As above you don't have one. This is extremely dangerous as the water is left to get too hot. A few years ago in my town a toddler died due to a fault with the immersion stat, it failed and continued to heat the water, which vented over and caused the header tank to get too hot and collapse. Boiler water came through the ceiling and killed the toddler in their sleep. Isolate and get a reputable plumber in to replace with a modern safe immersion