Electric Water Heaters - How They Work with Diagrams

Most of us take our water heater for granted.  At first glance there doesn’t seem to be much of interest.  After all, it’s basically an insulated tank full of water with a gas flame under it.  Add a thermostat, a pressure temperature relief valve, a dip tube, a drain valve, and you’ve got a water heater!

A water heater can exhibit some strange behavior

Sometimes water heaters behave in some interesting ways and can be puzzling when something goes wrong.  Let’s examine how an electric water heater works.

A typical 50 gallon heater consists of a tall cylindrical tank, surrounded with insulation, and enclosed in a sheet metal outer shell.  Usually the electrical connections are made on the top where there is a conduit fitting.

The thermostat controls the water temperature

Most electric water heaters have two thermostats, an upper and a lower. Typically the top and bottom electric heating elements each have their own thermostat and are wired in such a way that only one heating element heats up at a time. Normally the it's the upper thermostat that switches on first, and when the upper portion of the water is hot, the upper thermostat sends the electricity to the lower thermostat and element. If the lower thermostat is below the set point, then the lower heating element turns on.

High limit switch

If the water in the tank gets too hot, it will trip the high limit switch, (a circuit breaker). The high limit switch is located in the upper thermostat, and it usually has a red reset button. In order to get it working again the high limit switch must be manually reset by pushing in the red button.

The dip tube is critically important

At the inlet to the water heater there is what is called a “Dip tube”. The dip tube is a long skinny plastic tube that reaches down to the bottom of the tank.  Incoming cold water travels through the dip tube down to the bottom of the water heater.

Without the dip tube in place the cold water coming into the water heater can just travel a short distance to the side and leave through outlet. The water temperature will be much lower than normal, and it can seem as though you run out of hot water very quickly.

Cold water entering the heater is sent through the dip tube to the bottom of the tank where the thermostat is located.  Running even just a gallon out can cause the thermostat, to turn on the gas valve. The pilot light ignites the gas and the water heater begins heating up the water.

Water heater stacking

The hot water in a water heater tends to form layers with differing temperatures. When short uses occur frequently the heater can turn on and off and cause the water at the top of the heater to keep getting hotter and hotter each time. It’s called “stacking”. It’s because hot water, like hot air, rises.  It can get hot enough to cause the temperature / pressure safety valve to open and release hot water from the heater, even with the thermostat setting on low.

The TP valve is a safety device (TPR valve, T/P valve, T&P valve)

The safety valve often referred to as a TP valve, opens to relieve pressure if either the pressure gets too high or the temperature gets too high. This prevents the water heater from blowing up if something goes wrong. The TP valve usually is located on the side of the tank near the top. 

Normally as your water heater heats the water, the water expands as it’s heated, and as the water expands some of it gets pushed back into the source, such as the water main. Sometimes there is a check valve or some other obstruction between the heater and the source of cold water.  In that case, as the water expands, there is nowhere for it to go, and thus the pressure begins increasing.

Thermal expansion tank

If for some reason the TP valve does not open to relieve the pressure the water heater becomes a bomb and can explode with amazing power. One way to avoid problems with the T&P valve in this situation is to install an expansion tank at the inlet.

An expansion tank is a pressurized tank usually with a rubber bladder inside or a diaphragm in the middle.  As the water expands it moves into the expansion tank and doesn’t build up to dangerous pressure levels.

Usually local codes require the outlett of the T&P valve to be plumbed down to the floor, or outside to the ground, so no one can get scalded if the valve opens while they are nearby.

What is sediment and where does it come from?

One of the things that happen when you heat water is that dissolved minerals in the water can precipitate out and settle to the bottom. That is what sediment is.  You sometimes hear people say that you shouldn’t drink water that has passed through the water heater.  Obviously if some of the minerals settle out, then the hot water might have a lower mineral content, but that certainly can’t hurt you.

If it were un-healthy to drink water that has passed through the heater the NSF or FDA would be advising not to drink it.  There is simply no evidence that it is harmful in any way.  Now if you had lead pipes that would be another story.  Hot water could leach more lead out of the pipes than cold water.

If you live in an area with hard water, over time your sediment can build up to pretty significant levels.

If the buildup is large, the amount of water held by the water heater is reduced, so you could run out of water more quickly.

The anode protects against corrosion

Water heater tanks are internally coated with epoxy or have a glass lining. This is to prevent rusting and corrosion.  To further fight corrosion water heaters have a magnesium or aluminum rod called an anode inserted into them from the top. The idea is that the anode will corrode instead of the tank.

Smelly hot water -- rotten egg odor

Sometimes some anaerobic bacteria can get into the tank and will react with the magnesium or aluminum forming hydrogen sulfide gas which is what gives a rotten egg smell. This is more common with well water than from water mains.

Some people say that replacing the magnesium rod, the more common one, with aluminum can help. There are also some special (and expensive) anodes that are supposed to solve the problem. Anther method that works is to sterilize the tank and the hot water pipes with bleach or hydrogen peroxide.  However, the bacteria can come back, especially if you go away for a time and the water just sits in the heater for days or weeks.  Other than the awful smell the bacteria will not harm humans.

Drain valve - replace it with a ball valve

At the bottom of the tank there is a drain valve.  If you live in an area with hard water and you have a sediment problem, then you should probably replace the cheap plastic drain valve that most water heaters come with now days with a full flow ball valve.  This will make the tank much easier to drain and flush.

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