hot water getting into header tank

6 Oct 2013
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United Kingdom
I have hot water coming into my CH expansion/header tank via the cold make pipe and it overflows.
Not had CH on, it only does it when hot water is on ie. boiler on, pump on hot water valve open.
I have tried
1. Drain expansion/header tank, boiler off, pump off, valve shut, did not fill
2.Drained main cold tank to below level of expansion/header tank, did not fill
3. With main tank low, hot water low (no hot water at taps), turned on hot water. Make up pipe got hot and a small flow of water entered tank, switched off hot water and a gush of water entered tank followed by a small flow.
4. Did the same with main tank full and hot water at taps, did the same as above 3.
I have no air locks in system, the pump is set on 2, the water entering the expansion/header tank is clean, the ball cock is ok.
I think it is the coil in the hot water cylinder, but only does this when it is hot, the cylinder was new July 2011
Am I missing something before I replace the cylinder.
I have sent this before but seems to have got lost.
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I would suggest that you've got a blockage on the Heating side pipework, Don't lob a new cylinder in until last resort.
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Thanks for your replies.
Update from 7th October. Lowered water in expansion/header tank, and has only risen a little, keeping my eye on the tank.
The CH is not on, ie the CH valve not open.
The water is only going through the boiler, pump, hot water valve, coil and back to boiler.
Would this still mean it looks like a blockage in the CH.
My next approach is to empty the hot water cylinder and turn on the hot water control, if I get water coming from the drain on the cylinder and the tank level falls, I think it must be the coil.

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