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My Vaughn water heater is 20 years old and just recently started running out of hot water if I run two things at a time, such as the washer and a shower or dishwasher and wash dishes by hand. I know nothing about electronics, but was told by a friend who tested the elements with an ohmmeter that they "toned" as well as the lower thermostat, but the upper thermostat did not "tone". Does this make sense? Would I be running out of hot water if I had a faulty upper thermostat? Do you think I should just replace both elements since the tank is 20 years old?