Faulty Hot Water Cylinder???

[WSB]

I believe I have a faulty hot water cylinder.
Basically, my small header tank in the loft has been overflowing through the pipe to the outside.
Ball valve has been changed and tests done to see if the excess water is coming from the vent pipe but neither were guilty.
I switched off the heating controller to rule out an issue with the pump as well.
Still it overflows.... And I have left it quite a while.
I've been told that the cause of this can be perferations in the cylinder internal piping causing outer cylinder water to be forced in the pipeline and pushed back up into the small header tank.
Before I call a plumber to come and change my hot water cylinder, I just wanted to check with you all whether there's something else I should be considering or further tests I could do?
Thanks in advance.

 

[terryplumb]

You could bung the outlet from the cold water storage cistern in the loft ( or close off the water supply to it and drain it ). You would ,of course, then not have any domestic hot water. See if the level in the small f&e tank remains stable.
Unfortunately you would need to leave things in this state for quite some time.
How long does it take for f&e tank level to go from, say ,just below overflow level ,to running into overflow ? If that takes 4 hours ,you would need to run the test for a bit longer than that ,if your with me

 

[dilalio]

Isolate ball valve to f&e and drain it.
See if it fills again and how long it takes.
You're not running CH (hopefully) so there should be no expansion from that.