Sludge is a proliferation of dirt and rust, will restrict the flow of water around your radiator, curtail its performance, and increase the power bills.
Removing central heating sludge could imply that a heating system lasts for numerous years. First of all, the heating system must be off and cooled for at least an hour.
Manually flushing radiators, replacing offending radiators, or servicing the unit regularly might solve the issue.