What is central heating sludge?

Sludge, or magnetite, to give it its proper name, is a black substance in the central heating pipes made up of dissolved and rusted metal.

It forms when the water in the system reacts with the metal in radiators and pipes. Because sludge is heavier than water, it can settle in radiators and pipework and, if left to build up, it can damage the boiler and the system.