All the 50-liter top-loaders use the same heater - Diameter of the coil is about 7". A sensor is attached to the top of the heater. It is threaded through the 4 O-rings welded to the heater. The sensor ends up in a thermostat outside the chamber and it is set at 160C. If there is water covering the heater, the sensor will never detect 160C - highest set point is 126 or 128 for the HA300. If it detects 160C, that means there is insufficient water and an alarm will sound and the heater will turn off. This sensor detects insufficient water indirectly. To check if it is working or not, start a cycle and turn the thermostat knob down to 0C. At some point when the thermostat setting is below the water temperature, the alarm will sound with error message Er 1. This means the thermostat is working. If not, it is not working and there is no over-heating protection from this safety device. The Er 1 can only be trigger is a cycle is in progress - press START first.