F and E keeps filling with hot water

[theglovebox]

I have an issue with the fill and expansion tank in my loft. I have a conventional open vented system. Recently the tank was overfilling and the overflow pipe was not sealed properly so we had water leak through the ceiling. After some investigation I found the tank was filling with hot water from the expansion pipe (not the cold water feed). I have plumbing insurance and a plumber came out to fix. He could not answer why the tank kept filling but suggested fitting a new F and E tank and “see if it settles down”.
The tank still intermittently fills with hot water(the same as before), but strangely now the level never seems to go up and does not come near the overflow. I don’t think this is normal and have read lots of threads with similar, but not identical issues. For what it’s worth:
I have bled any air from the radiators
The heating is not being used currently
The hot water seems to be a normal colour
I cannot see any leaks in the system

My suspicion is there is a pin hole in the copper coil in the hot water cylinder, but the water is a normal colour and I can’t figure where the overfilling water is going. Does anyone have any thoughts or advice? I suspect I will get a plumber out again but hopefully one who can solve the underlying issue. Thanks in advance

 

[Exedon]

First this wants moving to Plumbing forum.
To answer your questions.
If water is stopping at a constant level in tank and not overflowing coil is probably OK.
You do urgently need to address the water pumping through open vent will cause rapid corrosion in your system.
Several possible causes partial blockage in cold feed/open vent pipework new pump recently fitted? Pump speed turned up?.
You need someone competent with open vent systems changing tank and hoping for best won't work!

 

[DiyNutJob]

Blockage or over pressure from pump? Reduce pump speed and see any thing changes. Insufficient pump speed will cause boiler to cycle on and off, boiler kettling, or some radiators not heating up.