By far the biggest cause of boiler breakdowns and poor efficiency is sludge in the system. This is cleared by power system flushing. This is noticed usually by the bottom of the radiators not heating up, and the radiator not giving out enough heat for the room.
But it not only affects the radiators heating up, eventually it will cause blockages in the boiler which will cause the hot water to go hot/cold, and other parts to fail – sometimes to the point where a new boiler is the only viable option. For this reason all boiler insurance policies will not cover any parts affected by sludge.