Central Heating Problem

Hi All,
I have an interesting problem that has just started with my CH system. I'll explain what I know about the system.
In the airing cupboard I have a copper tank which I assume feeds the system. Beneaf this there is a big insulated tank. The only pipe linking these two is an overflow from the insulated tank which enters at the top. The copper tank has a cold water feed operated with a ball cock, again at the top. At abou the same height there is an overflow pipe. There is also another pipe which exits the tank at the bottow and leads to somewhere else.
I have a boiler in the kitchen (ideal elan 2) which has two pipe enterting/exiting the top of the boiler. I assume that the pipe leading off from the copper tank goes to the boiler but I may be wrong with this.
The problem I am experienceing is that the copper tank is filling and overflowing. My initial thought was that the ball cock was faulty. I replaced this but the problem has persisted, the ball cock is not at fault. The water seems to enter the copper tank through the bottom pipe, and the water is a dirty sandy color.
Has anyone any idea what could be causing this to happen. Has anyone got a electronic copy of a manual or schematic for the boiler or CH diagrams that I can use to try trouble shoot the problem.
Many thanks
Barry.
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I presume that you have also got a smaller header tank in the attic - which acts as a fill and expansion tank for the primary circuit?
Your insulated tank holds your domestic hot water - which is heated by water from the boiler which passes through a heat exchanger coil inside the tank. The water in the primary circuit (i.e. the loop which goes to the boiler) is not supposed to mix with your domestic hot water - with heat transfer taking place through this sealed coil.
Your problem is that this sealed coil is leaking inside your tank. The greater static pressure generated by your header tank in the attic is forcing primary water (which is horrible grotty stuff which has picked up all sorts of corrosion from your radiators) to mix with your domestic hot water - which in turn is overflowing into what you describe as your copper tank.
If my diagnosis is correct - and I'm 99% confident that it is - you'll need to replace your insulated hot water cylinder. You can d-i-y this, if you know what you are doing - the tank itself costs about 85 from ScrewFix.
HTH, Roger
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To check, turn off mains, put hot water taps on, level of tank in the attic should go down.
Jon
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Probably a heat exchanger coil leak in the cylinder. This can only be fixed by replacement.
Christian.
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