Plumbing Puzzle

[Mr.Puzzle1]

I'm puzzled because cold water feed tank is overflowing a little, the ball valve is new and working OK, hot water is flowing back into the expansion tank and cold water is getting into the hot water supply pipe. THe CH is OK. The only thing I can think of is that there is a leak in the coil in the indirect cylinder but so far two different plumbers have been unable to fix this problem - its getting expensive! Could there be a shunt valve somewhere thast has stuck? I would be very grateful for any advice. Thanks.