Afternoon Ladies & Gents, I am having a repeat problem with my Grandma's Immersion Heater Thermostat. The thermostat is a 18" Tesla Dual safety variety from Screwfix. Grandma had a new cylinder and header tank fitted with immersion heater, no other source of hot water. I have replaced the thermostat on 4 occasions in as many months. I did notice that the thermostat terminal port had brown discolouration so checked for loose connections, one wire was loose so put it down to that. Since then it has stopped failed again. Can anyone suggest why this keeps occurring?