Recently, my Heat link has not switched the heating off, eventhough the stat says it should be off. This is happening randomly and the only way to turn the heating off is to kill the power. I suppose I need a new Heat Link.
It turned out that it wasn't the heat link. It was the actuator - thats the small motor that turns the valve in the hot water pipe on and off. It was getting stuck in the "on" position. Had to replace it.
