Bleeding a Radiator

Summary: Tips and information on how to correctly bleed a central heating if it is not heating up as it should and also how to drain a radiator if simple bleeding does not cure the issue you are having

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Bleeding a Radiator

Sometimes air can enter a central heating system and prevent it from working to its maximum efficiency. Normally you can tell this with a simple test. When your heating is on touch your radiator. If it is warm at the bottom and cold at the top then there may be air in it.. This usually happens when new water has been added from either the feed and expansion tank or simply when its topped up as a result of de furring or periodic maintenance.

Towels placed around radiator bleed valve

Towels placed around radiator bleed valve

Air can also be created in a central heating system by the blades of the heating pump spinning round. This air can cause air locks and stop the hot water and ultimately the heat from traveling around and completely filling your central heating system and in the case of radiators it can prevent the warm water from circulating from the base all the way up to the top. The usual remedy is to bleed your radiators or in some extreme cases totally drain them.

Bleeding and draining radiators can sometimes be a little awkward and messy so we advise that you have some old towels, a jug or pot and also some kitchen roll or a cloth. Lay the old towels on the floor around the drain plug area to capture any spillage (top image) occasionally old radiator water can be quite dirty and discoloured and if you have a light coloured carpet you don't want to risk the possibility of any spillage!

NOTE: Radiators should not be bled or removed with the heating on so turn it off. Not only should you do this to prevent boiling water from squirting all over you but by not turning the heating off and trying to bleed a radiator you could inadvertently suck more air into the system. Also, if your heating system features a header tank make sure that you have not cut off the water to this.

Old type radiator bleed valve

Old type radiator bleed valve

Depending on the layout of your home will depend on what radiator you start at. If you live in a house with 2 floors (an upstairs and a downstairs) you will have to start with the downstairs radiators first, beginning with the radiator that is furthest away from the boiler and once you have done this you can then move upstairs using the same method.

If you do have any air in your radiator it will rise to the top. On one side of the radiator there is a small square plug called a bleed valve. Depending on how old your radiators are will depend on what type of bleed you screw you have. The old type can be seen in the second image down to the right, with the new type valve in the fourth image down. If you have an old type valve you will need a special key (image to the right) to fit this bleed valve (imaginatively called a radiator bleed valve key) and it can be bought from the tools area to the right or by clicking on the previous link. If you have the new type valve then a bleed valve key should also fit this, but if you do not have one then you can use a flat head screwdriver (not ideal as you have a little more control with a bleed valve key).

Radiator bleed valve key

Radiator bleed valve key

Bleed valves can be quite delicate and on inspection you may find that yours is damaged and you are unable to loosen it with either a key or screwdriver. In this instance you may have to loosen the large nut that surrounds the bleed valve (can be seen in either of the 2 bleed valve images to the right) using a suitable sized spanner or an adjustable spanner. This can be quite tricky so only resort to this if you have no other option.

Holding your kitchen towel or cloth below the bleed valve (to catch any leakage) turn the key anti-clockwise about a quarter of a turn (old type valve or turn screwdriver anti-clockwise with the new type valve) and if there is air in the radiator you will hear a hissing sound as it escapes. As soon as the hissing stops a dribble of water will escape from the radiator. At this point close the key or screw very firmly. You have now bled all the air from the radiator and can now move on to the next one.

New type radiator bleed valve

New type radiator bleed valve

If your heating system features a combi boiler (or boiler that needs to be manually topped up) you may now need to top it up depending on how much water escaped during the bleeding process and/or how much air there was in the system.

If there was a long period of hissing before any water made an appearance then this could mean that your system is lacking water and needs re-filling. Different manufacturers may have differing methods of refilling their boiler systems so please refer to any documentation that came with your boiler on how to do this.

If bleeding the radiators does not solve the problem you are having with your central heating it may be that you need to remove and flush out and drain the radiators.

Radiator sludge produced from rust and debris in the water supply can grind central heating systems to a standstill. Use the following instructions to drain and remove your radiators without mess. Make sure this kit is to hand.

Draining a Radiator

The first task is to remove the radiator from the wall. Firstly identify the feed and return valves on your radiator and close them down fully. Slacken the bleed valve (described above) to remove any pressure, then re-tighten.

As in the example for bleeding radiators above, this can get a bit messy if you are not careful so make sure that the area you are working in is covered with old towels/sheets etc.... to ensure that you do not damage any carpeted surfaces. You will also need a tray of some kind that will sit on the floor below the pipe word and the union valve and catch any escaping water

Radiator Feed and Union Valves with catchment tray

Radiator Feed and Union Valves with catchment tray

Once you have protected the surrounding area and positioned your tray, hold the valve tightly with one spanner while you undo the union nut to the radiator with another. NOTE: For full and detailed instructions on how to do this see our removing a radiator project.

Water will begin to flow from the radiator. Should the water slow down do not undo the union nut any more. Open the bleed valve a little to let some air in, this will allow the water to flow out more easily. Do not undo the union nut or remove the bleed screw totally until the radiator is completely empty. The union may be re-tightened at any time during this process. If you have a large radiator and it begins to fill the catchment tray, close the union, empty the tray and repeat the above process.

When you are sure that the radiator is completely drained you can remove it safely and then take it out side and stand it upright to completely drain. There may also be the chance that your entire heating system will need power flushing. This is the process whereby pressurised water containing certain cleaning agents is pumped around your heating system to remove rust and dirt build up etc.... For a more detailed explanation of this process see the power flushing project

Take a look at our video sections on "bleeding radiators" and "removing a radiator" to watch some films that show how to tackle bleeding and draining radiators.

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