Bleeding a radiator is a fairly simple DIY job which can really improve the efficiency of your central heating system and it's usually something you can do yourself, without calling out an engineer.
Sometimes a radiator will feel warm at the bottom and cold at the top. This is because cold air has got into the system and is now trapped at the top of the radiator, obviously stopping the hot water from filling it.
So, you're heating the water, but it's not able to heat the radiator - or your room.
If you feel all your radiators with the central heating on, and a few of them are cold at the top, they probably need bleeding.
Follow these step by step instructions and it should be easy to fix;
1) First, find the bleed valve. Most radiators have a bleed valve (bleed screw) on the side at the top. Look at the end, or on the back of the radiator. It usually looks like a square nut, with a slot in it for a flat screwdriver.
2) Once you've found the bleed valve turn the central heating off and leave it until all the radiators are cold. Don't ever bleed the radiator with the central heating on.
3) Get an old towel or cloth just to mop up any water that may escape, (don't worry, if you follow these instructions only a small amount should come out).
4) Check that the valve at the bottom of the radiator is open, so water can flow in.
5) Using a radiator bleed key (you can get them from most DIY shops), or a screw driver if you have to, slowly turn the bleed screw anticlockwise until you hear the hissing noise of the air escaping from the radiator. Don't turn it any more, even if it seems to be taking a long time to fill up, be patient.
6) Eventually water will bubble out of the valve. At this point turn the valve clockwise, until the water stops, as the radiator is now full of water not air. You've just bleed your first radiator.
7) Now you may need to top up your boiler, depending on what type it is but if you have a look at your manual, it'll tell you how.
Sometimes you might not be able to use a bleed key on the bleed valve. The valve is usually made of quite a soft metal and so can become damaged, which is why it's best not to use a screw driver if you can avoid it. In this case, use a spanner to undo the large nut in which the bleed nut sits, but only do it if the valve is damaged.
Make sure you have the right sized spanner, and only loosen the nut until you hear air coming out. Be careful because if it falls out completely it'll be hard to get back in, and if it does, when the water starts pouring out, you'll definitely need a towel or two.
In either case check the colour of the water that comes out - if it's a bit black and murky, you might need your system flushing.
If your radiators keep on filling up with air, really you should get your central heating system serviced.