thermostat problem

HI, i replaced a very old header tank and my copper cylinder,the old cylinder boiled the water because the immersion thermo was the old single type with no overheat control,so all parts brand new, the prob i have is the new overheat stat, was constantly activating,so a second, new heating element and thermostat was fitted again, but still the problem remains,any ideas welcome thanks, Colin [vented cylinder /hot water only]

stat

We have no idea what the thermostat is controlling, but if the heat can heat the water by convection even with the thermostat shut off, it WILL overheat to the point where the high limit is activated. We cannot diagnose it from here, and with a vented tank, it sounds like a United Kingdom system.

overheat stat

You are correct, hj iam in the uk the copper tank 42 gallon is heated by an 27 inch heating element, the element is temp controlled by a replaceable internal 2 pin thermostat, which is wired in series to live feed has a built in secondary overheat button for protection,i have tried heat ranges from 60- 80 degrees, but the cutout still activates,after the heat gets to its setting or beyond the heater is connected to the mains E/L/ neutral in the usual way,the cylinder is gravity fed from an over head tank, as i have said i have replaced this new gear,twice [element and stat] still the fault remains, Colin