I can't understand why water keeps coming from overflow on my headder tank .
I have turned tap which fills tank OFF . When heating is on the level DROPS but then rises again once the water in system cools... can't understant this, I expect the reverse as hot water EXPANDS so would think it should rise when hot...
Can anyone expand on this.. Is it that I have leak in coil inside hot storage tank?

i can't understand why water keeps coming from overflow on my headder tank .
I have turned tap which fills tank OFF . when heating is on the level DROPS but then rises again once the water in system cools...... can't understant this, I expect the reverse as hot water EXPANDS so would think it should rise when hot.......
can anyone expand on this...... ? is it that I have leak in coil inside hot storage tank ??
The" fill tank", make up, could be leaking by, yes the coil in the storage tank may have a hole in it... shut off the IN/ OUt valves of the storage tank to verify if it leak. It could be one of the two or both are leaking

That is odd, my first look would be at the expansion tank.