my hot water cylinder is over heating and boiling, it over flows from the roof vent and is scalding hot from all taps and shower. have replaced thermostat several times and also the element, i switch the power off and wait a couple of days to turn it back on to re heat the tank. when i replace the thermostat i make sure i can hear it clicking on and off at the set temp but still randomly it over heats....this is a metal craft cylinder, 180 litres. any suggestions out there before i get the whole thing replaced?

Linkback: https://forum.plumbers.nz/q-and-a-hot-water-cylinders/44/hw-cylinder-is-overheating-to-boiling-point/1958/