Airlocks and how to bleed a radiator

Airlocks are a major problem in many radiator systems. When new water is added to a central heating system, a certain amount of air also enters the system. The action of the pump also draws out dissolved air from the water while slight leaks also allow air to slowly enter the system. Finally open systems (other than apartments, most systems are open systems) which are supplied with water from a tank small in the attic can cause problems where the tank is allowed to run dry for some reason, allowing air to leak into the system.

As air is lighter than water, it rises to high points in the system, this is especially noticeable within radiators. Air in radiators mean less water in the radiator and so can be detected as points which provide less heat and so are cooler to the touch, When the system is switched on, feel the top and then the bottom of the radiator – if the top is noticeably cooler than the bottom, there is a good likelihood that there is air in the radiator.

In very severe cases, the whole of the radiator may be filled with air so no difference in temperature between the bottom and the top, though the radiator in question will be cooler than the others.


How to bleed a radiator

  • If the central heating system is an open type, ensure that the water supply tank for the system in the attic has not run dry
  • If the central heating system is a sealed system, check the pressure and, if necessary, top it up as required. This is a bit more complicated so check out here for more advice.
  • When the system has heated up, switch it off completely. If you do not switch off the system, particularly the pump, there is a good chance more air will be drawn into the system while you are bleeding.
  • A “bleed key” is used to open the bleed valve. The bleed valve can be found at the top at one end of the radiator. A bleed key to fit the valve can be purchased in any good hardware shop.
  • Loosely wrap a piece of old cloth around the key to catch any expelled water. If you are careful there should be no need for a pot to catch water, but if it’s your first time, it might be prudent to have one just in case.
  • Slowly open the bleed valve by about half a turn anti clockwise. You should hear a hissing sound
  • When water starts to dribble out, close the valve. They are generally quite easily damaged so take care not to over tighten.radiator-bleed-valve

11 Responses to Airlocks and how to bleed a radiator

  1. Donncha says:

    Had to do that a few times but thankfully not in a while!

    There’s a handy airlock in our hotpress that I can open to let out air. Unfortunately air collects in that every time the heating is on which is worrying, especially as I think the system is supposed to be closed.

  2. Another bit of advice is to consider a central heating system flush. As the system ages, lots of crud (rust) circulates and starts to clog up the system. The worst part to be clogged being the heat exchanger. With a clogged exchanger that just about works, most of the heat from burning will go up the flue and not heat the water.

  3. william says:

    I have a radiator that does not have a bleeder do i get the air out of the radiator.

    • greennav says:

      Hi Billy,

      You will need to allow the water to flow for a while. Then refill the system. I hope its an open system. I also hope you have lots of pots!


  4. martine says:

    I live in a three storey house, the radiators on the top floor, 2 of them are cold and do not fill with water, no matter how much i try to bleed them. Air comes out and when I adjust others it allows more air to come out but not a great deal. These radiators are cold now and the ones on the middle floor are now not bleeding either! Help!

  5. Bleeding a radiator is one of the easiest things that people are able to do. I want to congratulate you for a well written tutorial.

  6. Thank you so much for making this guide, I’m sure people will find it very helpful. Bleeding a radiator is easy for people to do as long as they know what they are doing.

  7. jb says:

    hi have a oil burner heting system i did a clean out to flush the system to put p11 in after refill for a clean of the rads i added and refiled the system now i cant get all the rads back up to hot have beed them have 2 pumps on the system that are running the header tank is full and water being pumped to and from it still cant get the rads hot HELP before i go mad lol
    did refill and bleed from the ground up and all rads are full but no heat just warm

  8. It’s in point of fact a nice and useful piece of information. I am happy that you shared this useful info with us.
    Please stay us up to date like this. Thanks for sharing.

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