Don't know about your particular heater but you're aware many of them have to be calibrated? You have an external thermometer and note the tank temperature with it, then you turn the knob until the heater light just comes on or off and check to see whether the setpoint knob then needs to be adjusted so that a number representing the temp you have on your thermometer is now aligned with the setpoint arrow next to the knob. New heaters should have instructions for this if the heater works this way.
~~waterdrop~~