SLUDGE: If you have a radiator that is hot at the top but cold at the bottom, it isn't because air is being trapped in that unit. This is much more likely to be caused by so called 'radiator sludge' which is contaminants or debris accumulating at the bottom of the radiator and preventing it from being heated by the hot water in your system.
Radiator sludge is actually lots of little bits of rusted metal that has flaked off from the inside of your system and come to rest in the bottom of the radiator. When water comes into contact with metal, there is a chemical reaction that can cause the metal to corrode. Again, in order to stop this process of corrosion from occurring, it's important that your chemical inhibitor levels are topped up at all times.