How an electric hot water tank works
Larger electric HWTs are usually wired for 240v, with elements that are 4500 watts. This means that each element will draw 18.8 amps @ 12.8 ohms of resistence through the element. However most HWTs are wired so that only one element will run at any given time. Usually the top element will be on because it is most responsible for the heating of the water. The bottom element will come on and suppliment the top element to maintain heat in the tank.
Here is what a typical HWT element can look like:
So how does your electric HWT know when it needs to turn on and off to maintain a set water temperature!? With a thermostats!
Here is what a typical thermostat looks like:
As you can see there are some electrical connections, a red reset switch, and a white dial. That dial is used to set the desired temperature of the water in the tank. These thermostats are located at the top and bottom of the HWT and a either bolted or strapped right the inside metal liner of the tank.
So whenever you open up a hot water faucet cold water enters into your HWT through what is called the "dip tube". This dip tube runs from the top of the hot water tank to the bottom. This is to direct the incoming cold water into the bottom of the tank so it can be heated by the tank. The incoming cold water then pushes hot water out of the top of the tank (because only so much water will fit in the tank) and through your supply piping to your faucet!
As more and more cold water enters the tank the temperature of the tank will drop. The thermostats will pick up on the temperature drop and ONE of the elements to will turn on to begin heating up the water in the tank to the desired temperature.
Here is a picture to give you an idea of the overall picture:
What is the "anode" and "T&P valve" in the diagram above? Well the anode is actually called a "sacraficial anode" and it's job is to protect the tank from corrosion. The anode will corrode before the steel in the tank will corrode, thus extending the life of the tank. Your sacraficial anode will last anywhere from 3 years to 15 years depending on the quality of the water in your area. The only way to check the anode is to actually pull it out of the tank and inspect it.
The T&P valve is actually the "Temperature & Pressure Relief Valve". The purpose of this valve is to protect the hotwater tank from either over pressurizing or over heating. Here is a closer look:
As you can see the valve is threaded into the tank and the white thermostat sticks into the tank and is directly emersed in the water. As the water heats up that white thermostat expands and pushes the valve open so that it dumps out hotwater and lets in colde water through the bottom of the tank helping cool the tank down.
The valve also works the same on pressure. As the pressure in the tank builds up there is a spring inside the T&P valve that will be forced open letting out water and thus lowering the pressure inside the tank.
Most T&P valves for hotwater tanks are designed to open at 210*F or 150 psi. Which ever comes first! The T&P valve is by far the most important component on your HWT and probably in the entire plumbing system in your house. If your T&P fails the results could be deadly.
If the tank is over pressurized due to thermal expansion from VERY hot water, and that water reaches a temperature of greater then 212*F (which is the point where water becomes steam) there is a chance that your HWT will explode like a giant steam bomb if the tank is exposed to atmospheric pressure some-how.
Now this is very unlikely to occur because your HWT has more than one safety on it. It has the T&P and high limits that will kill power to the tank if it gets too hot. But parts do fail and you can't rely on the high limit by its self to safely protect you.
So if your T&P Valve is dripping water DO NOT attempt to plug off the valve. Call a professional to have the tank serviced and the problem diagnosed.
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So I hope you have learned something from this article! As you can see your HWT is actually a simple device that is extremely safe given proper maintenace!