On some designs of radiators you may even have a wheelhead design instead of a lockshield, if this is the case then also make a note of the number of turns and part turns needed
  • Turn on the next radiator, whilst leaving the previous one on, once you have confirmed that the radiator is working you can turn off the previous radiator
  • You may hear a gurgle of air as your radiator clears and the radiator may suddenly start to get hot, excellent work!
  • Repeat this exercise until all of the other radiators are returned back to their original settings and are functioning correctly. You may have to repeat this process several times with individual radiators however this will usually resolve the issues and leave your radiators ‘balanced’.
  • Make sure that you return the boiler thermostat to its original setting, turn the hot water setting back on if you disabled this in earlier steps and finally set the pump back to its original speed.
  • If at this point you have still not resolved the issue, then it is probably time to seek additional professional advice.

Fixing your own radiators can feel like a challenge, but learning a few of these key skills now can help to save you countless hours of hassle in the long run, not to mention the money that you will save. Understanding why your central heating system is not working, and learning what symptoms to look out for, will help you keep your central heating system fully functional, all throughout the year.  If you do face any issues when trying to rectify any airlock problems yourself, always consult a professional. It is important that if you feel unsure, or do not feel confident when working on your radiators, that you speak with a professional. It is better to call a plumber out for something small rather than cause any accidental damage.


Hopefully from the above you will have a good understanding of what causes an airlock and how the various issues can be resolved. Remember that it is impossible to bleed every last bubble of air out of a system, but having a small amount should not adversely affect normal operation. Air in the system only becomes an issue when feel the radiators not heating to the very top, the pressure drops, or you start to hear strange banging or knocking noises coming from the pipes. By following the steps above you should be able to ensure both efficient and reliable operation of your heating system. There is nothing worse than waking up to a cold bedroom or a cold home and these simple steps should help to avoid this. Being able to understand why your central heating system is not working, and how you can fix this will be a valuable skill that you can and will use for years to come. We hope that you have found this information useful, and that you get to use these new found skills and experience the satisfaction of having fixed something yourself, time to sit down, congratulate yourself and enjoy a nice warm property!