If your hot water heater is not keeping water heated, it may not be the heater itself that is the problem. The water is actually heated by a heating element inside the tank that acts like a thermostat. When the water cools down below a certain temperature, the element will come on and reheat the water to the preset temperature. Over time, due to frequent use, the element can become corroded and wear out. You can do some simple tests to see if your heating element is the cause of your lack of hot water.
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Determine which of the two heating elements in your water heater could be the problem by turning on the water at the faucet and feeling the temperature of the water. If the water starts off cold, it could be the upper element, but if it starts off warm and turns cold, it could be the lower element.
Turn off the breaker switch to the hot water heater at the breaker box. If the heater is an electric unit, unplug it from the outlet. Remove the upper and lower access panels to the heating elements on the side of your water heater by using a screwdriver to undo the cover plate.
Unfasten one of the electrical wires attached to the upper heating element terminal with a screwdriver and move the wire out of the way, so that it doesn't contact any metal pieces. Set the multimeter to home position.
Touch both of the multimeter probes to the terminals on the front of the heating element and check the results. If your multimeter reads 7.5 ohms or more, the heating element is working properly; if the meter is reading "infinity" or "1", the element needs replacing.
Reattach the electrical wire to the heating element, then repeat step 4 for the lower heating element. If you find that either of the heating elements are bad, replace the element to get your water heater working again.