Electric Water Heaters - How to Test, Fix, Heating Elements, Thermostats, High Limit Switches, TPR valve Complete With Diagrams

Topics covered include TPR valves, testing heating elements, dip tube, Anodes, High Limit switches, Thermostats, Upper and Lower, Circuit Breaker, drain valve and tank.
 

All about electric water heaters, how they work, how to fix and repair them including how to test heating elements.

TPR Valve - Temperature Pressure Relief Valve is needed for water heater safety

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 tank, 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.

Sacrificial 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.

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

Thermostat, Upper and Lower

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.

Heating Elements

Typically there are two heating elements in an electric water heater, one near the top, and one near the bottom.  The heating elements are controlled by the upper and lower thermostats and the high limit switch.

Testing a water heater's heating elements

Be sure the power is shut off at the circuit breakers. Disconnect one of the two wires from the heating element.  Now using an ohm meter measure the resistance between the two terminals.

If the heating elements are good then you will get a reading of less than 50 ohms.  If the heating element is burned out the meter will read infinity, or open circuit.

The meter should also give you an open circuit reading between the terminals and the metal housing of the water heater element.

For more water heater information: Water Heater Information

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