Most common parts of the central heating to get damaged by sludge.
Pump - which cannot run properly when blocked up and burns out.
Boiler - simply stops working when full of sludge. Has many internal parts and narrowing's, all of which can get damaged or blocked.
Radiators - rust and start leaking, can also not heat up as the water cannot flow.
Radiator taps – will not allow flow to radiators when blocked.
Two-port valves – cannot open and close properly, either jamming in the permanently open or closed position this will also burn out the valve's actuator motor.