Unless your central heating system has sufficient corrosion inhibitor from the day it is installed, the water starts to deteriorate. The steel and iron components will start to rust. This leads to the formation of magnetite and a build up of sludge which is evidenced by the water in the system becoming black. As the build up of sludge increases it can block waterways and seriously impair the efficiency of your heating system, leading to higher energy bills and ultimately failure of pumps, radiator valves or the boiler itself.

If you have radiators which have cold spots at the bottom you probably have a build up of sludge and flakes of rust blocking the waterways within the radiator. If your radiators have cold spots at the top it may just be due to air in the system or it could be caused by hydrogen gas which forms due to a chemical reaction which is a part of the corrosion process. On top of this if you live in a hard water area and your system has not been regularly treated it may well also have a build up of lime scale which only serves to exacerbate the problem.