The first thing to learn about electric water heaters, and I suppose anything you are thinking of working on, is how they work. Like any machine there are various elements to be considered, pun intended. It has been my goal to avoid sounding encyclopedic in these pages and I feel that presenting this information may be a greater challenge to this goal than I have yet faced. All of the ground must be covered however so that none are left behind, even at the risk of becoming mundane. There is nothing for it than but to press on. I will list the parts of the common electric water heater as I discuss how they perform in concert. On subsequent pages I will go over the individual parts.
I did well during my long career of motorcycle riding, having only one wreck. I was ever conscious of the raw potential of the machine and the inherent danger of being so very exposed on the road. Line voltage has a much more subtle and therefore more deadly potential. When it is so easy and yet so dangerous to make a mistake it becomes of paramount importance to follow and never deviate from protocol.
The standing storage type water heater is a vessel installed in the cold water system of a structure. It is the starting point of the hot water system. Except for its initial filling, water will only flow into the tank when water is flowing out of it. This may seem obvious but I have had occasion to explain that the tank isn't empty just because there is no hot water to be had when it fails to heat water.
What you see when you look at an electric water heater is the jacket the tank is encased in. The tank itself is the important part and there is plenty to know about it.
A water heater is required to have an inlet service valve to isolate it from the cold water system during service or emergencies.
Early water heaters were designed to be connected to the cold water supply at the bottom of the tank, and the hot water outlet at the top of the tank. This was supremely logical since like hot air, which is lighter than cold air, hot water rises above cold water. When manufacturers decided to abandon this practice and put both the inlet and outlet ports at the top of the water heater they still needed the incoming cold water to dwell at the bottom of the vessel so as not to spoil, with cold water, the pool of hot water floating at the top of the tank. To get the incoming cold water to the bottom of the tank they installed a shunt into the inlet orifice. This shunt is referred to as a "dip tube".
Manufactures install metal anode rods in water heaters. They work as sacrificial anodes and are there to extend the life of the tank.
Temperature and Pressure Relief Valve
Every closed vessel that stores hot water must have a temperature and pressure relief valve. It will be threaded into the upper portion of the tank where the hottest water dwells.
Standing storage tanks have drain down valves located at their bases. these valves are referred to as boiler drains.
Thermal Expansion Tank
A thermal expansion tank is a separate vessel that is plumbed into the cold water system at the water heater to keep the temperature and pressure relief valve from opening due to thermal expansion.
There are usually two thermostats on an electric water heater, an upper and a lower. They work together to start and stop the flow of the second leg of 120 volt power to the elements. Note, each element always has at least one leg of 120 volt power energized when the system is on. Nothing is off unless the entire system is off and tested to prove that it is off.
The actual heating of water in an electric water heater is done by the elements that protrude into the tank and are connected to and powered by the thermostats.