CWS tank overflow problem

[joe27]

The cold water storage tank in the loft is overflowing, I have replaced the ballcock, what seems to be happening is hot water from the cylinder is getting forced back up into the cws tank and causing the overflow, the central heating header tank runs at a slight trickle and as soon as I isolate the water feed into the central heating header tank the cws storage tank overflow subsides, I have also replaced the ballcock in the central heating header tank, the hot water runs discoloured from the taps, could I be looking at a split heat exchanging coil in the cylinder? Any help much appreciated, thank you.

 

[SilentMike]

it is a indirect cylinder you have?

turn of the cold and drain it down you will prove it is what you think.
There no where else the two can mix

 

[Best]

Does look like the coil, by your description, is letting the highest head water pass (I assume the expansion tank is the highest?). A lot of the old hot cylinders near 40 years & more, didn't have a coil, but a sort of cylinder inside the hot tank & it was prone to leak, more than a coil would.
Only other thing that can cause the cwt to overflow, is if you fitted a mixer tap that was faulty anywhere in the house & it was supplied with mains cold water & hot gravity water & no check valves on it's pipes. Seen this happening with a monobloc sink mixer which while turned fully off let the waters mix in the base of the body. New tap sorted it. Probably, though, you need a new hot cylinder.

 

[AWheating]

as the water is discoloured i would think the two systems are mixing which would point towards the coil passing.

 

[joe27]

it is a indirect cylinder you have?

turn of the cold and drain it down you will prove it is what you think.
There no where else the two can mix

thank you