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This guide assumes that the hot water heaters were properly installed and were operating correctly before any problems developed.
Most electric hot water heaters have two thermostats, one near the top of the tank and one near the bottom, and are covered by removable metal cover plates. The thermostats are pressed firmly against the bare metal wall of the hot water heaters tank.
The top thermostat usually has a high limit switch that will trip if the water gets too hot. When it trips it shuts off the electricity to both the upper and lower heating elements.
To reset the high limit switch there is usually a red button that you must press. When the upper limit switch trips it is often an indication that something else has gone wrong with the heater.
First remove the upper thermostat cover plate and check the high limit switch button. If it has tripped, press it to reset it.
Check for power at the input terminals of the upper thermostat. (usually marked L2 and L2) If no power is present then check your circuit breakers or wiring to the water heater.
If there is no hot water then the upper thermostat should be applying power to the upper heating element. Check for voltage at the upper heating element. If there is power at the heating element but the element is not getting hot then the heating element needs to be replaced.
If there is no power at the upper heating element then the upper thermostat needs to be replaced.
When the top of the tank is hot the upper thermostat removes power from the upper heating element and transfers the power to the lower thermostat and heating element. If the lower thermostat is defective, then the lower portion of the tank will not be heated and the supply will be greatly reduced.
Check for power at the upper thermostat terminals where the power is sent to the lower thermostat and heating element. If there is no power then the upper thermostat should be replaced. If there is power then check for power at the lower heating element. If there is no power at the lower heating element then replace the lower thermostat.
If there is power to the lower heating element then it should be getting hot. If it is not, replace it. Another possibility is a broken dip tube. Check for a broken dip tube and replace if necessary.
A likely cause is one of the thermostats. Check for power at the top heating element. If power is present at the top heating element, the top thermostat is defective. If power is present at the lower heating element then the lower thermostat is defective.
Another possible cause is if the thermostats are not pushed firmly against the tank.
The relief valve will operate if either the temperature is to high or if the pressure gets too high.
If the inlet is fed directly from the main with no pressure relief valves or check valves between them then when the water heater heats it expands and simply flows back toward the main.
If however there is a blockage such as a check valve or pressure reducing valve with a defective bypass then the increase in water volume has no where to go and the pressure will increase dramatically.
Placing an expansion tank in the line at the inlet will give the increased volume someplace to go and prevent operation of the relief valve.
Another possibility is that the City water pressure has increased above the setting of the relief valve.
Scale can build up on the heating elements causing all sorts of noises to occur while heating. Try removing the heating elements and either clean or replace them.
Certain types of bacteria can react with the magnesium anode rod resulting in a rotten egg odor. Clean the tank using chlorine bleach or changing the anode rod to aluminum usually will solve the problem.
Replace the tank.