Radiator Sludge Query

[Prudie]

A plumber has said a radiator he's had to replace was full of black sludge and so the rest of the house is probably the same. I have 2 Questions:
What's the problem with leaving it?
Where on earth does it come from?

[ChillDoubt]

It's a residue from where the rust that slowly corrodes the radiators from inside begins to settle in the bottom of them, especially during summer months.

If you leave it then it will build to a point where the heating system becomes blocked. You can either have them cleaned out now and prolong the life of the rads and the system or leave it until the system breaks down and you STILL have to have them cleaned or replaced. Your choice!

[buildersmate]

If you do nothing, the quantity of black sludge will just increase until it slows and finally blocks the flow of hot-water through the pipe.

You stop more sludge forming by putting enough inbihitor in the sealed radiator system water to stop the corrosion occurring - your plumber isn't very good if you weren't even told that.

If the radiators left are showing no signs of leaking (pin-hole sized lumps of rust start appearing on the outside just before) you don't really need to change all the rads now.

Leaving the black sludge in there won't cause the existing corrosion to get any worse (provided you put inhibitor in), but you really need to get rid of the existing sludge using a dissolving chemicals, then flush with clean water several times, then insert inhibitor.

Quite a simple procedure though time-consuming. It doesn't need a costly plumber to do it - a handyperson who understands the issue would be perfectly capable.

[Prudie]

Thank you both for comprehensive answers.