Old central heating system upgrade

Parkhall

So my oil central heating system is about 30 years old and I am investigating getting a new condenser boiler.

I the meantime I need to do something about the radiators. A couple of them have completely stopped working and the rest are very unreliable. I'm guessing that 30 years of sludge accumulation is the problem. Perhaps also dodgy valves.

My first instinct is to just replace the lot and flush the system but some of the radiators look OK externally. So what to do? Replace the lot or just flush the system and replace the completely dead ones?