Electric Water Heaters - How an Electric Water Heater Works

TPR Valve

Electric water heaters have a TPR valve (Temperature Pressure Relief) or T&P valve, which is a safety device that releases pressure if the pressure or the temperature reach an unsafe level. All tank type units must have a TPR valve, otherwise they could blow up.

There is a manual release lever located on the valve. According to most water heater manufacturers, TPR valves should be periodically tested by lifting the manual release lever at least once a year. 

Sometimes the TPR valve is mounted on the top of the water heater, and sometimes it is mounted on the side of the tank.

Due to the scalding potential of the discharge from the TPR valve, the outlet from the valve should be piped to a safe area.  Typically TPR valve discharges are piped down to within 6 inches of the floor or even outside of the dwelling at near ground level.  Check with your local building department for local building code requirements.

Anode
The sacrificial anode is a metal rod usually magnesium or aluminum which helps prevent corrosion of  the electric water heaters tank.  Electrolysis eats away the anode instead of the tank. 

Once the anode is completely gone the tank itself begins to corrode, so you should check your anode and replace it if needed. The anode is screwed into the top of the tank and can in theory be easily replaced.  In real life, getting any fitting unscrewed after many years can be quite challenging. 

Sometimes the anode is built into a special outlet fitting.

Dip Tube
A dip tube is a long slender plastic tube that drops down into the inlet fitting, usually with a small hole (about 1/8" dia.) near the top of the tube.

The dip tube directs incoming cold water to the bottom of the water heater tank, preventing pre-mature mixing of incoming cold water with the out going hot water. If your dip tube is broken, it will seem as though you run out of hot water much too quickly.

 
Thermostat
The thermostat senses when the temperature in the tank drops below a certain set temperature and causes the electric heating elements to come on. When the heater reaches the desired temperature the thermostat shuts off the elements. 

Usually the top and bottom elements each have their own thermostat and are wired so that only one element comes on at a time. Normally the upper thermostat comes on first, and then when the top is hot, the upper thermostat re-directs the electricity to the lower thermostat and element. 

High limit switch
When the tank gets too hot it trips the high limit switch, (a circuit breaker). The high limit switch is in the top thermostat and usually has a red button. In order to get it working again the high limit switch must be manually reset by pushing in the red button.

Drain Valve
A drain valve allows draining periodically for removal of sediment, or for replacement. In areas with high mineral content it is recommended to drain at least 5 gallons from the drain valve every six months or so to prevent sediment build up.  Plastic drain valves are common.  Be careful not to break it. I'd recommend replacing it with a brass ball valve. A ball valve has a bigger opening through it making draining sediment clumps much easier. 

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