I just gave my unvented cylinder (Redring LWC series) a good look over
recently (want to make sure there's nowt wrong with it before its annual
service, just in case I find a cowboy).

Thinking it might be a good idea, I drained it, only to be rather amazed
by the sludge that came out. About 10 litres of water that looked like
pea-soup, after which I got reasonably clear water. Wondering where this
stuff was coming from, I paid particular heed to the large label stating
that the cylinder was fitted with a sacrificial anode.

I can't believe the sludge is coming from the water - the water here in
Sheffield is extremely soft, and not even my kettle seems to have
developed any real scale. Presumably then, the sludge is coming from the
anode.

How long are these anodes supposed to last? There is no obvious access
to the anode, and the installation and maintenance instructions make no
reference to the anode, other than that it is there. Presumably the
anode is attached to the actual cylinder - but that is buried beneath
foam insulation, which is covered by a riveted casing.

I dread to ask a 'certified installer' how much an anode replacement
would cost, especially if it's going to involve drilling out the rivets,
and removing the insulation to get to it.

Or do I just have to accept that this type of cylinder is designed with
obsolescence and limited life span in mind - and should be replaced
after about 10 years?

Mark