Water in your system reacts with the steel in radiators to create a black inky like sludge called magnetite. This product of internal corrosion of the radiators has the disadvantage of blocking valves, boiler parts, pipes and radiators making the system less efficient or, if the water in your system is very contaminated it can cause the heating system to fail.
Sludge in your heating system can cause to blockages and corrosion which will lead to both inefficiency and / or breakdown. In extreme cases it can be so damaging that a complete boiler replacement is needed.
Parts that are particularly susceptible to ‘sludge’ in the system are the boiler’s heat exchanger, radiator valves and the impeller on the system pump. Reducing the useful cross sectional area of pipework, it can also prevent the free and efficient flow of hot water around your system. As sludge settles in the bottom and middle of radiators it prevents an even heat distribution and radiators will be slow to heat up. Over the summer season while radiators are not used, sludge can settle at the bottom of rad valves so that, come the autumn and radiators are switched back on, some will not work or will not get to proper temperatures. It is common for some rads to be much cooler at the bottom and middle because of sludge buildup where flow is weakest through the rad; hot water rises as it pumps through the rad and sludge build up prevents an even flow through the whole radiator.
Blocked heat exchangers in your boiler, including the plate heat exchanger on combi boilers restrict or prevent free flow through the boiler as the water is heated; this causes the boiler to overheat and the boiler to switch off heating as the thermistors detect that the water has already reached very hot temperatures. Other symptoms of a blocked plate exchanger on your boiler include varying temperature of water at hot taps caused by an uneven flow through the system.