fitted a new immersion tank in my daughter flat three months ago,
thermostat is turned down to about 55, but during the day water rises into header tank ( direct cylinder ) and gos out of over flow i have put in new ball valve ( second one ) water level when cold is 75mm below over flow
the ball valve is not leaking, so is it just water some getting hot then rising enough to go out of over flow or would it be a faulty thermostat
which would keep getting hotter
Expansion of water when heated would not cause a 3" rise on the header tank. I am assuming you do mean the tank is over filling and overflowing to outside and not the expansion pipe overflowing into the header tank.
I have seen this problem before, and it may be due to a mixer tap problem. Either there is a fault in the tap which allows water from the cold to flow to the hot side or with the difference between the hot and cold pressures, when both the hot and cold taps are turned on together, water from the cold is back fed up the hot pipe filling up the header tank.
water fills up slowly over a couple of hours, also water is not hot quite cold
i think she said she had turned off cold water tap, and it was still doing it
so you could be right in saying the mixer tap wants replaceing
There are few places on a conventional system where pressurised water can cross from one side to another. So if you can discount everywhere else, only the tap is left. Having said that, I have only ever seen these symptoms twice in 30 years and then only with brand new taps.
If the mixer tap has isolator valves does turning them off cure the fault?
No not really - even if the regs say you should fit one if mixer taps are installed in combined high / low pressure systems. Experience says that NRV's can sometimes stop the flow entirely in low pressure gravity system.