The function of a thermostat in any electrical device, a water heater included, is to switch on or off the power supply to the equipment as it reaches a preset temperature.
The back of a thermostat has a bi-metallic disk that is hard-pressed to the side of the tank. As the name suggests, it is made up of two metals, having different coefficients of expansion, meaning they contract or expand at different rates varying with temperature.
It causes the metal to bend at predetermined temperature. When used along with a switch, it controls the temperature of the equipment using the thermostat. You might have heard a clicking sound on devices like a coffee maker or a washing machine. That click is caused due to back and forth movement of the bimetallic strip operating the switch.
Electric water heaters necessarily require heating elements for heating water. Some of the elements operate on 240 volts while others need 120 volts for their operation. Domestic water heaters of smaller capacity usually have just one heating element, but the larger heaters are equipped with two elements.
Single-Element Water Heaters
Electric water heaters with single element are the simplest. They use one heating element along with a single thermostat. The thermostat has a control arrangement that allows to set the desired temperature to which water needs to be heated.
As the temperature of the water falls below the adjusted setting, the thermostat turns the power on for the element, which continues heating water till the temperature reaches the pre-adjusted setting. At that point supply to the element is cut off, and the cycle starts over once again.
Dual-Element Water Heaters
Dual-element electric water heaters are provided with two thermostats, one each for the top and the base element. The top thermostat controls the upper element as also the lower thermostat. The two elements are designed not to operate simultaneously.
Water heaters are supplied water from the base. A fill tube leaves from the intake at the top of the tank to the base of the tank, allowing cold water to enter from the base, while hot water leaves from the top, from where it leaves the tank at its outlet as required.
The primary element is the upper one. It initiates the operation of heating water at the top, making it available for use. In the beginning, when the tank is full of cold water, the primary thermostat at the top switches on the top element and keeps it on till the water has reached a temperature as already set on the thermostat.
The primary thermostat then switches off supply to the top element, sending current to the lower, secondary thermostat that switches on the other element at the base. The lower element continues heating water in the lower part of the tank till it attains the set temperature. That means the entire volume of water in the tank is hot but both the elements are off.
Water used from the top of the tank pushes cold water into the bottom of the tank, forcing hot water at the bottom to move up. As the top thermostat is still off, the thermostat at the base switches on the lower element again to heat the additional cold water till it has reached the set temperature.
If adequate amount of hot water is used, the tank is once again filled with cold water, the top thermostat will switch on the top element and turn off the thermostat at the base, starting the cycle once again.